Trafficking of the Vasopressin and Oxytocin Prohormone Through the Regulated Secretory Pathway : Trafficking of the vasopressin/oxytocin prohormone

The events in the biogenesis of secretory granules after the budding of a dense-cored vesicle from the trans-Golgi network (TGN) were investigated in the neuroendocrine cell line PC12, using sulfate- labeled secretogranin II as a marker. The TGN-derived dense-cored vesicles, which we refer to as immature secretory granules, were found to be obligatory organellar intermediates in the biogenesis of the mature secretory granules which accumulate in the cell. Immature secretory granules were converted to mature secretory granules with a half-time of approximately 45 min. This conversion entailed an increase in their size, implying that the maturation of secretory granules includes a fusion event involving immature secretory granules. Pulse- chase labelling of PC12 cells followed by stimulation with high K+, which causes the release of secretogranin II, showed that not only mature, but also immature secretory granules were capable of undergoing regulated exocytosis. The kinetics of secretion of secretogranin II, as well as those of a constitutively secreted heparan sulfate proteoglycan, were reduced by treatment of PC12 cells with nocodazole, suggesting that both secretory granules and constitutive secretory vesicles are transported to the plasma membrane along microtubules. Our results imply that certain membrane proteins, e.g., those involved in the fusion of post-TGN vesicles with the plasma membrane, are sorted upon exit from the TGN, whereas other membrane proteins, e.g., those involved in the interaction of post-TGN vesicles with the cytoskeleton, may not be sorted.

Subtilisin E, an alkaline serine protease consisting of a single polypeptide chain of 275 amino acids is produced from a pre-pro-protein. The pre-sequence functions as the signal peptide for protein secretion across the membrane. Deletion of the pro-sequence yields mature but inactive subtilisin: the 77-amino acid pro-sequence must precede the mature subtilisin to guide the latter into an active conformation. Pro-subtilisin denatured in 6 M guanidine-HCl can be self-processed to the active enzyme intramolecularly, with concomitant cleavage of the pro-sequence, when dialysed against renaturing buffer. We have constructed an active-centre mutant of pro-subtilisin (Asp 32----Asn) which is not processed to active enzyme, unlike the wild-type pro-subtilisin, because intramolecular processing is prevented. Here we report an intermolecular pathway for the refolding of the inactive mature protein to an active enzyme in vitro with the aid of exogenously added pro-sequence. We establish conditions under which the mature inactive form, as well as acid-denatured subtilisins Carlsberg and BPN', can be renatured by the mutant pro-subtilisin.