Title of Invention: SINGLE-CHAIN FUSION POLYPEPTIDE COMPRISING TRAIL Abstract: The present invention refers to single-chain fusion proteins comprising three soluble TNF superfamily (TNFSF) cytokine domains and nucleic acid molecules encoding these fusion proteins. The fusion proteins are substantially non-aggregating and suitable for therapeutic, diagnostic and/or research applications. Detailed Description: This application is a continuation of U.S. Ser. No. 15/172,393, filed Jun. 3, 2016; which is a continuation of U.S. application Ser. No. 14/320,261, filed Jun. 30, 2014, now U.S. Pat. No. 9,359,420; which is a continuation of U.S. application Ser. No. 13/902,328, filed May 24, 2013, now U.S. Pat. No. 8,921,519; which is a continuation of U.S. application Ser. No. 13/055,109, filed Mar. 10, 2011, now U.S. Pat. No. 8,450,460; which is a National Stage of International Application PCT/EP2009/059269, filed Jul. 18, 2009, published Jan. 28, 2010, under PCT Article 21(2) in English; which claims the priority of EP 08013112.1, filed Jul. 21, 2008. The contents of the above applications are incorporated herein by reference in their entirety. REFERENCE TO SEQUENCE LISTING, TABLE OR COMPUTER PROGRAM The Sequence Listing is concurrently submitted herewith with the specification as an ASCII formatted text file via EFS-Web with a file name of Sequence_Listing.txt with a creation date of Jun. 30, 2014, and a size of 150 kilobytes. The Sequence Listing filed via EFS-Web is part of the specification and is hereby incorporated in its entirety by reference herein. DESCRIPTION The present invention refers to single-chain fusion proteins comprising three soluble TNF superfamily (TNFSF) cytokine domains and nucleic acid molecules encoding the fusion proteins. The fusion proteins are substantially non-aggregating and suitable for therapeutic, diagnostic and/or research applications. STATE OF THE ART It is known that trimerization of TNFSF cytokines, e.g., the CD95 ligand (CD95L), is required for efficient receptor binding and activation. Trimeric complexes of TNF superfamily cytokines, however, are difficult to prepare from recombinant monomeric units. WO 01/49866 and WO 02/09055 disclose recombinant fusion proteins comprising a TNF cytokine and a multimerization component, particularly a protein from the C1q protein family or a collectin. A disadvantage of these fusion proteins is, however, that the trimerisation domain usually has a large molecular weight and/or that the trimerisation is rather inefficient. Schneider et al. (J Exp Med 187 (1989), 1205-1213) describe that trimers of TNF cytokines are stabilised by N-terminally positioned stabilisation motifs. In CD95L, the stabilisation of the receptor binding domain trimer is presumably caused by N-terminal amino acid domains which are located near the cytoplasmic membrane. Shiraishi et al. (Biochem Biophys Res Commun 322 (2004), 197-202) describe that the receptor binding domain of CD95L may be stabilised by N-terminally positioned artificial α-helical coiled-coil (leucine zipper) motifs. It was found, however, that the orientation of the polypeptide chains to each other, e.g. parallel or antiparallel orientation, can hardly be predicted. Further, the optimal number of heptad-repeats in the coiled-coil zipper motif are difficult to determine. In addition, coiled-coil structures have the tendency to form macromolecular aggregates after alteration of pH and/or ionic strength. WO 01/25277 relates to single-chain oligomeric polypeptides which bind to an extracellular ligand binding domain of a cellular receptor, wherein the polypeptide comprises at least three receptor binding sites of which at least one is capable of binding to a ligand binding domain of the cellular receptor and at least one is incapable of effectively binding to a ligand binding domain of the cellular receptor, whereby the single-chain oligomeric polypeptides are capable of binding to the receptor, but incapable of activating the receptor. For example, the monomers are derived from cytokine ligands of the TNF family, particularly from TNF-α. WO 2005/103077 discloses single-chain fusion polypeptides comprising at least three monomers of a TNF family ligand member and at least two peptide linkers that link the monomers of the TNF ligand family members to one another. Recent experiments, however, have shown that these single-chain fusion polypeptides show undesired aggregation. It was an object of the present invention to provide single-chain fusion proteins comprising at least three TNF cytokine domains which allow efficient recombinant manufacturing combined with good stability concerning aggregation. SUMMARY OF THE INVENTION The present invention relates to a single-chain fusion polypeptide comprising: (i) a first soluble TNF-family cytokine domain, (ii) a first peptide linker, (iii) a second soluble TNF-family cytokine domain, (iv) a second peptide linker, and (v) a third soluble TNF-family cytokine domain, which is substantially non-aggregating. The invention further relates to a nucleic acid molecule encoding a fusion protein as described herein and to a cell or a non-human organism transformed or transfected with a nucleic acid molecule as described herein. The invention also relates to a pharmaceutical or diagnostic composition comprising as an active agent a fusion protein, a nucleic acid molecule, or a cell as described herein. The invention also relates to a fusion protein, a nucleic acid molecule, or a cell as described herein for use in therapy, e.g., the use of a fusion protein, a nucleic acid molecule, or a cell as described herein for the preparation of a pharmaceutical composition in the prophylaxis and/or treatment of disorders caused by, associated with and/or accompanied by dysfunction of TNFSF cytokines, particularly proliferative disorders, such as tumours, e.g. solid or lymphatic tumours; infectious diseases; inflammatory diseases; metabolic diseases; autoimmune disorders, e.g. rheumatoid and/or arthritic diseases; degenerative diseases, e.g. neurodegenerative diseases such as multiple sclerosis; apoptosis-associated diseases or transplant rejections. DESCRIPTION OF THE FIGURES FIG. 1 Domain structure of the inventive single-chain fusion polypeptide. I., II., III. soluble TNF-family cytokine domains. FIG. 2 Schematic picture representing the general structure of TNF-SF proteins. ▪▪▪ cell membrane, N-terminus located within the cell, 1. anti-parallel β-fold of receptor-binding domain (RBD), 2. interface of RBD and cell membrane, 3. protease cleavage site. FIG. 3 Schematic picture representing the structure of the native TNF-SF trimer. Cylindric structures represent RBDs, N-termini connect RBD with the cell membrane. FIG. 4 Schematic picture representing the structure of three soluble domains comprising the receptor-binding domain of a TNF cytokine. I., II., III. soluble TNF-family cytokine domains. FIG. 5 Trimerisation of the soluble domains comprising the RBD of a TNF cytokine, characterised in that the N- and C-termini of the three soluble domains form a surface. FIG. 6 Schematic picture representing the structure of the single-chain TNF-SF comprising all or a part of the stalk-region illustrating the requirement of longer linkers to compensate for the distance to the N-terminus of the next soluble domain. FIG. 7 scFv-TNF-SF fusion protein known from the art. FIG. 8 Fc-TNF-SF fusion protein known from the art. FIG. 9 9A Single-chain fusion polypeptide comprising an additional Fab antibody fragment. 