Multiple pathways for vacuolar sorting of yeast proteinase A

The sorting of the yeast proteases proteinase A and carboxypeptidase Y to the vacuole is a saturable, receptor-mediated process. Information sufficient for vacuolar sorting of the normally secreted protein invertase has in fusion constructs previously been found to reside in the propeptide of proteinase A. We found that sorting of such a hybrid protein is dependent on the vacuolar protein-sorting receptor Vps10p. This was unexpected, as strains disrupted for VPS10 sort more than 85% of the proteinase A to the vacuole. Consistent with a role for Vps10p in sorting of proteinase A, we found that 1) overproduction of Vps10p suppressed the missorting phenotype associated with overproduction of proteinase A, 2) overproduction of proteinase A induced missorting of carboxypeptidase Y, 3) vacuolar sorting of proteinase A in a deltavps10 strain was readily saturated by modest overproduction of proteinase A, and 4) Vps10p and proteinase A interact directly and specifically as shown by chemical cross-linking. Interestingly, overexpression of two telomere-linked VPS10 homologues, VTH1 and VTH2 suppressed the missorting phenotypes of a deltavps10 strain. However, disruption of the VTH1 and VTH2 genes did not affect the sorting of proteinase A. We conclude that proteinase A utilizes at least two mechanisms for sorting, a Vps10p-dependent path and a Vth1p/Vth2p/Vps10p-independent path.