protein phosphorylation is the most abundant post - translational modification and a key regulatory step in eukaryotic signal transduction ( reviewed in hunter , 2007 ; temporini et al . , 2008 ; gnad et al . , it governs diverse processes such as metabolism , cell cycle , development , hormone signaling , and pathogen - perception ( reviewed in cohen , 1982 ; ranjeva and boudet , 1987 ; xing et al . , 2002 ; cozzone , 2005 ; reiland et al . , 2009 ) . in plants , most phosphorylation events occur in serine ( 85% ) or threonine ( 10% ) residues ( sugiyama et al . , 2008 ; reiland et al . , 2009 ; nakagami et al . among other post - translational modifications phosphorylation of these residues usually occurs in intrinsically disordered regions ( iakoucheva et al . , intrinsically disordered regions are flexible and extended protein segments that have no ordered secondary structure under physiological conditions . the discovery of intrinsically disordered proteins led to a revision of the structure  function paradigm , which states that a protein can only fulfill its biological function by folding into a unique and structured state . their importance is underlined by an exponential growth in the number of intrinsically disordered proteins described in the literature within the last decades ( sickmeier et al . , 2007 ) . these regions exhibit large net charges at neutral ph as a consequence of harboring many non - interacting charged groups and few hydrophobic residues . additionally , their sequences are enriched in disorder - promoting residues ( d , k , r , s , q , p , and e ) and devoid of order - promoting residues ( w , y , f , i , v , l , and t ) . these features concomitantly result in low compactness and globularity but high flexibility of the respective region . proteins harboring long stretches of intrinsic disorder are frequently involved in essential processes such as transcriptional and translational activation , chromatin remodeling , and signal transduction ( dyson and wright , 2005 ; radivojac et al . substrate interaction , where both partners can dissociate rapidly and efficiently . hypothetically , such combination could be achieved in a coupled binding and folding mechanism consisting of disorder - to - order transitions . in these transitions , the originally disordered region is locked in the binding conformation instead of exploring different conformations free in solution . this results in a reduction of entropy in the free energy of binding , which translates into a weaker binding ( low affinity ) compared with interactions that involve already ordered proteins with equivalent exposed surface areas ( dyson and wright , 2005 ) . the specificity is determined by the extension and complementarity of the interaction surface area , which in the case of intrinsically disordered regions , is large even when the interaction domains themselves may be small ( mittag et al . , 2009 ) . plants , as sessile organisms , require efficient signal transduction cascades to adapt to constantly changing environmental conditions . these cascades are part of complex networks involving hundreds of kinases and thousands of kinase substrates ( stone and walker , 1995 ; dissmeyer and schnittger , 2011 ) . accordingly , it has been predicted that 29% of the arabidopsis thaliana proteome contains intrinsically disordered regions that are 50 residues or longer . moreover , it has been estimated that around 8% of the total a. thaliana proteome is comprised of completely intrinsically disordered proteins , which mainly serve function as dna and rna binding proteins , stress response modulators and signaling components , among others ( dunker et al . , 2000 ) . based on the current literature , only few plant signaling components have been experimentally characterized with respect to their intrinsic disorder . according to the disprot database , chaperones , molecular assembly proteins , and molecular recognition effectors are over - represented classes of intrinsically disordered proteins in plants . examples of chaperones are the a. thaliana erd10 and erd14 proteins belonging to the dehydrin family that are induced by abiotic stress ( mouillon et al . , 2006 ; kovacs et al . , 2008 ) . furthermore , the n - terminal region of the a. thaliana hy5 bzip transcription factor involved in photomorphogenesis ( yoon et al . , 2006 ) and the hordeum vulgare ( barley ) senescence - associated nac transcription factor ( kjaersgaard et al . , 2011 ) are examples of intrinsically disordered proteins involved in molecular assembly . the group of intrinsically disordered molecular recognition effectors is represented by a number of a. thaliana chloroplast translocases ( richardson et al . , 2009 ) , the c - terminal region of the alb3 membrane insertase ( falk et al . , 2010 ) and the n - terminal domain of della proteins that are involved in gibberellic acid signaling ( sun et al . , 2010 ) . recently , remorin proteins have emerged as new components of signal transduction cascades ( jarsch and ott , 2011 ) . remorins are plant - specific proteins present in all land plants including ferns and mosses and constitute a multi - gene family that comprises six subgroups . all described members tightly associate to the plasma membrane in specialized compartments known as membrane rafts and/or can be enriched in detergent insoluble membranes ( raffaele et al . , 2009 ; lefebvre et al . , these rafts have been suggested to constitute platforms for signal transduction , pathogen infection , and apoptosis , among other processes ( simons and toomre , 2000 ; jarsch and ott , 2011 ; simon - plas et al . , 2011 ) . remorin proteins do not show any significant sequence similarity to other described proteins or domains ( raffaele et al . they are characterized by the presence of a conserved c - terminal region ( pfam domain remorin_c ; pf03763 ) that encodes a predicted coiled - coil motif and a putative membrane - anchoring motif . on the other hand , the n - terminal region is extremely diverse and , for example , in the case of members belonging to the phylogenetic group 3 even absent . the current annotation of this region ( pfam domain remorin_n ; pf03766 ) can thereby not be applied to the whole remorin family but is restricted to subgroup 1b . among the few motifs that can be predicted in these n - terminal regions are putative phosphorylation sites for serine / threonine protein kinases ( i.e. , pka , pikk , gsk3 , mapk ; raffaele et al . the potato remorin strem1.3 was originally discovered as a plasma membrane associated protein , which is strongly phosphorylated upon treatment with polygalacturonic acid ( farmer et al . , 1989 ) . in a. thaliana differential phosphorylation of the group 1 remorins atrem1.2 ( at3g61260 ) and atrem1.3 ( at2g45820 ) is dependent on the presence of the nbs lrr resistance protein rpm1 ( widjaja et al . , 2009 ) and triggered upon perception of microbe - associated molecular pattern ( mamps ; benschop et al . , 2007 ) , respectively . the predominant detection of phosphopeptides from group 1b remorins in proteomic screens certainly reflects the high abundance of these members in plant cells . however , the identification of phosphopeptides from several other remorin proteins in a number of independent studies suggests phosphorylation to be common feature of this protein family . generally , phosphorylation of remorins appears to be restricted to serine and threonine residues despite putative phosphorylation of few tyrosine residues examples are phosphopeptides of two a. thaliana ( atrem1.2 and atrem1.3 ) and two medicago truncatula [ mtrem1.1 ( mtc60319 ) and mtrem1.2 ( mtc00278 ) ] 1b remorins that were detected in independent studies ( benschop et al . , 2007 ; sugiyama et al . , 2008 ; whiteman et al . , 2008 ; li et al . interestingly , phosphorylation of a serine residue that is conserved across group 1b , was reported in all of these four proteins ( s88 , s66 , s85 , and s76 in atrem1.2 , atrem1.3 , mtrem1.1 , and mtrem1.2 , respectively ) , which suggests functional conservation in this amino acid position . an additional feature is that all sites are located in the lowly conserved n - terminal region . in case of groups with longer n - terminal regions , e.g. , group 5 and group 6 , several other phosphopeptides have been detected . in group 5 , phosphorylation of one a. thaliana ( atrem5.1 ; at1g45207 ) and two oryza sativa ( rice ) remorins ( os08g36760 and os02g52810.1 ) have been reported ( benschop et al . , 2007 ; nuhse et al . , 2007 ; sugiyama et al . , 2008 ; nakagami et al . , a similar situation is present in group 6 , where phosphorylation of four a. thaliana [ atrem6.1 ( at2g02170 ) , atrem6.2 ( at1g30320 ) , atrem6.4 ( at4g36970 ) , and atrem6.5 ( at1g67590 ) ] and two m. truncatula ( medtr4g115770 , medtr5g046550 ) remorins was described in non - conserved residues ( benschop et al . , 2007 ; nuhse et al . , 2007 ; sugiyama et al . , 2008 ; reiland et al . , 2009 ; grimsrud et al . , 2010 ; nakagami et al . , 2010 ) . furthermore , a serine residue ( s48 ) in the n - terminal region of the symbiotic remorin ljsymrem1 ( remorin group 2 ) of lotus japonicus is phosphorylated by the kinase domains of the receptor proteins nfr1 and symrk in vitro ( toth et al . , 2012 ) . it was described earlier that a combination of low mean hydrophobicity and relatively high net charge results in the absence of a compact structure in proteins under physiological conditions ( uversky et al . , 2000 ) . thus , both parameters can be used to discriminate disordered from folded proteins . investigating a subset of 84 remorin proteins distributed throughout the six phylogenetic groups revealed a common pattern that suggests a tendency toward intrinsic disorder with respect to their net charge and hydrophobicity ( figure 1a ) . only 15 remorins did not fulfill the disorder criteria according to the charge - hydrophobicity phase space this supports an overall clear tendency toward intrinsic disorder ( figure 1a ) but also highlights the diversity among remorin proteins . black circles depict intrinsically disordered proteins described in the literature ( data taken partially from uversky et al . , 2000 ) . the solid black line represents border between intrinsically disorder proteins and folded proteins and is described by the formula : r  =  2.785 h 1.151 , where r and h are the mean net charge and the mean net hydrophobicity , respectively . predictions were performed using the pondr vl - xt program on a set of 17 group 1b remorins , which belong to arabidopsis thaliana , mesembryanthemum crystallinum , medicago truncatula , nicotiana tabaccum , populus trichocarpa , solanum lycopersicum , and solanum tuberosum . gray dots indicate residues that have been found to the phosphorylated in atrem1.2 , atrem1.3 , mtrem1.1 , and mtrem1.2 . the red dot indicates the residue that was described to be phosphorylated and conserved among all group 1b remorin proteins . the asterisk marks a conserved region of order within the disordered n - terminal region . the alignment was performed using the clustalw algorithm on the full - length protein sequences . for simplicity only the n - terminal region and the first residues of the c - terminal region non - conserved phosphorylation sites are depicted in blue , whereas the conserved phospho - serine residue is indicated in red . asterisks ( * ) indicate positions that have a single , fully conserved residue . a colon ( :) indicates conservation between amino acids with highly similar chemical properties . a period ( . ) indicates conservation between amino acids with weakly similar chemical properties . deeper insights into the intrinsic disorder profiles of remorins belonging this