A member of the eukaryotic subtilisin family (PC3) has the enzymic properties of the type 1 proinsulin-converting endopeptidase

Abstract

PC3, a mammalian homologue of the yeast subtilisin-like proteinase Kex2, was expressed in Xenopus oocytes and its activity was characterized. PC3 cleaved human proinsulin at one of the two dibasic sites (KTRR32 but not LQKR65). The specificity, inhibitor profile, pH optimum (5.5) and Ca2"-dependence (Ko5 = 2.5-3 mM) paralleled those of the insulingranule type 1 endopeptidase activity, suggesting a role for PC3 in the conversion of prohormones.

Sorting and processing of secretory proteins

“…The data for PC2 activity in COS cells are in agreement with the specificity of type II activity studied in vitro [181]. Intriguingly, whereas PCl appears able to cleave proinsulin at both junctions in COS cells, it can only cleave at the B-chain/C-peptide junction in vitro [230], and a similar selective substrate specificity has been shown for the type I enzyme in vitro [181]. It is, however, difficult to compare the in vivo and in vitro studies.…”

“…There is a precise alignment of Asp, His and Ser in the catalytic domains of the different members, although a highly conserved Asn residue has been replaced by Asp in PC2, an interesting mutation since this residue has been shown to play an important role in catalysis in the bacterial subtilisins [227,228]. PCl and PC2 have an optimal enzyme activity at acidic pH [229][230][231][232], while furin has a broad, neutral pH optimum [233]. This family of enzymes has also in common the fact that Ca2l is an essential requirement, although the concentration dependency varies greatly [181].…”

“…As discussed above, in rat insulinoma granules a type I activity cleaves at the Arg-Arg sequence at the B-chain/C-peptide junction, and a type 2 activity cleaves at the Lys-Arg sequence at the C-peptide/A chain junction [181,294]. It has been shown that type II activity corresponds to PC2 [268], and there is good evidence that type I activity is identical with PCI [230,235,250]. Several independent studies have shown the importance of basic residues in proinsulin conversion.…”

Proinsulin processing by the subtilisin-related proprotein convertases furin, PC2, and PC3.

“…Moreover, its clearcut localization to the islets of Langerhans supports a role for this protease in proinsulin processing. Recent studies of Bailyes et al (48) analysis (data not shown) and which require a more acidic environment for optimal activity (36,48 …”

Processing of pro‐islet amyloid polypeptide (proIAPP) by the prohormone convertase PC2

“…The fact that IAPP is co-localized with insulin in fl-cell granules, and human proinsulin is processed by the combined action of PC2 and PC3 [13] suggests that these enzymes might be involved in proIAPP processing. Alternatively, proIAPP may be processed in a separate compartment, possibly by furin.…”