Mutants of the Saccharomyces cerevisiae VPS genes CCZ1 and YPT7 are blocked in different stages of sporulation

The CCZ1 gene is a member of the class B VPS (vacuolar protein sorting) genes and it is engaged in the last stage of delivery of multiple kinds of cargo to the yeast vacuole. In the process of fusion of the multivesicular body (MVB) with the vacuole, Ccz1p forms a complex with Ypt7p. Both genes are non-essential for vegetative growth, but their deletions cause a complete block in spore formation. The results of this study indicate that ccz1Δ cells initiate the meiotic program, properly proceed through premeiotic DNA replication and through the pairing of homologous chromosomes, but fail to progress through the first meiotic divisions and arrest in prophase I with a single nucleus. The mutant cells are defective in spindle formation as well as in duplication and/or separation of the SPBs. ypt7Δ cells, on the other hand, cannot execute DNA synthesis. We also show that expression of a mutated variant of the YPT7 gene suppresses the sporulation and autophagy defects of ccz1Δ cells to a quantitatively similar level, suggesting that restoration of autophagy in the ccz1Δ strain is sufficient to enable its sporulation.