Association of prohormone convertase 3 with membrane lipid rafts

Prohormone convertase 3 (PC3) is a neuroendocrine-specific member of the subtilisin-kexin family, involved in the intracellular processing and maturation of prohormones and proneuropeptides. PC3 is synthesised as a proprotein that undergoes two different cleavages resulting in the mature PC3 and the enzymatically active PC3 DeltaC. In vitro translated proPC3 and proPC3 DeltaC bind to trans-Golgi network (TGN)/granule-enriched membranes from the AtT20 neuroendocrine cell line in a pH-dependent manner suggesting both a dominant role for the pro-region in membrane association and that the C-terminal region is not essential. However, while PC3 bound to membranes the majority of PC3 DeltaC did not, suggesting that either the pro-region or the C-terminal region of PC3 is required for membrane association. Removal of peripheral membrane proteins did not affect the binding properties of any of the in vitro translated proteins. Chromaffin granule membranes (CGMs) were used to study the binding characteristics of endogenous PC3 and its active C-terminal truncated counterpart (PC3 DeltaC). Incubation of CGMs with Triton X-100 did not completely solubilise either of these forms of PC3. Moreover, both PC3 and PC3 DeltaC remained associated with detergent-resistant membrane microdomains, termed lipid rafts, purified from CGMs. The data raise the possibility that PC3 and PC3 DeltaC are sorted to the regulated secretory pathway via their association with membrane lipid rafts.