Proprotein convertase expression and localization in epidermis: evidence for multiple roles and substrates

Specific proteolysis plays an important role in the terminal differentiatio n of keratinocytes in the epidermis and several types of proteases have bee n implicated in this process. The proprotein convertases (PCs) are a family of Ca2+-dependent serine proteases involved in processing and activation o f several types of substrates. In this study we examined the expression and some potential substrates of PCs in epidermis, Four PCs are expressed in e pidermis: furin, PACE4, PC5/6 and PC7/8. Furin is detected in two forms, ei ther with or without the transmembrane domain, suggesting occurrence of pos t-translational cleavage to produce a soluble enzyme. In addition the furin active site has differential accessibility in the granular layer of the ep idermis relative to the basal layer, whereas antibodies to the transmembran e domain stain both layers. These findings suggest that furin has access to different types of substrates in granular cells as opposed to basal cells. PC7/8, in contrast, is detected throughout the epidermis with antibodies t o both the transmembrane and active site and no soluble form observed. A pe ptide PC inhibitor (dec-RVKR-CMK) inhibits cleavage of Notch-1, a receptor important in cell fate determination that is found throughout the epidermis . Profilaggrin, found in the granular layer, is specifically cleaved by fur in and PACE4 in vitro at a site between the amino terminus and the first fi laggrin repeat. This work suggests that the PCs play multiple roles during epidermal differentiation.