subgroup were gained by using the pondr vl - xt algorithm . alignment of the pondr vl - xt disorder prediction scores that were obtained for 17 group 1b remorin proteins distributed over seven different species showed that the n - terminal region is intrinsically disordered in all selected sequences ( figure 1b ) . additionally , a segment of predicted order was located around positions 7090 within this predicted unstructured region in 6 out of 17 group 1b remorins ( figure 1b ) . conservation of this segment anticipates its functional importance . furthermore , molecular recognition motives involved in protein protein interactions may coincide with segments of local order within disordered regions ( oldfield et al . however , on protein sequence level , remorin n - terminal regions show little conservation even within the same phylogenetic group . in contrast , the c - terminal region is conserved throughout the remorin protein family . structurally , the latter region in group 1b remorins is mostly ordered ( figure 1b ) . interestingly , the transition between disordered and ordered segments coincides well with the borders between the phylogenetically defined n- and c - terminal regions that were described previously ( raffaele et al . , 2007 ) . an overlay of experimentally determined phosphorylation sites on the disorder prediction plot of group 1b remorins showed that they cluster in the disordered n - terminal region ( figures 1b , c ) . expansion of such analyses to group 5 and 6 remorins with experimentally determined phosphoserines and phosphothreonines revealed that these residues predominantly accumulate within long stretches of predicted disorder . in contrast , phosphorylation sites predicted with the netphos 2.0 server are randomly distributed along remorin protein sequences ( figure 2 ) . this reinforces the idea that serine and threonine phosphorylation occurs preferentially in flexible regions ( iakoucheva et al . , 2004 ) . the disphos 1.3 predictor incorporates intrinsic disorder prediction to improve the discrimination between phosphorylation sites . black and white boxes under the plots indicate the n- and c - terminal regions , respectively . disorder prediction for a. thaliana remorin proteins was performed using the pondr vl - xt algorithm . disordered regions exhibit different patterns with respect to the distribution of gained point mutations and generally higher rates of insertions and deletions compared to ordered proteins . this is probably due to their sequence composition and the fact that , despite high rates of sequence evolution , functionality is maintained by retaining domain flexibility under physiological conditions ( brown et al . , 2011 ) . an interesting example is symrem1 , a group 2 remorin protein , which has been shown to be involved in bacterial infection during root nodule symbiosis and was proposed as a novel signaling component in this mutualistic interaction ( lefebvre et al . , 2010 ; toth et al . , 2012 ) two proteins , encoded by orthologous genes , have been described in the phylogenetically closely related model legumes m. truncatula and l. japonicus . in line with their evolutionary proximity essential signaling proteins are highly conserved on amino acid level : e.g. , the lrr receptor - like kinases ( rlks ) nfp / nfr5 ( 82.8% similarity ) , the lysm - rlks dmi2/symrk ( 87.6% similarity ) , the calcium-/calmodulin - dependent kinases dmi3/ccamk ( 92.2% similarity ) and the putative transcription factor nsp2 ( 83.4% similarity ) . in contrast the orthologous remorins mtsymrem1 and ljsymrem1 exhibit an overall similarity of only 67.1% . this is mainly due to low sequence conservation in their predicted intrinsically disordered n - terminal regions ( 38.3% ) , while their ordered c - terminal regions with a similarity of 85.3% reflect the evolutionary distance between these two legumes ( toth et al . , 2012 ) . to understand signal transduction processes and regulatory functions on a structural and mechanistic level , detailed analysis of intrinsically disordered proteins is inevitable . remorins that have been proposed to be components of plant innate immunity signal transduction , represent prominent examples for intrinsically disordered proteins . analysis of their intrinsically disordered regions may help to unravel their so far unknown physiological roles in the future . however , accumulating evidence suggests that their molecular functions may be as diverse as their protein sequences . the two common features of remorins , phosphorylation proclivity within their n - terminal regions and intrinsic disorder , make them ideal candidates to elicit signaling functions . the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
plant - specific remorin proteins reside in subdomains of plasma membranes , originally termed membrane rafts . 
 they probably facilitate cellular signal transduction by direct interaction with signaling proteins such as receptor - like kinases and may dynamically modulate their lateral segregation within plasma membranes . 
 recent evidence suggests such functions of remorins during plant 
 microbe interactions and innate immune responses , where differential phosphorylation of some of these proteins has been described to be dependent on the perception of the microbe - associated molecular pattern ( mamp ) flg22 and the presence of the nbs  lrr resistance protein rpm1 . 
 a number of specifically phosphorylated residues in their highly variable and intrinsically disordered n - terminal regions have been identified . 
 sequence diversity of these evolutionary distinct domains suggests that remorins may serve a wide range of biological functions . here 
 , we describe patterns and features of intrinsic disorder in remorin protein and discuss possible functional implications of phosphorylation within these rapidly evolving domains .