A C-TERMINAL DOMAIN CONSERVED IN PRECURSOR PROCESSING PROTEASES IS REQUIRED FOR INTRAMOLECULAR N-TERMINAL MATURATION OF PRO-KEX2 PROTEASE

The Kex2 protease of the yeast Saccharomyces cerevisiae is the prototy pe of a family of eukaryotic subtilisin homologs thought to process pr ohormones and other precursors in the secretory pathway. Deletion anal ysis of Kex2 protease shows that a sequence of 154-159 residues carbox yl to the subtilisin domain is essential for the formation of active e nzyme. Disruption of this region, termed the 'P-domain', blocks the no rmally rapid intramolecular cleavage of the N-terminal pro-segment of pro-Kex2 protease in the endoplasmic reticulum (ER). The C-terminal bo undary of the P-domain coincides closely with the endpoint of similari ty between Kex2 protease and its mammalian homologues. The conservatio n of and functional requirement for the P-domain sharpens the distinct ion between a 'Kex2 family' of processing enzymes and degradative 'sub tilases', and implies that the Kex2-related enzymes have in common ent irely novel structural features that are important in the maturation o f precursor polypeptide substrates. Failure to cleave the N-terminal p ro-domain, due either to truncation of the P-domain or to mutation of the active site histidine or serine, results in stable, intracellular retention of pro-enzyme, apparently in the ER. Thus pro-Kex2 protease appears to contain an ER retention signal which is removed or destroye d by cleavage of the pro-domain.