9B Single-chain fusion polypeptide comprising an additional scFv antibody fragment. FIG. 10 Dimerization of two N-terminally fused scFc fusion polypeptides via disulfide bridges. FIG. 11 Dimerization of two C-terminally fused scFc fusion polypeptides via disulfide bridges. FIG. 12 Dimerization of single-chain fusion polypeptides via a linker. FIG. 13 Single-chain fusion polypeptide comprising an additional Fab antibody fragment further fused to a second fusion polypeptide or to a scFv fusion polypeptide. FIG. 14 Dimerization of two scFab fusion polypeptides via disulfide bridges. FIG. 15 N-terminally fused scFc fusion polypeptides further comprising a Fv and/or Fab antibody fragment. FIG. 16 C-terminally fused scFc fusion polypeptides further comprising a Fv and/or Fab antibody fragment. FIG. 17 SEC analysis of recombinantly expressed, purified TNF-SF members under native conditions. Exemplarily shown are two SEC analyses of purified TNF-SF members on a Superdex200 column under native condition (e.g.: PBS, pH 7.4). The diagrams show the absorption at 280 nm (mAU) plotted against the elution volume (ml). The filled arrow indicates the elution peak for the fraction containing defined, soluble trimeric TNF-SF protein. The triangle indicates the elution peak for the oligomerised TNF-SF. The open arrow indicates the void volume of the SEC-column that contains protein-aggregates, which are too big to be separated (>800 kDa). A: TNF-SF protein Aggregation Diagram A exemplarily shows an analysis of a TNF-SF protein preparation that contains a high amount of oligomerised/aggregated protein (indicated by the high amount of protein eluting in the void volume and the high amount of oligomeric protein). B: TNF-SF protein defined soluble protein Diagram B exemplarily shows an analysis for a TNF-SF protein preparation that contains almost exclusively defined soluble protein (indicated by the absence of protein eluting in the void volume and by the very limited amount of protein eluting as oligomer). FIG. 18 SEC analysis of recombinantly expressed, affinity purified Fab-scTRAILR2-SSSS. SEC analysis of Fab-scTRAILR2-SSSS on a Superdex200 column using PBS, pH 7.4. The diagram shows the absorption at 280 nm (mAU) plotted against the elution volume (ml). The protein elutes as a distinct peak with an elution volume of 14.56 ml, corresponding to an apparent MW of 68 kDa. No additional protein peaks with lower retention volume, indicating oligomerised/aggregated protein, could be observed. FIG. 19 SEC analysis of recombinantly expressed, affinity purified Fab-scTRAILR2-SNSN. SEC analysis of Fab-scTRAILR2-SNSN on a Superdex200 column using PBS, pH 7.4. The diagram shows the absorption at 280 nm (mAU) plotted against the elution volume (ml). The protein elutes as a distinct peak with an elution volume of 14.12 ml, corresponding to an apparent MW of 87 kDa. No additional protein peaks with lower retention volume, indicating oligomerised/aggregated protein, could be observed. FIG. 20 SEC analysis of recombinantly expressed, affinity purified Fab-scTRAILwt-SNSN. SEC analysis of Fab-scTRAILwt-SNSN on a Superdex200 column using PBS, pH 7.4. The diagram shows the absorption at 280 nm (mAU) plotted against the elution volume (ml). The protein elutes as a distinct peak with an elution volume of 13.99 ml, corresponding to an apparent MW of 94 kDa. A small additional protein hwest@example.net ml could be observed. The apparent Mw of this peak corresponds to about 270 kDa, indicating a defined trimerisation of Fab-scTRAILwt-SNSN. The total protein amount of the hwest@example.net ml accounts for <3% of the total protein. More than 97% of the analysed Fab-scTRAILwt-SNSN has a defined soluble state (correct assembly of the three receptor binding modules). The hwest@example.net ml corresponding to a MW of 28 kDa contains Fab-light-chain polypeptide and was not included for the analysis of peak areas. FIG. 21 Human scTRAIL Linker glycosylation A: Amino acid sequence of the linker(s) used to combine the receptor binding modules of single chain TRAIL constructs. Gly281 encodes the last amino acid of a respective receptor binding module, the sequence GSGN/SGN/SGS (SEQ ID NOs: 52-54) encodes the linker sequence, Arg121 encodes the first amino acid of the following TRAIL receptor binding domain. The designed linker sequences contains two putative N-linked glycosylation sites at position 1 or 2 as indicated. These positions were permutated as indicated (version I, II, III, SEQ ID NOs: 52-54). B: Combination of linker positions: The scTRAIL molecules contain three homologue modules (grey barrels) that are connected with linker 1 (SEQ ID NOs: 52-54) and linker 2 (SEQ ID NOs: 52-54) as indicated. Each of the two linkers, can be designed for N-linked glycosylation as described in “A”. A complete set of 9 different proteins containing all possible combinations of linkers can be designed based on the sequences shown in B for linker 1 and 2. (Six of these proteins were expressed—see “C”). C: Nomenclature of scTRAIL constructs expressed to test the influence of different linker sequences on glycosylation FIG. 22 Western Blot analysis of recombinant scTRAIL constructs Single chain TRAIL proteins with different linker sequences were recombinantly expressed, separated by SDS-PAGE and transferred to a PVDF-membrane. Bound proteins were detected with a mouse monoclonal antibody recognising the Strep-Tag followed by a Peroxidase-conjugated secondary anti-mouse antibody. Different TRAIL variants were loaded as indicated. Note the MW-shift indicating differential glycosylation of scTRAIL-linker variants. FIG. 23 Cell culture supernatant of HEK293 cells, transiently expressing scCD95L (SEQ ID NO:27) was collected