the first insect protein with the capacity to bind mammalian insulin and insulin - like peptides had a serendipitous discovery eight years ago . a 27 kda protein from the fall armyworm spodoptera frugiperda was uncovered as an insulin - binding activity in insect - cell - conditioned media during attempts to purify fragments of the insulin receptor from sf9 cells . the protein was purified and identified , allowing subsequent identification of its single drosophila homolog , imp - l2 . in a recent study in journal of biology , honegger and coworkers   present the first , and long - awaited , in vivo functional characterization of this insect insulin / insulin - like growth factor ( igf ) binding protein . the insulin / igf signaling ( iis ) pathway is an evolutionarily conserved neuroendocrine signaling pathway that regulates a plethora of metazoan functions and traits , both during development and in the adult . in model animals ranging from the nematode worm and the fruit fly to the mouse , iis affects growth and development , metabolic / energy homeostasis , stress resistance , reproduction and lifespan [ 3 - 5 ] . the cellular iis cascade is initiated by the extracellular binding of an insulin / igf - like ligand to an insulin - type receptor , resulting in the activation of its intracellular tyrosine kinase domain and the subsequent sequential activation of phosphoinositide 3-kinase ( pi 3-kinase ) and protein kinase b ( akt ) and inactivation of the forkhead box - o transcription factors . the active receptor also activates the extra - cellular signal - regulated kinase ( erk ) , and the akt branch of the pathway interacts with the target of rapamycin ( tor ) pathway . although there are numerous variants of the intracellular iis components in mammals , in invertebrates these are mainly encoded by single genes . on the other hand , mammals have only three ligands , insulin , igf - i and igf - ii , whereas there are 38 in the caenorhabditis elegans genome   and seven in drosophila . dissecting the functions of all these paralogs may give insights into how this pathway regulates such diverse aspects of animal physiology . drosophila and other model organisms have provided valuable insights into the mechanisms and effects of iis . however , an important aspect of the extracellular regulation of the pathway has not been dissected in drosophila : the binding of ligands by extracellular binding proteins . in mammals , igf - i and igf - ii are bound in vivo by igf binding proteins ( igfbps ) . igfbps act as regulators of the activity of igfs , by prolonging their half - life , altering their local and systemic availability and , through high - affinity binding , sequestering them from the receptor . mammals have six igfbps that can bind igfs with high affinity , as well as several igfbp - related proteins ( igfbp - rp ) with somewhat lower affinity for igfs . igfbps and igfbp - rps belong to a protein superfamily sharing sequence homology predominantly in their amino - terminal portion , which is thought to be involved in igf binding . the complexity of the igf - igfbp system and how it affects iis has not been examined in invertebrates because no orthologs of igfbp have been identified  that is , until recently . iis is an important regulator of growth , and overexpression of the drosophila insulin receptor in the eye during development results in hyperplasia ( overgrowth ) of the eye . honegger and coworkers   used this phenotype , which had previously been shown to be sensitive to the availability of drosophila insulin - like peptides ( dilps ) , to screen for negative regulators of iis . the authors identified imaginal morphogenesis protein - late 2 ( imp - l2 )   as a strong negative regulator of iis . the amino acid sequence of imp - l2 indicates that it is a secreted protein of the immunoglobulin superfamily , with homologs in other invertebrates . the carboxy - terminal portion of imp - l2 is similar to that of the human igfbp - rp1 ( also known as igfbp7 ; figure 1 ) , leading to the exciting possibility that the screen might have identified a fly igfbp . indeed , imp - l2 had previously been shown to bind human igf - i , igf - ii and insulin in vitro with high affinity , but its binding to dilps and its potential role in fly iis had not been examined . honegger and coworkers   therefore set out to determine whether imp - l2 is functionally equivalent to igfbps . sequence comparison of imp - l2 , its invertebrate homologs , and igfbp - rp1 . the sequences of imp - l2 ( drosophila ) , insulin - related peptide binding protein ( ibp ; s. frugiperda ) , zig-4 ( c. elegans ) and igfbp - rp1 ( human ) were aligned using clustalw2 . residues identical or similar in at least three sequences are highlighted in black and gray , respectively . the two immunoglobulin - like domains are indicated by a gray bar and the region in igfbp - rp1 that has the most similarity to igfbps by a black bar below the sequences . if