Structural and Functional Analysis of Vac8p-Vac17p Complex for Organelle Inheritance in the Budding Yeast Saccharomyces cerevisiae

Abstract

Cell division is a process during which a mother cell is divided into two cells. During this process, not only genetic materials (genetic inheritance) but also almost all organelles (organelle inheritance) are shared by the two cells. In the budding yeast Saccharomyces cerevisiae, a portion of vacuoles move towards the emerging bud during mitosis, and this process is termed vacuole inheritance. Vacuole inheritance is mediated mainly by the three proteins Vac8p, Vac17p, and Myo2p. The motor protein Myo2p interacts with Vac17p, and the Myo2p-Vac17p complex then interacts with Vac8p, a vacuolar membrane protein, delivering a portion of vacuole-derived vesicles to the bud along the actin cable. Also, Vac8p interacts with the nuclear membrane protein Nvj1p to form NVJ (Nucleus-Vacuole Junction), which is a membrane contact site between the nucleus and vacuole. NVJ is required for the piecemeal microautophagy of the nucleus (PMN), a yeast-specific type of nucleophagy. In addition, Vac8p binds to Atg13p to mediate Cvt pathway (cytoplasm-to-vacuole targeting pathway) which moves the cytoplasmic protein Ape1p into the lumen of vacuole. In this study, based on X-ray crystal structures of the Vac8p-Vac17p complex, I found that vacuole inheritance is largely impaired when the amino acid residues important for Vac8p-Vac17p interaction are mutated (the L31 and N234 of Vac8p; the L336 and F339 of Vac17p). The structures of the Vac8p-Vac17p complex suggest that vacuole inheritance and Vac8p-mediated autophagy occur in a mutually-exclusive manner because Vac17p interaction with Vac8p prevents Vac8p homo-dimerization, which is required for Vac8p-mediated autophagy-like processes, Cvt and PMN.