ELECTRON-MICROSCOPIC IMMUNOCYTOCHEMICAL EVIDENCE FOR THE INVOLVEMENT OF THE CONVERTASES PC1 AND PC2 IN THE PROCESSING OF PROINSULIN IN PANCREATIC B-CELLS

Endoproteolytic cleavage of pairs of basic amino acids is the key mech anism in the specific processing of precursor hormone molecules. Two e ndoproteases, PC1 (or PC3) and PC2, have recently been implicated in t he conversion of proinsulin. Using antibodies against these proteases and proinsulin, followed by protein A-gold complex, we performed an im munocytochemical study for precise identification of the subcellular c ompartments involved in the processing of insulin. Both PC1 and PC2 im munoreactivities followed a pattern of gradually increasing density al ong the secretory pathway, being higher in the immature granules, Proi nsulin labeling was detected in the Golgi apparatus and in the coated immature secretory granules located mainly in the Golgi area. Using do uble labeling, we demonstrated the presence of PC1 and/or PC2 in the m ajority of proinsulin-rich granules. In addition, we provided evidence that PC1 and PC2 are co-localized within the same granules. Co-expres sion of PC1 and PC2 with proinsulin in islet beta-cells indicates that these proteases are actively involved, probably in a sequential manne r, in the conversion of proinsulin into insulin.