Roles of septins in prospore membrane morphogenesis and spore wall assembly in Saccharomyces cerevisiae

The highly conserved family of septin proteins has important functions in cytokinesis in mitotically proliferating cells. A different form of cytokinesis occurs during gametogenesis in Saccharomyces cerevisiae, in which four haploid meiotic products become encased by prospore membrane (PSMs) and specialized, stress-resistant spore walls. Septins are known to localize in a series of structures near the growing PSM, but previous studies noted only mild sporulation defects upon septin mutation. We report that directed PSM extension fails in many septin-mutant cells, and, for those that do succeed, walls are abnormal, leading to increased susceptibility to heating, freezing, and digestion by the Drosophila gut. Septin mutants mislocalize the leading-edge protein (LEP) complex required for normal PSM and wall biogenesis, and ectopic expression of the LEP protein Ssp1 perturbs mitotic septin localization and function, suggesting a functional interaction. Strikingly, extra copies of septin CDC10 rescue sporulation and LEP localization in cells lacking Sma1, a phospholipase D-associated protein dispensable for initiation of PSM assembly and PSM curvature but required for PSM extension. These findings point to key septin functions in directing efficient membrane and cell wall synthesis during budding yeast gametogenesis.