In an effort to find additional Furin-like enzymes, the polymerase chain reaction was used successfully to detect and amplify conserved sequences within the catalytic domain of Furin and Kex2. In 1990, Seidah et al., identified, in mouse pituitary, the cDNA of two additional PC-related enzymes that were called PC1 and PC2 . At approximately the same time, Smeekens and Steiner identified in human insulinoma a cDNA coding for PC2 . The human and mouse PC1 genes (PCSK1) are localized on chromosomes 5 and 13, respectively, whereas the PC2 gene (PCSK2) is localized on human chromosome 20 and on mouse chromosome 2 (Figure 3). The corresponding protein of the full-length cDNA of PC1 is a 751-residue protein and the cDNA of PC2 encodes a 638-residue protein. Contrary to Kex2 and Furin, both PC1 and PC2 lack a transmembrane domain (Figure 1) [15, 16]. In 1991, using similar approaches, Smeekens et al., identified a PC-related enzyme highly expressed in the mouse AtT20 anterior pituitary cell line that unfortunately was called PC3 , since it turned out to be identical to PC1 [18, 19, 20]. Studies in various laboratories revealed that PC1 and PC2 process peptide hormones and neuropeptide precursors within the dense core vesicles of the regulated secretory pathway of the brain and the neuroendocrine system [21,22]. Although PC1 and PC2 are structurally very similar, each convertase has definite substrate site preferences. Among the major substrates of these enzymes are proopiomelanocortin (POMC), proinsulin and proglucagon [23, 24]. Regulation of the activity of PC1 occurs by both its N-and C-terminal domains. Following its N-terminal autocatalytic cleavage within the endoplasmic reticulum, the 84 kDa form of PC1 is transported to the trans Golgi Network (TGN) and secretory granules to undergo two other autocatalytic cleavages, one within the inhibitory prosegment and the other at its carboxy-terminal domain to generate the fully active 66-kDa form , the major form found in islets of Langerhans and in secretory granules of AtT20 cells . Although PC2 is also autocatalytically processed prior to activation like PC1 (Figure 2), the removal of its prosegment is less efficient and PC2 slowly exits from the ER as a zymogen (proPC2) and is processed to PC2 only in immature secretory granules. This difference in the time course of activation of PC1 and PC2 was reportedly linked to pH and calcium levels . The cleavage of proPC1 to the 84 kDa PC1 occurs at a neutral pH and is calcium-independent, whereas PC2 is activated much more slowly in the immature secretory granules at pHs 5-6 in a calcium-dependent fashion . As a consequence of the different temporal activation of PC1 and PC2 in cells expressing both enzymes, PC1 will cleave precursors before PC2, leading to an ordered cleavage mechanism that may explain the first cleavage of POMC into P-LPH and then into ^-endorphin, peptide products that require the consecutive action of PC1 and PC2, respectively [27, 28].
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