PC5-A-MEDIATED PROCESSING OF PRO-NEUROTENSIN IN EARLY COMPARTMENTS OFTHE REGULATED SECRETORY PATHWAY OF PC5-TRANSFECTED PC12 CELLS

Among the members of the proprotein convertase (PC) family, PC1 and PC 2 have well established roles as prohormone convertases. Another good candidate for this role is PC5-A that has been shown to be present in the regulated secretory pathway of certain neuroendocrine tissues, but evidence that it can process prohormones is lacking. To determine whe ther PC5-A could function as a prohormone convertase and to compare it s cleavage specificity with that of PC1 and PC2, we stably transfected the rat pheochromocytoma PC12 cell line with PC5-A and analyzed the b iosynthesis and subcellular localization of the enzyme, as well as its ability to process pro-neurotensin/neuromedin N (pro-NT/NN) into acti ve peptides, Our data showed that in transfected PC12 cells, PC5-A was converted from its 126-kDa precursor form into a 117-kDa mature form and, to a lesser extent, into a C-terminally truncated 65-kDa form of the 117-kDa product. Metabolic and immunochemical studies showed that PC5-A was sorted to early compartments of the regulated secretory path way where it colocalized with immunoreactive NT. Furthermore, pro-NT/N N was processed in these compartments according to a pattern that diff ered from that previously described in PC1- and PC2-transfected PC12 c ells. This pattern resembled that previously reported for pro-NT/NN pr ocessing in the adrenal medulla, a tissue known to express high levels of PC5-A. Altogether, these data demonstrate for the first time the a bility of PC5-A to function as a prohormone convertase in the regulate d secretory pathway and suggest a role for this enzyme in the physiolo gical processing of pro-NT/NN.