Roles for the canonical polarity machinery in thede novoestablishment of polarity in budding yeast spores

Abstract

ABSTRACTBudding in the yeastSaccharomyces cerevisiaeoccurs at a single site pre-determined by cortical landmarks deposited during prior budding. During mating between haploid cells in the lab, external pheromone cues override the cortical landmarks to drive polarization and cell fusion. By contrast, in haploid gametes (called spores) produced by meiosis, a pre-determined polarity site drives initial polarized morphogenesis independent of mating partner location. Spore membranes are madede novoso existing cortical landmarks were unknown, as were the mechanisms by which the spore polarity site is made and how it works. We find that the landmark canonically required for distal budding, Bud8, stably marks the spore polarity site along with Bud5, a GEF for the GTPase Rsr1 that canonically links cortical landmarks to the conserved Cdc42 polarity machinery. Cdc42 and other GTPase regulators arrive at the site during its biogenesis, after spore membrane closure but apparently at the site where membrane synthesis began, and then these factors leave, pointing to a discrete “functionalization” step. Filamentous actin may be required for initial establishment of the site, but thereafter Bud8 accumulates independent of actin filaments. These results suggest a distinct polarization mechanism that may provide insights into gamete polarization in other organisms.SIGNIFICANCE STATEMENTDormant budding yeast spores possess a single, stable cortical site that marks the location where polarized growth occurs upon dormancy exit. It was not known how the site forms or which molecules comprise it.Using fluorescently tagged proteins in living cells undergoing sporulation, the authors found proteins canonically involved in polarization of non-spore cells arriving at the polarity site in a choreographed manner and required for site function.These findings point to a distinct polarity mechanism from non-spore cells and raise new questions about polarity protein interactions with membranes that may be applicable to gametogenesis in other organisms.