Yang S, Ayscough K R, Drubin D G
Department of Molecular and Cell Biology, University of California, Berkeley 94720, USA.
J Cell Biol. 1997 Jan 13;136(1):111-23. doi: 10.1083/jcb.136.1.111.
Saccharomyces cerevisiae cells select bud sites according to one of two predetermined patterns. MATa and MAT alpha cells bud in an axial pattern, and MATa/alpha cells bud in a bipolar pattern. These budding patterns are thought to depend on the placement of spatial cues at specific sites in the cell cortex. Because cytoskeletal elements play a role in organizing the cytoplasm and establishing distinct plasma membrane domains, they are well suited for positioning bud-site selection cues. Indeed, the septin-containing neck filaments are crucial for establishing the axial budding pattern characteristic of MATa and MAT alpha cells. In this study, we determined the budding patterns of cells carrying mutations in the actin gene or in genes encoding actin-associated proteins: MATa/alpha cells were defective in the bipolar budding pattern, but MATa and MAT alpha cells still exhibit a normal axial budding pattern. We also observed that MATa/alpha actin cytoskeleton mutant daughter cells correctly position their first bud at the distal pole of the cell, but mother cells position their buds randomly. The actin cytoskeleton therefore functions in generation of the bipolar budding pattern and is required specifically for proper selection of bud sites in mother MATa/alpha cells. These observations and the results of double mutant studies support the conclusion that different rules govern bud-site selection in mother and daughter MATa/alpha cells. A defective bipolar budding pattern did not preclude an sla2-6 mutant from undergoing pseudohyphal growth, highlighting the central role of daughter cell bud-site selection cues in the formation of pseudohyphae. Finally, by examining the budding patterns of mad2-1 mitotic checkpoint mutants treated with benomyl to depolymerize their microtubules, we confirmed and extended previous evidence indicating that microtubules do not function in axial or bipolar bud-site selection.
酿酒酵母细胞根据两种预定模式之一选择出芽位点。MATa和MATα细胞以轴向模式出芽，而MATa/α细胞以双极模式出芽。这些出芽模式被认为取决于细胞皮质中特定位点的空间线索的定位。由于细胞骨架元件在组织细胞质和建立不同的质膜结构域中发挥作用，它们非常适合定位出芽位点选择线索。事实上，含septin的颈部细丝对于建立MATa和MATα细胞特有的轴向出芽模式至关重要。在这项研究中，我们确定了肌动蛋白基因或编码肌动蛋白相关蛋白的基因发生突变的细胞的出芽模式：MATa/α细胞在双极出芽模式上存在缺陷，但MATa和MATα细胞仍表现出正常的轴向出芽模式。我们还观察到，MATa/α肌动蛋白细胞骨架突变体的子细胞将其第一个芽正确定位在细胞的远端极，但母细胞随机定位其芽。因此，肌动蛋白细胞骨架在双极出芽模式的产生中起作用，并且是MATa/α母细胞中正确选择出芽位点所特需的。这些观察结果和双突变研究的结果支持了这样的结论，即不同的规则支配着MATa/α母细胞和子细胞中的出芽位点选择。有缺陷的双极出芽模式并不妨碍sla2-6突变体进行假菌丝生长，这突出了子细胞出芽位点选择线索在假菌丝形成中的核心作用。最后，通过检查用苯菌灵处理以解聚其微管的mad2-1有丝分裂检查点突变体的出芽模式，我们证实并扩展了先前的证据，表明微管在轴向或双极出芽位点选择中不起作用。