and used to stimulate Jurkat cells at varying concentrations. The supernatant was used either directly without further modifications or an anti-Streptag antibody (2 microgram/ml) was added to cross-link the scCD95L protein. Jurkat cells were incubated with HEK293 cell culture supernatant for three hours at 37°, lysed and analysed for caspase activity. Only cell supernatant that contained cross-linked scCD95L-St increased caspase activity in Jurkat cells, indicating that scCD95L alone does not form higher order aggregates able to be pro-apoptotic. FIG. 24 The protein scCD95L (SEQ ID NO:27) can be produced by transient transfection of HEK293 cells, stable transfection of other eukaryotic cells or by expression using prokaryotic cells. The recombinant protein can be affinity purified by using StrepTactin Sepharose matrix. Bound protein can be eluted with a buffer containing desthio-biotin. FIG. 2 shows a silver stained SDS-PAGE of the elution fractions (lanes 1 to 5; fraction 2 is positive) of the affinity purification. The elution fraction containing scCD95L could be applied to size exclusion chromatography (SEC). It is expected, that the protein shows only a low aggregate content. FIG. 25 Cell culture supernatants of HEK293 cells, transiently expressing single chain TRAIL proteins with different linkers (derived from SEQ ID NO: 28) were collected and used to stimulate Jurkat cells at varying dilutions (exemplarily, a dilution of 1:8 is shown in this figure). The supernatants were used either directly without further modifications or an anti-Streptag antibody (2 microgram/ml Strep MAB Immo) was added to cross-link the scTRAIL proteins. Jurkat cells were incubated with HEK293 cell culture supernatant for three hours at 37°, lysed and analysed for caspase activity. Cell culture supernatant that contained cross-linked scTRAILwt proteins induced an increased caspase activity in Jurkat cells, indicating that scTRAILwt proteins alone do form only a low amount of higher order aggregates able to be pro-apoptotic. FIG. 26 Influence of the module succession of scTRAIL-construct components on their expression rate of Fab-scTRAIL fusion proteins. Western blot of HEK293T cell culture supernatants from transient expression experiments. The polypeptide chains necessary for the formation of the Fab-scTRAIL proteins were either expressed separately (lanes 1 to 10) or alternatively co-expression experiments were performed (lanes 11-13). After reducing SDS-PAGE, proteins were transferred to a nitrocellulose membrane and proteins containing a Streptag were detected, using an anti-Streptag specific mAB as primary AB. The light-chain-scTRAIL(R2-specific) proteins were secreted even in the absence of the accessory heavy chain (lanes 1-4). In contrast, the heavy-chain-scTRAIL(R2-specific) fusion proteins were not secreted in the absence of the accessory light chain (lanes 5-8). As exemplified in lane 13, the heavy-chain-scTRAIL(R2-specific) fusion proteins were only secreted in the presence of the light chain. FIG. 27 Cell culture supernatants of HEK293T cells, transiently expressing scTRAILwt-Fc fusion proteins with different linkers were collected and used to stimulate Jurkat cells at varying dilutions. The supernatants were used directly without further modifications. Jurkat cells were incubated with HEK293T cell culture supernatant for three hours at 37°, lysed and analysed for caspase activity. There was already a pronounced pro-apoptotic capacity present in the scTRAILwt-Fc containing supernatants, indicating that scTRAILwt-Fc fusion proteins alone do form dimeric assemblies able to be pro-apoptotic. FIG. 28 It is well known that the use of artificially cross-linked or a membrane-bound ligand of the TNF superfamily has superior bioactivity as compared to soluble, homotrimeric ligand. Thus the local enrichment of single chain TRAIL (scTRAIL) constructs on cells that express the antigen Her2 via the Her2-selective Fab-fragment (“Pertuzumab”) fused to these scTRAIL proteins should increase their cytotoxic bioactivity. Likewise, the blocking of the Her2 binding sites on cells by pre-incubation with the Her2-specific Fab-fragment (Pertuzumab-Fab) only should decrease the cytotoxic bioactivity of Fab-scTRAIL fusion proteins. As shown in A, scTRAIL constructs induce the death of HT1080 cells, as the viability decreases with increasing protein concentration. In accordance, the pre-incubation of HT1080 cells with the Fab-fragment (Pertuzumab-Fab), followed by co-incubation with the Fab-scTRAIL constructs (Fab-scTRAILR2-SNSN or Fab-scTRAILwt-SNSN) overnight, reduced the cytotoxic activity of the Fab-scTRAIL constructs (B), whereas the Fab only induced no cell death (Pertuzumab-Fab). This means that the Fab-scTRAIL constructs bind to HT1080 cells via the Fab fragment thus increasing the cytotoxic bioactivity of scTRAIL. DETAILED DESCRIPTION OF THE INVENTION According to the present invention a substantially non-aggregating fusion polypeptide comprising at least three soluble TNF family ligand domains connected by two peptide linkers is provided. The term “non-aggregating” refers to a monomer content of the preparation of ≧50%, preferably ≧70% and more preferably ≧90%. The ratio of monomer content to aggregate content may be determined by examining the amount of aggregate formation using size-exclusion chromatography (SEC). The stability concerning aggregation may be determined by SEC after defined time periods, e.g. from a few to several days, to weeks and months under different storage conditions, e.g. at 4° C. or 25° C. For the fusion protein, in order to be classified as substantially non-aggregating, it is preferred that the monomer content is as defined above after a time period of several days, e.g. 10 days, more preferably after several weeks, e.g. 2, 3 or 4 weeks, and most preferably after several months, e.g. 2 or 3 months of storage at 4° C., or 25° C. As an increase of e.g. the apoptosis inducing potential in the case of scCD95L on human Jurkat cells correlates with its aggregation state, the stability of the fusion polypeptide concerning aggregation may also be determined by examining the biological activity of the fusion polypeptide. The single-chain fusion polypeptide may comprise additional domains which may be located at the N- and/or C-termini thereof. Examples for additional fusion domains are e.g. single-chain antibodies or antibody fragments or other targeting molecules or a further cytokine domain, e.g. an interleukin. The single-chain fusion protein comprises three soluble domains derived from a cytokine of the TNF superfamily. Preferably, those