Figure 1.

(A) Alignment of the propeptide sequences of the β-type insulin-like genes in C. elegans, except for that of INS-1 and INS-10. The partial protein sequences only exclude the predicted signal peptides (predomain) from the predicted full-length prepropeptide sequences. The blue box outlines the furin-like cleavage site RXXR (also see B legend); the first arginine residue is mutated to cysteine in daf-28(sa191). The red boxes indicate the aligned four pairs of cysteine residues. Hydrophobic residues important for helix formation are in bold. The numbers in green are the percentages of identical residues of each propeptide shared with DAF-28. (B) Three groups of C. elegans insulins classified on the basis of predicted proteolytic processing sites. All preproinsulins in C. elegans at least contain a signal peptide (predomain) and A and B domains. The signal peptide is removed by signal peptidase to form a proinsulin, and the A and B domains become part of a mature insulin. (a) DAF-28 and eight other β-type insulins are predicted to have a cleaved F-peptide between the signal peptide and the B domain; the predicted proteolytic processing site between the F-peptide and the B domain is RXXR, which is recognized by a proprotein convertase similar to mammalian protease furin. The consensus and the minimal sequences for furin cleavage are RXRR and RXXR, respectively. (b) β-Type INS-1 and γ-type INS-18 are the only two insulins in C. elegans that are predicted to bear a C-peptide. The C-peptide is flanked by a dibasic residue cleavage site on either side, which are predicted to be recognized by a proprotein convertase similar to the mammalian protease PC2. (c) The remaining proinsulins in C. elegans do not have obvious endoproteolytic sites. (d) Human insulin has a cleaved C-peptide.