Cellular distributions of the prohormone processing enzymes PC1 and PC2.

The prohormone convertases PC1 (also known as sPC3) and PC2 are known to mediate the proteolytic conversion of inactive neuropeptide and hormone precursors to bioactive peptide products. In this study we have used sucrose density centrifugation to determine the subcellular distributions of the various forms of PC1 and PC2 in three different cell types, AtT-20, beta TC3, and PC12 cells. The former two cell lines naturally express PC enzymes, while PC12 cell clones expressing PCs were obtained by stable transfection. Our data show considerable cell-line specific variation in PC processing, with PC12 cells exhibiting the most complete processing of both enzyme precursors. While in all cell lines mature forms of both enzymes were stored within particles having the same buoyant density as secretory granule markers, in some cell lines substantial amounts of mature PC1 and PC2 were also associated with the Golgi marker. Processing of the two PC precursors was not interdependent since PC12 cells expressing only one of the two PCs were fully capable of enzyme maturation. Interestingly, analysis of intracellular processing of an endogenous peptide precursor, proneurotensin, revealed that transfected PC1, but not PC2, showed enzymatic activity against this precursor.