soluble domains are derived from a mammalian, particularly human cytokine including allelic variants and/or derivatives thereof. The soluble domains comprise the extracellular portion of a TNFSF cytokine including the receptor binding domain without membrane located domains. Proteins of the TNF superfamily are anchored to the membrane via an N-terminal portion of 15-30 amino acids, the so-called stalk-region. The stalk region contributes to trimerization and provides a certain distance to the cell membrane. However, the stalk region is not part of the receptor binding domain (RBD). Importantly, the RBD is characterised by a particular localisation of its N- and C-terminal amino acids. Said amino acids are immediately adjacent and are located centrally to the axis of the trimer. The first N-terminal amino acids of the RBD form an anti-parallel beta-strand with the C-terminal amino acids of the RBD (FIGS. 2 and 3). Thus, the anti-parallel beta-strand of the RBD forms an interface with the cell membrane, which is connected to and anchored within the cell membrane via the amino acids of the stalk region. It is highly preferred that the soluble domains of the single-chain fusion protein comprises a receptor binding domain of the TNF-SF cytokine lacking any amino acids from the stalk region (FIGS. 4 and 5). Otherwise, a long linker connecting the C-terminus of one of the soluble domains with the N-terminus of the next soluble domain would be required to compensate for the N-terminal stalk-region of the next soluble domain (FIG. 6), which might result in instability and/or formation of aggregates. A further advantage of such soluble domains is that the N- and C-terminal amino acids of the RBD are not accessible for any anti-drug antibodies. Preferably, the single-chain fusion polypeptide is capable of forming an ordered trimeric structure comprising at least one functional binding site for the respective cytokine receptor. The fusion polypeptide may comprise one, two or three functional cytokine receptor binding sites, i.e. amino acid sequences capable of forming a complex with a cytokine receptor. Thus, at least one of the soluble domains is capable of binding to the corresponding cytokine receptor. In one embodiment, at least one of the soluble domains is capable of receptor activation, whereby apoptotic and/or proliferative activity may be effected. In a further embodiment, one or more of the soluble domains are selected as not being capable of receptor activation. The soluble domain may be derived from TNF superfamily members, e.g. human TNFSF-1 to -18 and EDA-A1 to -A2 as indicated in Table 1, preferably from LTA (SEQ ID NO:1), TNFα (SEQ ID NO:2), LTB (SEQ ID NO:3), OX40L (SEQ ID NO:4), CD40L (SEQ ID NO:5), CD95L (SEQ ID NO:6), CD27L (SEQ ID NO:7), CD30L (SEQ ID NO:8), CD137L (SEQ ID NO:9), TRAIL (SEQ ID NO:10), RANKL (SEQ ID NO:11), TWEAK (SEQ ID NO:12), APRIL 1 (SEQ ID NO:13), APRIL 2 (SEQ ID NO:14), BAFF (SEQ ID NO:15), LIGHT (SEQ ID NO:16), TL1A (SEQ ID NO:17), GITRL (SEQ ID NO:18), EDA-A1 (SEQ ID NO:19) and EDA-A2 (SEQ ID NO:20). Preferred soluble domains of the respective proteins are indicated in Table 1 (NH2-aa to COOH-aa) and, e.g., comprise amino acids 59-205, 60-205 or 64-205 of LTA (SEQ ID NO:1), 86-233 of TNFα (SEQ ID NO:2), 82-244 or 86-244 of LTB (SEQ ID NO:3), 52-183 or 55-183 of OX40L (SEQ ID NO:4), 112-261, 117-261 or 121-261 of CD40L (SEQ ID NO:5), 51-193 or 56-193 of CD27L (SEQ ID NO:7), 97-234, 98-234 or 102-234 of CD30L (SEQ ID NO:8), 86-254 of CD137L (SEQ ID NO:9), 161-317 of RANKL (SEQ ID NO:11), 103-249, 104-249, 105-249 or 106-249 of TWEAK (SEQ ID NO:12), 112-247 of APRIL 1 (SEQ ID NO:13), 112-250 of APRIL 2 (SEQ ID NO:14), 140-285 of BAFF (SEQ ID NO:15), 91-251, 93-251 or 97-251 of TL1A (SEQ ID NO:17), 52-177 of GITRL (SEQ ID NO:18), 245-391 of EDA-A1 (SEQ ID NO:19), 245-389 of EDA-A2 (SEQ ID NO:20). More preferably, the soluble domains are derived from CD95L, TRAIL or LIGHT. In an especially preferred embodiment, the soluble domains are selected from human CD95L, particularly starting from amino acids 144, 145 or 146 and comprise particularly amino acids 144-281 or 145-281 or 146-281 of SEQ ID NO:6 or human TRAIL, particularly starting from amino acids 120-122 and comprise particularly amino acids 120-281, 121-281 or 122-281 of SEQ ID NO:10. Optionally, amino acid Lys145 of SEQ ID NO:6 may be replaced by a non-charged amino acid, e.g. Ser or Gly. Optionally, amino acid Arg121 of SEQ ID NO:10 may be replaced by a non-charged amino acid, e.g. Ser or Gly. In a further preferred embodiment, the soluble domains are selected from human LIGHT, particularly starting from amino acids 93, 94 or 95 of SEQ ID NO:16 and particularly comprise amino acids 93-240, 94-240 or 95-240 of SEQ ID NO:16. As indicated above, the soluble domains may comprise the wild-type sequences as indicated in SEQ ID NO: 1-20. It should be noted, however, that it is possible to introduce mutations in one or more of these soluble domains, e.g. mutations which alter (e.g. increase or decrease) the binding properties of the soluble domains. In one embodiment, soluble domains may be selected which cannot bind to the corresponding cytokine receptor. An example of such a mutation is a replacement of amino acid Y218 in human CD95L (SEQ ID NO:6) by another amino acid, e.g. R, K, S or D. Further, a mutation may be introduced which alters the binding to other cellular and/or extracellular components, e.g. the extracellular matrix. An example of such a mutation is a replacement of amino acid K177 in CD95L (SEQ ID NO: 6) by another amino acid, e.g. E, D or S. In a further preferred embodiment of the invention, the soluble cytokine domain (i) comprises a mutant of the cytokine of the TNF superfamily or a receptor binding domain thereof which binds and/or activates TRAIL-receptor 1 (TRAILR1) and/or TRAIL-receptor 2 (TRAILR2). The binding and/or activity of the mutant may be, e.g., determined by the assays as described in van der Sloot et al. (PNAS, 2006, 103:8634-8639), Kelley et al. (J. Biol. Chem., 2005, 280:2205-2215), or MacFarlane et al. (Cancer Res., 2005, 65: 11265-11270). The mutant may be generated by any technique and is known by the skilled person, e.g., the techniques described in van der Sloot et al. (PNAS, 2006, 103:8634-8639), Kelley et al. (J. Biol. Chem., 2005, 280:2205-2215), or MacFarlane et al. (Cancer Res., 2005, 65: 11265-11270) and may comprise any type of structural mutations, e.g., substitution, deletion, duplication and/or insertion of an amino acid. A preferred embodiment is the generation of substitutions. The substitution may