Many physiologically important proteins are expressed as large inactive polypeptides, examples of which include growth factors, growth factor receptors, hormones, peptide neurotransmitters, viral glycoproteins and several metalloproteinases. One way in which the majority of these proproteins can be activated is by their limited proteolytic cleavage at dibasic residues, a reaction catalyzed by members of the kex2
/subtilisin-like proprotein convertase family (kexins or PC) of serine endoproteinases. In vertebrates at least seven different members of the family have been identified and include furin, PC1/3, PC2, PC4, PC6, PC7 and PACE4. Our laboratory identified the first C. elegans member of the family as bli-4
which encodes at least four gene products that arise by alternative slicing. The combined efforts of ourselves, the genome sequencing consortium and the laboratory of Ian Dickerson at the University of Miami School of Medicine, have identified a further three nematode members of the family. These three members include genes with sequence similarity to furin and PC7, both of which reside on the right arm of chromosome I, and a PC2 homologue on chromosome V. We are interested in determining the role of each of the kexins during C. elegans development and towards this end we are attempting to isolate mutants for these three genes using the targeted approach described by Gary Moulder and Bob Barstead at the recent Worm meeting at Madison, WI (and personal communication). Continued studies on the bli-4
gene products suggests that the major role of these convertases is the processing of cuticle collagens during development, although we have not ruled out the possibility that other substrates are cleaved. We expect the furin homologue like its vertebrate counterpart, to recognize a large number of proproteins since this member of the PC family appears to be the major role player. Curiously the C. elegans gene differs markedly at the carboxy-terminus when compared with that from Drosophila and vertebrate proteins in that the worm protein does not contain a transmembrane domain even though the protease domain is highly conserved. Vertebrate PC2 is active in the regulated secretory pathway and has been shown to process a number of prohormones and peptide neural transmitters. PC7, like furin, is expressed ubiquitously in mammalian cells and is thought to be largely responsible, amongst other roles, for the processing of viral glycoproproteins including infection by HIV. Intriguingly, the nematode gene F32A7.6 is expressed as part of an operon with unc-54
. The kexin gene is trans-spliced to SL2 which is consistent with it being the downstream gene in the operon. Obviously, expression of the kexin gene under the control of the unc-54
promoter raises the possibility that the role of the convertase is highly restricted to muscle tissue and we have recently isolated candidate mutants which may help us determine the function of this serine endoproteinase.