PROTEOLYTIC PROCESSING OF CHROMOGRANIN-B AND SECRETOGRANIN-II BY PROHORMONE CONVERTASES

Two experimental approaches were used to study the processing of chrom ogranin B and secretogranin II by prohormone convertases. In GH(3) cel ls various prohormone convertases were overexpressed together with the substrate chromogranin B by use of a vaccinia virus infection system. PC1 appeared to be by far the most active enzyme and converted chromo granin B to several smaller molecules, including the peptide PE-11. In brain this peptide is cleaved physiologically from chromogranin B. So me processing of chromogranin B and formation of free PE-11 were also observed with PC2 and PACE4. Furin produced larger fragments, whereas PC5-A and PC5-B had negligible effects. As a second model, PC12 cells were stably transfected with PC1 or PC2 to investigate the processing of endogenous chromogranins. Both enzymes effectively cleaved chromogr anin B and secretogranin II, liberating the peptides PE-11 and secreto neurin, respectively. However, in transfection experiments the ability to generate the free peptides was more pronounced with PC2 than with PC1. The extent of proprotein processing achieved by prohormone conver tases apparently differed depending on the experimental system applied . This suggests that in vivo mechanisms to support and fine-tune the a ctivity of the processing enzymes exist, which might be overlooked by using only one methodological approach.