affect at least one amino acid of the cytokine of the TNF superfamily or a receptor binding domain thereof as described herein. In a preferred embodiment, the substitution may affect at least one of the amino acids of TRAIL, e.g., human TRAIL (e.g., SEQ ID NO: 10). Preferred substitutions in this regard affect at least one of the following amino acids of human TRAIL of SEQ ID NO:10: R130, G160, Y189, R191, Q193, E195, N199, K201, Y213, T214, S215, H264, I266, D267, D269. Preferred amino acid substitutions of human TRAIL of SEQ ID NO:10 are at least one of the following substitutions: R130E, G160M, Y189A, Y189Q, R191K, Q193S, Q193R, E195R, N199V, N199R, K201R, Y213W, T214R, S215D, H264R, I266L, D267Q, D269H, D269R, or D269K. The amino acid substitution(s) may affect the binding and/or activity of TRAIL, e.g., human TRAIL, to or on either the TRAILR1 or the TRAILR2. Alternatively, the amino acid substitution(s) may affect the binding and/or activity of TRAIL, e.g., human TRAIL, to or on both, the TRAILR1 and the TRAILR2. The binding and/or activity of the TRAILR1 and/or TRAILR2 may be affected positively, i.e., stronger, more selective or more specific binding and/or more activation of the receptor. Alternatively, the binding and/or activity of the TRAILR1 and/or TRAILR2 may be affected negatively, i.e., weaker, less selective or less specific binding and/or less or no activation of the receptor. Examples of mutants of TRAIL with amino acid substitution(s) of the invention that affect binding and/or activation of both TRAILR1 and TRAILR2 may be found, e.g., in Table 1 of MacFarlane et al. (cf. above) and may comprise a human TRAIL mutant with the following two amino acid substitutions of SEQ ID NO: 10 Y213W and S215D or with the following single amino acid substitution: Y189A. Examples of mutants of TRAIL with amino acid substitution(s) of the invention that affect binding and/or activation of TRAILR1 may be found, e.g., in Table 1 of MacFarlane et al. (cf. above) and may comprise a human TRAIL mutant with the following four amino acid substitutions of SEQ ID NO: 10 N199V, K201R, Y213W and S215D or with the following five amino acid substitutions: Q193S, N199V, K201R, Y213W and S215D, or may be found in Table 2 of Kelley et al. (cf. above) and may comprise a human TRAIL mutant with the following six amino acid substitutions: Y213W, S215D, Y189A, Q193S, N199V, and K201R, or with Y213W, S215D, Y189A, Q193S, N199R, and K201R. Examples of mutants of TRAIL with amino acid substitution(s) of the invention that affect binding and/or activation of TRAILR2 may be found, e.g., in Table 1 of MacFarlane et al. (cf. above) or in Table 2 of Kelley et al. (cf. above) and may comprise a human TRAIL mutant with the following six amino acid substitutions of SEQ ID NO: 10: Y189Q, R191K, Q193R, H264R, I266L, and D267Q, or may be found in Table 2 of van der Sloot et al. (cf. above) and may comprise a human TRAIL mutant with the following single amino acid substitution: D269H, or with the following two amino acid substitutions: D269H and E195R or D269H and T214R. Thus one preferred embodiment is a fusion protein as described herein wherein at least one of the soluble domains comprises a mutant of TRAIL or of a receptor binding domain thereof which binds and/or activates TRAILR1 and/or TRAILR2. Further examples of mutants of TRAIL, which show reduced TRAIL induced receptor aggregation are H168 (S, T, Q), R170 (E, S, T, Q) and H177 (S, T). One preferred embodiment of a fusion protein comprising a mutant of TRAIL or of a receptor binding domain as described herein is a fusion protein wherein component (i) comprises at least one amino acid substitution, particularly as indicated below. Such an amino acid substitution affects at least one of the following amino acid positions of human TRAIL (SEQ ID NO: 10): R130, G160, H168, R170, H177, Y189, R191, Q193, E195, N199, K201, Y213, T214, S215, H264, I266, D267, D269. Such an amino acid substitution is at least one of the following: R130E, G160M, H168 (S, T, Q), R170 (E, S, T, Q), H177 (S,T), Y189A, Y189Q, R191K, Q193S, Q193R, E195R, N199V, N199R, K201R, Y213W, T214R, S215D, H264R, I266L, D267Q, D269H, D269R, or D269K. A preferred TRAIL-R2 selective domain comprises amino acid substitutions Y189Q, R191K, Q193R, H264R, I266L and D267Q. A preferred TRAIL-R1 selective domain comprises amino acid substitutions Y189A, Q193 S, N199V, K201R, Y213W and S215D. The single-chain fusion molecule of the present invention comprises additionally three soluble cytokine domains, namely components (i), (iii) and (v). According to the present invention, it was surprisingly found that the stability of a single-chain TNF family cytokine fusion polypeptide against aggregation is enhanced, if the second and/or third soluble TNF family cytokine domain is an N-terminally shortened domain which optionally comprises amino acid sequence mutations. Thus, preferably, both the second and the third soluble TNF family cytokine domain are N-terminally shortened domains which optionally comprise amino acid sequence mutations in the N-terminal regions, preferably within the first five amino acids of the N-terminus of the soluble cytokine domain. These mutations may comprise replacement of charged, e.g. acidic or basic amino acids, by neutral amino acids, particularly serine or glycine. In contrast thereto, the selection of the first soluble TNF family cytokine domain is not as critical. Here, a soluble domain having a full-length N-terminal sequence may be used. It should be noted, however, that also the first soluble cytokine domain may have an N-terminally shortened and optionally mutated sequence. In a preferred embodiment of the present invention, the soluble TNF family cytokine domains (i), (iii) and (v) are soluble CD95L domains, particularly soluble human CD95L domains. The first soluble CD95L domain (i) may be selected from native, shortened and/or mutated sequences. The N-terminal sequence of the first domain (i) may e.g. start between amino acid Glu142 and Val146 of human CD95L, wherein Arg144 and/or Lys145 may be replaced by a neutral amino acid, e.g. by Ser or Gly. The second and third soluble CD95L domains (iii) and (v), however, are selected from shortened and/or mutated sequences. Preferably, at least one of the soluble CD95L domains, (iii) and (v), has an N-terminal sequence which starts between amino acid Arg144 and Val146 of human CD95L, and wherein Arg144 and/or Lys145 may be replaced by a neutral amino acid, e.g. by Ser and/or Gly. In an especially preferred embodiment, the second and third soluble CD95L domain start with an N-terminal