Caenorhabditis elegans contains a family of putative insulin-like genes proposed to regulate dauer arrest and senescence. These sequences often lack characteristic sequence features of human insulin essential for its folding, structure, and function. Here, we describe the structure and receptor-binding properties of INS-6, a singlechain polypeptide expressed in specific neurons. Despite multiple nonconservative changes in sequence, INS-6 recapitulates an insulin-like fold. Although lacking classical receptor-binding determinants, INS-6 binds to and activates the human insulin receptor. Its activity is greater than that of an analogous single-chain human insulin analog. Received November 12, 2002; revised version accepted January 24, 2003. Development of Caenorhabditis elegans can arrest to form a specialized third-stage larva. Such dauer ("enduring") larvae can survive up to eight times longer than the organism's normal 2-wk lifespan (Blaxter and Bird 1997). An adaption to limited food supply, increased population density, or elevated temperature (Riddle and Albert 1997), the dauer stage is triggered by chemosensory cues under the joint control of insulin-related and TGF--related signaling pathways (Ren et al. 1996;Kimura et al. 1997). On sensing favorable conditions for growth and reproduction, the dauer resumes development into adulthood. Genetic screens for dauer arrest have led to identification of C. elegans orthologs of the insulin receptor (daf-2) and post-receptor signaling pathway, including the catalytic subunit of PI-3-kinase (age-1), serine-threonine kinases Akt/PKB (akt-1 and akt-2) and a forkhead transcription factor (daf-16; Guarente and Although genetic screens for diapause or long-lived phenotypes did not reveal DAF-2 ligands, search of the C. elegans genome by sequence-and structure-based algorithms has uncovered a family of 37 putative insulinrelated (ins) genes (Pierce et al. 2001). These sequences are classified into three classes, designated ␣, , and ␥, based on pattern of cysteines (Pierce et al. 2001). ␣-Class proteins lack an A6-A11 disulfide bridge, an invariant feature of vertebrate insulins and insulin-like growth factors (Baker et al. 1988). -Class proteins appear to retain insulin's three canonical disulfide bridges, plus a fourth, whose location is variable. ␥-Class proteins contain three canonical (or pseudo-canonical) disulfide bridges. Because these sequences diverge from mammalian insulins at conserved sites required for biological activity, their classification as insulin-like has been presumptive. Genetic evidence is provided elsewhere in this issue that an ins gene (daf-28) functions in the daf-2 pathway of dauer arrest (Li et al. 2003).We describe here the chemical synthesis and structure of a -class INS protein. Because it lacks dibasic processing sites (as utilized in cleavage of proinsulin; Steiner 1998), the protein was prepared as a single chain. The polypeptide folds to form a well-defined structure with four disulfide bridges. Despite marked sequence divergen...