Biosynthesis of the Vacuolar Yeast Glycoprotein Carboxypeptidase Y

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Abstract
The vacuolar glycoprotein carboxypeptidase Y (Mr 61 000) is synthesized via a larger precursor (Mr 67 000). About 35% of the precusor protein can be recovered in the sedimentingfraction of the cell homogenate. A large part of the radioactivity precipitable by antibodies directed against carboxypeptidase Y is redistributed from the sedimenting fraction into the soluble one as the precursor is converted to the enzyme. The precursor proteinis not identical with the enzyme‐inhibitor complex described previously by Hayashi et al. [Agr. Biol. Chem. 33, 196 (1969)]. The incorporation of radioactivemannose into the precursor indicates that the glycosylation takes place before or immediately after the completion of the syntheses of the polypeptide chain. The precursor is converted by trypsin in vitro to a protein identical in size to carboxypeptidase Y. The extra piece released by trypsin probably contains four phenylalanine and at least 10 leucine residues. Of three yeast proteinases tested in vitro only the proteinase B catalyzed conversion of the precursor to a protein identical in size to carboxypeptidase Y. The conversion in vivo, which presumably is an example of intracellular‐limitedproteolysis. proceeds with a half‐life of about 6 min. It is assumed that the precursor is an inactive pro‐enzyme, procarboxypeptidase Y, rather than a pre‐enzyme and that, as inactive hydrolase, it exists only during the time required for the intracellular transport to the vacuole, where the pro‐enzyme is activated.

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