sequence selected from: (a) Arg144-(Gly/Ser) 145-Val (146) (b) (Gly/Ser) 144-Lys145-Val (146) and (c) (Gly/Ser) 144-(Gly/Ser) 145-Val (146). Further, it is preferred that the CD95L domain ends with amino acid Leu 281 of human CD95L. The soluble CD95L domain may comprise a mammalian, e.g. a human wild-type sequence. In certain embodiments, however, the CD95L sequence may comprise a mutation which results in a reduction or complete inhibition of the binding to the extracellular matrix, e.g. a mutation at position Lys177, e.g. Lys177→Glu, Asp or Ser and/or a mutation which reduces and/or inhibits binding to the CD95L receptor, e.g. a mutation at position Tyr218, e.g. Tyr218→Arg, Lys, Ser, Asp. In certain embodiments of the present invention, one of the three soluble CD95L modules is a sequence variant with a reduced receptor binding. In other embodiments, two of the modules contain mutations resulting in reduced receptor binding. In a further preferred embodiment of the present invention, the soluble TNF family cytokine domains (i), (iii) and (v) are soluble TRAIL domains, particularly soluble human TRAIL domains. The first soluble TRAIL domain (i) may be selected from native, shortened and/or mutated sequences. Thus, the first soluble TRAIL domain (i) has an N-terminal sequence which may start between amino acid Glu116 and Val122 of human TRAIL, and wherein Arg121 may be replaced by a neutral amino acid, e.g. by Ser or Gly. The second and third soluble TRAIL domains (iii) and (v) have a shortened N-terminal sequence which preferably starts between amino acid Gly120 and Val122 of human TRAIL and wherein Arg121 may be replaced by another amino acid, e.g. Ser or Gly. Preferably, the N-terminal sequence of the soluble TRAIL domains (iii) and (v) is selected from: (a) Arg121-Val122-Ala123 and (b) (Gly/Ser)121. The soluble TRAIL domain preferably ends with amino acid Gly281 of human TRAIL. In certain embodiments, the TRAIL domain may comprise internal mutations as described above. In a further preferred embodiment of the present invention, the soluble TNF family cytokine domains (i), (iii) and (v) are soluble LIGHT domains, particularly soluble human LIGHT domains. The first soluble LIGHT domain (i) may be selected from native, shortened and/or mutated sequences. Thus, the first soluble LIGHT domain (i) has an N-terminal sequence which may start between amino acid Glu91 and Ala95 of human LIGHT. The second and third soluble LIGHT domains (iii) and (v) have a shortened N-terminal sequence which preferably starts between amino acid Pro94 and Ala95 of human LIGHT. The soluble LIGHT domain preferably ends with amino acid Val240. Components (ii) and (iv) of the single-chain fusion polypeptide are peptide linker elements located between components (i) and (iii) or (iii) and (v), respectively. The flexible linker elements have a length of 3-8 amino acids, particularly a length of 3, 4, 5, 6, 7, or 8 amino acids. The linker elements are preferably glycine/serine linkers, i.e. peptide linkers substantially consisting of the amino acids glycine and serine. In cases in which the soluble cytokine domain terminates with S or G (C-terminus), e.g. human TRAIL, the linker starts after S or G. In cases in which the soluble cytokine domain starts with S or G (N-terminus), the linker ends before this S or G. It should be noted that linker (ii) and linker (iv) do not need to be of the same length. In order to decrease potential immunogenicity, it may be preferred to use shorter linkers. In addition it turned out that shorter linkers lead to single chain molecules with reduced tendency to form aggregates. Whereas linkers that are substantially longer than the ones disclosed here may exhibit unfavourable aggregations properties. If desired, the linker may comprise an asparagine residue which may form a glycosylation site Asn-Xaa-Ser. In certain embodiments, one of the linkers, e.g. linker (ii) or linker (iv) comprises a glycosylation site. In other embodiments, both linkers (iv) comprise glycosylation sites. In order to increase the solubility of the scTNF-SF proteins and/or in order to reduce the potential immunogenicity, it may be preferred that linker (ii) or linker (iv) or both comprise a glycosylation site. Preferred linker sequences are selected from GSGSGSGS (SEQ ID NO:52), GSGSGNGS (SEQ ID NO:53), GGSGSGSG (SEQ ID NO:21), GGSGSG (SEQ ID NO:22), GGSG (SEQ ID NO:23), GGSGNGSG (SEQ ID NO:24), GGNGSGSG (SEQ ID NO:25) and GGNGSG (SEQ ID NO:26) The fusion protein may additionally comprise an N-terminal signal peptide domain, which allows processing, e.g. extracellular secretion, in a suitable host cell. Preferably, the N-terminal signal peptide domain comprises a protease cleavage site, e.g. a signal peptidase cleavage site and thus may be removed after or during expression to obtain the mature protein. Further, the fusion protein may additionally comprise a C-terminal element, having a length of e.g. 1-50, preferably 10-30 amino acids which may include or connect to a recognition/purification domain, e.g. a FLAG domain, a Strep-tag or Strep-tag II domain and/or a poly-His domain. Further, the fusion polypeptide may additionally comprise N-terminally and/or C-terminally a further domain, e.g. a targeting domain such as a single-chain antibody or an antibody fragment domain. Specific examples of suitable antibodies are anti-tumour antibodies, such as antibodies against EGFR-family members. Suitable examples of other targeting molecules are cytokines, such as interleukins. Examples of specific fusion proteins of the invention are SEQ ID NOs: 27, 28, 29, 43, 45, 47, 49 and 51. A further aspect of the present invention relates to a nucleic acid molecule encoding a fusion protein as described herein. The nucleic acid molecule may be a DNA molecule, e.g. a double-stranded or single-stranded DNA molecule, or an RNA molecule. The nucleic acid molecule may encode the fusion protein or a precursor thereof, e.g. a pro- or pre-proform of the fusion protein which may comprise a signal sequence or other heterologous amino acid portions for secretion or purification which are preferably located at the N- and/or C-terminus of the fusion protein. The heterologous amino acid portions may be linked to the first and/or second domain via a protease cleavage site, e.g. a Factor Xa, thrombin or IgA protease cleavage site. Examples of specific nucleic acid sequences of the invention are SEQ ID NOs: 30, 31 32, 44, 46, 48 and 50. The nucleic acid molecule may be operatively linked to an expression control sequence, e.g. an expression control sequence which allows expression of the