Regulation of exocytotic events by centrosome-analogous structures. In: Keränen S, Jäntti J, editors. Regulatory Mechanisms of Intracellular Membrane Transport. Berlin: Springer Berlin Heidelberg; 2004. pp. 193-207. [DOI: 10.1007/b98734]

The SpoMBe pathway drives membrane bending necessary for cytokinesis and spore formation in yeast meiosis

Precise control over organelle shapes is essential for cellular organization and morphogenesis. During yeast meiosis, prospore membranes (PSMs) constitute bell-shaped organelles that enwrap the postmeiotic nuclei leading to the cellularization of the mother cell's cytoplasm and to spore formation. Here, we analysed how the PSMs acquire their curved bell-shaped structure. We discovered that two antagonizing forces ensure PSM shaping and proper closure during cytokinesis. The Ssp1p-containing coat at the leading edge of the PSM generates a pushing force, which is counteracted by a novel pathway, the spore membrane-bending pathway (SpoMBe). Using genetics, we found that Sma2p and Spo1p, a phospholipase, as well as several GPI-anchored proteins belong to the SpoMBe pathway. They exert a force all along the membrane, responsible for membrane bending during PSM biogenesis and for PSM closure during cytokinesis. We showed that the SpoMBe pathway involves asymmetric distribution of Sma2p and does not involve a GPI-protein-containing matrix. Rather, repulsive forces generated by asymmetrically distributed and dynamically moving GPI-proteins are suggested as the membrane-bending principle.

Dynamic organization of the actin cytoskeleton during meiosis and spore formation in budding yeast

During sporulation in Saccharomyces cerevisiae, the four daughter cells (spores) are formed inside the boundaries of the mother cell. Here, we investigated the dynamics of spore assembly and the actin cytoskeleton during this process, as well as the requirements for filamentous actin during the different steps of spore formation. We found no evidence for a polarized actin cytoskeleton during sporulation. Instead, a highly dynamic network of non-polarized actin cables is present underneath the plasma membrane of the mother cell. We found that a fraction of prospore membrane (PSM) precursors are transported along the actin cables. The velocity of PSM precursors is diminished if Myo2p or Tpm1/2p function is impaired. Filamentous actin is not essential for meiotic progression, for shaping of the PSMs or for post-meiotic cytokinesis. However, actin is essential for spore wall formation. This requires the function of the Arp2/3p complex and involves large carbohydrate-rich compartments, which may be chitosome analogous structures.

Spore number control and breeding in Saccharomyces cerevisiae: a key role for a self-organizing system

Spindle pole bodies (SPBs) provide a structural basis for genome inheritance and spore formation during meiosis in yeast. Upon carbon source limitation during sporulation, the number of haploid spores formed per cell is reduced. We show that precise spore number control (SNC) fulfills two functions. SNC maximizes the production of spores (1-4) that are formed by a single cell. This is regulated by the concentration of three structural meiotic SPB components, which is dependent on available amounts of carbon source. Using experiments and computer simulation, we show that the molecular mechanism relies on a self-organizing system, which is able to generate particular patterns (different numbers of spores) in dependency on one single stimulus (gradually increasing amounts of SPB constituents). We also show that SNC enhances intratetrad mating, whereby maximal amounts of germinated spores are able to return to a diploid lifestyle without intermediary mitotic division. This is beneficial for the immediate fitness of the population of postmeiotic cells.

Molecular interactions position Mso1p, a novel PTB domain homologue, in the interface of the exocyst complex and the exocytic SNARE machinery in yeast

In this study, we have analyzed the association of the Sec1p interacting protein Mso1p with the membrane fusion machinery in yeast. We show that Mso1p is essential for vesicle fusion during prospore membrane formation. Green fluorescent protein-tagged Mso1p localizes to the sites of exocytosis and at the site of prospore membrane formation. In vivo and in vitro experiments identified a short amino-terminal sequence in Mso1p that mediates its interaction with Sec1p and is needed for vesicle fusion. A point mutation, T47A, within the Sec1p-binding domain abolishes Mso1p functionality in vivo, and mso1T47A mutant cells display specific genetic interactions with sec1 mutants. Mso1p coimmunoprecipitates with Sec1p, Sso1/2p, Snc1/2p, Sec9p, and the exocyst complex subunit Sec15p. In sec4-8 and SEC4I133 mutant cells, association of Mso1p with Sso1/2p, Snc1/2p, and Sec9p is affected, whereas interaction with Sec1p persists. Furthermore, in SEC4I133 cells the dominant negative Sec4I133p coimmunoprecipitates with Mso1p-Sec1p complex. Finally, we identify Mso1p as a homologue of the PTB binding domain of the mammalian Sec1p binding Mint proteins. These results position Mso1p in the interface of the exocyst complex, Sec4p, and the SNARE machinery, and reveal a novel layer of molecular conservation in the exocytosis machinery.