imp - l2 is a functional equivalent of igfbp , it should negatively regulate growth , and this effect should not be restricted to the cells producing it but should be cell non - autonomous . indeed , honegger and coworkers   found that weak , ubiquitous overexpression of imp - l2 yielded smaller flies . when clones of cells in the drosophila eye were made to overexpress imp - l2 in an otherwise wild - type fly , their cell specification and patterning were not affected , but the clones were small in size and this reduction also seemed to affect the neighboring cells . furthermore , overexpression of imp - l2 in the eye resulted not only in smaller eyes but also in reduction in the size of the whole fly and a developmental delay . similarly , overexpression in the larval fat body reduced the size of the whole organism . the latter observation may , however , be confounded by the possibility that fat - body - restricted downregulation of iis could affect energy homeostasis and thus organism growth . honegger and coworkers   also looked at the in vivo levels of phosphatidylinositol ( 3,4,5)trisphosphate , the secondary messenger produced by pi 3-kinase , and demonstrated that , as would be expected of an igfbp , imp - l2 overexpression can alter signaling downstream of the insulin receptor . to further confirm imp - l2 as a bona fide igfbp equivalent , honegger et al . examined its interaction with dilp2 , the most potent growth regulator of all the dilps . as expected , dilp2 and imp - l2 were found to antagonize each other genetically . weak ubiquitous overexpression of dilp2 during development caused a body and organ size increase that was exacerbated in flies with only one copy of the imp - l2 gene . strong overexpression of either dilp2 or imp - l2 alone resulted in lethality , but strong simultaneous overexpres - sion of both allowed wild - type - sized flies to develop . furthermore , the authors showed that the imp - l2 protein can bind its native partner , dilp2 , in vitro . the data presented by honneger and coworkers   argue strongly that imp - l2 is functionally equivalent to mammalian igfbps , opening the way to analysis of the functions of this class of iis regulators in flies . examination of loss - of - function alleles showed that imp - l2 is required for body size determination during normal growth . imp - l2 was induced in the fat body when larvae were starved and loss of imp - l2 function resulted in a failure to decrease iis and caused starvation - sensitivity . a detailed examination of the role of imp - l2 in adult physiology has yet to be made , but some hints exist as to the function of this protein in the adult . concomitantly , the imp - l2 transcript is upregulated , indicating that imp - l2 may be part of a gonad - derived signaling that modulates whole - body iis . it will be important to establish the similarities and differences between the mammalian igf - igfbp system and the drosophila dilp - imp - l2 system . characterization of the dilps at the protein level , and of whether and how they form complexes with imp - l2 , will be important . it is interesting in this respect that the homology between igfbp - rp1 and imp - l2 does not extend into the amino - terminal , igfbp - like portion of igfbp - rp1 ( see figure 1 ) , thought to be required for igf and insulin binding . it will be important to determine functional similarities between imp - l2 and igfbp - rp1 , especially now that the importance of igfbp - rp1 as a tumor suppressor has been highlighted . furthermore , it may be interesting to determine whether imp - l2 , like some igfbps , has functions independent of dilp binding , opening up the possibility of using drosophila to understand how these ligand - independent functions are effected . it will also be interesting to examine whether imp - l2 , like mammalian igfbps , can act both locally and systemically and whether its activity is regulated by proteolysis . a similarity to the mammalian system , in which most igf - i or igf - ii circulates as part of ternary complexes of igf , igfbp3 and the acid - labile subunit ( als ) , was uncovered by the recent characterization of the drosophila als , which appears to form a trimeric complex with dilp2 and imp - l2 . the number of questions that remain only demonstrates how important the work by honneger and coworkers   has been in opening up the field of study of igfbp in drosophila . the study of imp - l2 in such a genetically amenable system will surely yield results relevant to the understanding of mammalian igfbps . we acknowledge funding by the wellcome trust ( lp ) and a marie curie fellowship ( na ) . we thank iain robinson for critically reading the manuscript .
insulin - like growth factor ( igf ) binding proteins provide a layer of complexity to the insulin / igf signaling system in mammals , but only now , in a recent study in journal of biology , has one such protein been functionally characterized in drosophila .