DIFFERENTIAL CLEAVAGE OF PROVASOPRESSIN BY THE MAJOR MOLECULAR-FORMS OF SPC3

We have investigated the roles of full-length and carboxyl-terminus-tr uncated forms of the subtilisin-like prohormone convertase SPC3 in the processing of the radiolabeled vasopressin and oxytocin precursors, i n vitro, We found SPC3 cleaves provasopressin al both the vasopressin- neurophysin and neurophysin-glycopeptide processing sites. Prooxytocin is cleaved by SPC3 at the oxytocin-neurophysin cleavage sits However, our results reveal differences in processing of provasopressin by the different molecular forms of SPC3, In incubations where the rate of a utocatalytic carboxyl-terminus truncation of SPC3 was dramatically red uced, 86-kDa SPC3, which has an unprocessed carboxyl terminus, cleaved provasopressin at the neurophysin-glycopeptide junction, Cleavage at the vasopressin-neurophysin junction only occurred with the appearance of carboxyl-terminus-truncated forms of the enzyme, Incubations conta ining 64-kDa SPC3 or 64-kDa SPC3-T, a recombinant form of SPC3 truncat ed 14 amino acids beyond the conserved carboxyl-terminal ''P-domain,'' rapidly cleaved provasopressin at both the vasopressin-neurophysin an d neurophysin-glycopeptide junctions. Our results also suggest that pr ooxytocin is unable to be cleaved by the 86-kDa form of SPC3. We propo se that SPC3 should be considered as a candidate endoprotease in the b iosynthesis of vasopressin, Furthermore, we suggest thar the carboxyl terminus of SPC3 alters the cleavage specificity of SPC3.