REQUIREMENT OF THE PROPEPTIDE FOR IN-VIVO FORMATION OF ACTIVE YEAST CARBOXYPEPTIDASE-Y

Deletions have been constructed in the region encoding the 91-amino ac id propeptide of the vacuolar enzyme carboxypeptidase Y of Saccharomyc es cerevisiae, and in vivo effects of these mutations on the intracell ular transport of the mutant proenzymes have been examined. Deletions did not include the vacuolar targeting signal, and none of the mutated forms of procarboxypeptidase Y was found to be secreted. All deletion s, however, re suited in a decreased rate of transport of the truncate d proenzymes from the endoplasmic reticulum to the Golgi apparatus. Up to 29 residues close to the N terminus can be removed without complet ely eliminating transport of the mutated proenzymes to the vacuole. Ho wever, the C-terminal part of the propeptide contains elements which a re essential, since two small deletions, of 9 and 15 residues, respect ively, within this area re suited in loss of carboxypeptidase Y activi ty. This region is, however, not sufficient for efficient formation of active carboxypeptidase Y, since truncated precursors in which the va cuolar targeting signal was fused to the C-terminal part of the proreg ion did not give rise to active enzyme. Based on the results, we propo se that the carboxypeptidase Y propeptide plays an essential role in g uiding the proper folding of the protein in vivo and that many parts o f the propeptide contribute, in an additive way, to this function.