nucleic acid molecule in a desired host cell. The nucleic acid molecule may be located on a vector, e.g. a plasmid, a bacteriophage, a viral vector, a chromosomal integration vector, etc. Examples of suitable expression control sequences and vectors are described for example by Sambrook et al. (1989) Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Press, and Ausubel et al. (1989), Current Protocols in Molecular Biology, John Wiley & Sons or more recent editions thereof. Various expression vector/host cell systems may be used to express the nucleic acid sequences encoding the fusion proteins of the present invention. Suitable host cells include, but are not limited to, prokaryotic cells such as bacteria, e.g. E. coli, eukaryotic host cells such as yeast cells, insect cells, plant cells or animal cells, preferably mammalian cells and, more preferably, human cells. Further, the invention relates to a non-human organism transformed or transfected with a nucleic acid molecule as described above. Such transgenic organisms may be generated by known methods of genetic transfer including homologous recombination. A further aspect of the present invention relates to a pharmaceutical or diagnostic composition comprising as the active agent at least one fusion protein, a respective nucleic acid encoding therefore, or a transformed or transfected cell, all as described herein. At least one fusion protein, respective nucleic acid encoding therefore, or transformed or transfected cell, all as described herein may be used in therapy, e.g., in the prophylaxis and/or treatment of disorders caused by, associated with and/or accompanied by dysfunction of TNF-SF cytokines, particularly proliferative disorders, such as tumours, e.g. solid or lymphatic tumours; infectious diseases; inflammatory diseases; metabolic diseases; autoimmune disorders, e.g. rheumatoid and/or arthritic diseases; degenerative diseases, e.g. neurodegenerative diseases such as multiple sclerosis; apoptosis-associated diseases or transplant rejections. The term “dysfunction of TNF-SF cytokines” as used herein is to be understood as any function or expression of a TNF-SF cytokine that deviates from the normal function or expression of a TNF-SF cytokine, e.g., overexpression of the TNF-SF gene or protein, reduced or abolished expression of the TNF-SF cytokine gene or protein compared to the normal physiological expression level of said TNF-SF cytokine, increased activity of the TNF-SF cytokine, reduced or abolished activity of the TNF-SF cytokine, increased binding of the TNF-SF cytokine to any binding partners, e.g., to a receptor, particularly a CD95 or TRAIL receptor or another cytokine molecule, reduced or abolished binding to any binding partner, e.g. to a receptor, particularly a CD95 or TRAIL receptor or another cytokine molecule, compared to the normal physiological activity or binding of said TNF-SF cytokine. The composition may be administered as monotherapy or as combination therapy with further medications, e.g. cytostatic or chemotherapeutic agents, corticosteroids and/or antibiotics. The fusion protein is administered to a subject in need thereof, particularly a human patient, in a sufficient dose for the treatment of the specific conditions by suitable means. For example, the fusion protein may be formulated as a pharmaceutical composition together with pharmaceutically acceptable carriers, diluents and/or adjuvants. Therapeutic efficacy and toxicity may be determined according to standard protocols. The pharmaceutical composition may be administered systemically, e.g. intraperitoneally, intramuscularly or intravenously or locally, e.g. intranasally, subcutaneously or intrathecally. Preferred is intravenous administration. The dose of the fusion protein administered will of course be dependent on the subject to be treated, on the subject's weight, the type and severity of the disease, the manner of administration and the judgement of the prescribing physician. For the administration of fusion proteins, a daily dose of 0.001 to 100 mg/kg is suitable. Examples 1. Manufacture of a Single-Chain CD95L Fusion Protein (scCD95L) In the following, the general structure of the recombinant proteins of the invention (FIG. 1) is shown exemplified for the receptor binding domain of the human CD95 ligand. 1.1 Polypeptide Structure A) Amino acids Met1-Ser21 IgKappa-signal peptide, assumed signal peptidase cleavage site after amino acid Gly20 B) Amino acids Glu22-Leu161 First soluble cytokine domain of the human CD95 ligand (CD95L; amino acids 142-281 of SEQ ID NO: 6 including a K145S mutation). C) Amino acids Gly162-Gly169 First peptide linker element. D) Amino acids Arg170-Leu307 Second soluble cytokine domain of the human CD95 ligand (CD95L; amino acids 144-182 of SEQ ID NO: 6 including a K145S mutation). E) Amino acids Gly308-315 Second peptide linker element. F) Amino acids Arg316-Leu453 Third soluble cytokine domain of the human CD95 ligand (CD95L; amino acids 144-281 of SEQ ID NO: 6 including a K145S mutation). G) Amino acid Gly457-Lys472 Peptide linker with a Strep-tag II motif. The amino acid sequence of sc CD95L is shown in SEQ ID NO. 27. The fusion polypeptide comprises first and second peptide linkers having the sequence GGSGSGSG (SEQ ID NO: 21). Further preferred linker sequences are SEQ ID NOs: 22-26 as described above. It should be noted that the first and second peptide linker sequences need not to be identical. The signal peptide sequence (A) may be replaced by any other suitable, e.g. mammalian signal peptide sequence. The Strep-tag II motif (G) may be replaced by other motifs, if desired, or deleted. As shown in FIG. 23, cell culture supernatant of HEK293 cells, transiently expressing scCD95L (SEQ ID NO:27) was collected and used to stimulate Jurkat cells at varying concentrations. The supernatant was used either directly without further modifications or an anti-Streptag antibody (2 microgram/ml) was added to cross-link the scCD95L protein. Only cell supernatant that contained cross-linked scCD95L-St increased caspase activity in Jurkat cells, indicating that scCD95L alone does not form higher order aggregates able to be pro-apoptotic. 1.2 Gene Cassette Encoding the Polypeptide The synthetic gene may be optimised in view of its codon-usage for the expression in suitable host cells, e.g. insect cells or mammalian cells. A preferred nucleic acid sequence is shown in SEQ ID NO: 30. 1.3 Cloning Strategy The synthetic gene may be cloned, e.g. by means of a restriction enzyme hydrolysis into a suitable expression vector. 