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The biosynthesis of the prohormone convertase PC2 was studied in Chinese hamster ovary cells stably transfected with PC2 cDNA (CHO/PC2) and in rat insulinoma cells (Rin5f). The major form of PC2 synthesized by CHO/PC2 cells was a 75-kDa protein corresponding to proPC2; this protein was retained intracellularly for 2-4 h following synthesis, suggesting prolonged intracellular residence. In contrast, the major form of PC2 within Rin cells initially exhibited a molecular mass of 72 kDa and was then progressively converted to a 64-kDa species. This 64-kDa species, which required 1-2 h to be released, was the major PC2 form detectable in Rin cell medium. Calcium-dependent benzyloxycarbonyl-Arg-Ser-Lys-Arg-aminomethylcoumarin cleaving activity was found in spent Rin cell medium; this activity could be immunoprecipitated with a carboxyl-terminal PC2 antibody, but not with preimmune serum. In neither cell line did intracellular PC2 become endoglycosidase H-resistant over time. PC2 released from Rin cells was also endoglycosidase H-sensitive. Microsequencing and endoglycosidase H results indicate that 75-kDa CHO cell PC2 and 72-kDa Rin cell PC2 both represent proPC2. We speculate that (a) PC2 undergoes unusual glycosylation, which may be related to its slow release from cells, and (b) the 64-kDa molecule detectable in spent Rin cell medium represents the enzymatically active form of PC2. |