2. Manufacture of a Single-Chain TRAIL Fusion Protein (Sc TRAIL Wt) 2.1 Polypeptide Structure A) Amino acids Met1-Gly20 Ig-Kappa-signal peptide, assumed signal peptidase cleavage site after amino acid Gly 20. B) Amino acids Gln21-Gly 182 First soluble cytokine domain of the human TRAIL ligand (TRAIL, amino acid 120-281 of SEQ ID NO:10) C) Amino acids Gly183-Ser 190 First peptide linker element, wherein the two amino acids designated X are both S or one is S and the other one is N. D) Amino acids Arg191-Gly351 Second soluble cytokine domain of the human TRAIL ligand (TRAIL, amino acids 121-281 of SEQ ID NO:10) E) Amino acids Gly 352-Ser359 Second peptide linker element wherein the two amino acids designated X are both S or one is S and the other one is N. F) Amino acids Arg360-Gly520 Third soluble cytokine domain of the human TRAIL ligand (TRAIL, amino acids 121-Gly281 of SEQ ID NO:10). G) Amino acids Gly521-Lys538 Peptide linker element with a Streptag II motif. The amino acid sequence of sc TRAIL wt is shown in SEQ ID NO: 28. The indicated linkers may be replaced by other preferred linkers, e.g. as shown in SEQ ID NOs: 21.26. It should be noted that the first and second peptide linkers do not need to be identical. The signal peptide sequence (A) may be replaced by any other suitable, e.g. mammalian signal peptide sequence. The Strep-tag II motif (G) may be replaced by other motifs, if desired, or deleted. Cell culture supernatants of HEK293 cells, transiently expressing single chain TRAIL proteins with different linkers (derived from SEQ ID 28, in total nine different linker combinations) were collected and used to stimulate Jurkat cells at varying dilutions (exemplarily, a dilution of 1:8 is shown in FIG. 25). The supernatants were used either directly without further modifications or an anti-Streptag antibody (2 microgram/ml Strep MAB Immo) was added to cross-link the scTRAILwt proteins. Jurkat cells were incubated with HEK293 cell culture supernatant for three hours at 37°, lysed and analysed for caspase activity. Cell culture supernatant that contained cross-linked scTRAILwt proteins induced an increased caspase activity in Jurkat cells (results shown on the right hand side of the graph), indicating that scTRAILwt proteins alone do form only a low amount of higher order aggregates able to be pro-apoptotic. 2.2 Gene Cassette Encoding the Polypeptide The synthetic gene may be optimised in view of its codon usage for the expression in suitable host cells, e.g. insect cells or mammalian cells. A preferred nucleic acid sequence is shown in SEQ ID NO: 31. 3. Manufacture of a Single-Chain Mutated TRAIL Fusion Protein (scTRAIL (R2-Specific)) In the following, the structure of a single-chain TRAIL polypeptide comprising a mutation for selective binding to TRAIL receptor R2 is shown. 3.1 Polypeptide Structure A) Amino acids Met1-Ser29 Ig-Kappa signal peptide, assumed signal peptidase cleavage site after amino acid Gly20 and peptide linker B) Amino acids Arg29-Gly190 First soluble cytokine domain of the human TRAIL ligand (TRAIL, amino acids 121-281 of SEQ ID NO: 10 including the mutations Y189Q, R191K, Q193R, H264R, I266L and D267Q) C) Amino acid Gly191-Ser198 First peptide linker element, wherein the amino acids designated X are as indicated in Example 2 D) Amino acids Arg199-Gly359 Second soluble cytokine domain of the human TRAIL ligand (TRAIL amino acids 121-281 of SEQ ID NO: 10 including the mutations as indicated in B) E) Amino acids Gly360-Ser367 Second peptide linker element, wherein the amino acids X are as indicated in Example 2 F) Amino acids Arg368-Gly528 Third soluble cytokine domain of the human TRAIL ligand (TRAIL, amino acids 121-281 of SEQ ID NO: 10 including the mutations as indicated in B) G) Amino acids Gly529-Lys546 Peptide linker with a Strep-tag II motif The amino acid sequence of scTRAIL(R2-specific) is shown in SEQ ID NO: 29. The indicated linkers may be replaced by other preferred linkers, e.g. as shown in SEQ ID NOs: 21-26. It should be noted that the first and second peptide linkers do not need to be identical. The signal peptide sequence (A) may be replaced by any other suitable, e.g. mammalian signal peptide sequence. The Streptag II motif (G) may be replaced by other motifs, if desired, or deleted. 3.2 Gene Cassette Encoding the Polypeptide The synthetic gene may be optimised in view of its codon usage for the expression in suitable host cells, e.g. insect cells or mammalian cells. A preferred nucleic acid sequence is shown in SEQ ID NO: 32. 4. Expression and Purification a) Cloning, Expression and Purification of Fusion Polypeptides Hek293T cells grown in DMEM+GlutaMAX (GibCo) supplemented with 10% FBS, 100 units/ml Penicillin and 100 μg/ml Streptomycin were transiently transfected with a plasmid containing an expression cassette for a fusion polypeptide. In those cases, where a plurality of polypeptide chains is necessary to achieve the final product, e.g. for the Fab-scTNF-SF fusion proteins (FIG. 9A), the expression cassettes were either combined on one plasmid or positioned on different plasmids during the transfection. Cell culture supernatant containing recombinant fusion polypeptide was harvested three days post transfection and clarified by centrifugation at 300×g followed by filtration through a 0.22 μm sterile filter. For affinity purification Streptactin Sepharose was packed to a column (gel bed 1 ml), equilibrated with 15 ml buffer W (100 mM Tris-HCl, 150 mM NaCl, pH 8.0) or PBS pH 7.4 and the cell culture supernatant was applied to the column with a flow rate of 4 ml/min. Subsequently, the column was washed with 15 ml buffer W and bound polypeptide was eluted stepwise by addition of 7×1 ml buffer E (100 mM Tris HCl, 150 mM NaCl, 2.5 mM Desthiobiotin, pH 8.0). Alternately, PBS pH 7.4 containing 2.5 mM Desthiobiotin can be used for this step. The protein amount of the eluate fractions was quantitated and peak fractions were concentrated by ultrafiltration and further purified by size exclusion chromatography (SEC). SEC was performed on a Superdex 200 column using an Äkta chromatography system (GE-Healthcare). The column was equilibrated with phosphate buffered saline and the concentrated, Streptactin-purified polypeptide was loaded onto the SEC column at a flow rate of 0.5 ml/min. The elution profile of the polypeptide was monitored by absorbance at 280 nm. For determination of the apparent molecular weight of purified fusion polypeptide under native conditions a Superdex 200 column was loaded with standard proteins of known molecular weight. Based on the elution volume of the standard proteins a
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