Molecular Cellular Developmental Biology Program, The Ohio State University, Columbus, OH 43210.
Department of Mathematics, City University of Hong Kong, Kowloon, Hong Kong.
Mol Biol Cell. 2019 Sep 15;30(20):2543-2557. doi: 10.1091/mbc.E19-02-0106. Epub 2019 Aug 14.
The Cdc42 guanosine triphosphatase (GTPase) plays a central role in polarity development in species ranging from yeast to humans. In budding yeast, a specific growth site is selected in the G1 phase. Rsr1, a Ras GTPase, interacts with Cdc42 and its associated proteins to promote polarized growth at the proper bud site. Yet how Rsr1 regulates cell polarization is not fully understood. Here, we show that Rsr1-GDP interacts with the scaffold protein Bem1 in early G1, likely hindering the role of Bem1 in Cdc42 polarization and polarized secretion. Consistent with these in vivo observations, mathematical modeling predicts that Bem1 is unable to promote Cdc42 polarization in early G1 in the presence of Rsr1-GDP. We find that a part of the Bem1 Phox homology domain, which overlaps with a region interacting with the exocyst component Exo70, is necessary for the association of Bem1 with Rsr1-GDP. Overexpression of the GDP-locked Rsr1 interferes with Bem1-dependent Exo70 polarization. We thus propose that Rsr1 functions in spatial and temporal regulation of polarity establishment by associating with distinct polarity factors in its GTP- and GDP-bound states.
CDC42 鸟嘌呤核苷酸三磷酸酶（GTPase）在从酵母到人类的各种物种中发挥着核心作用，参与极性的发展。在出芽酵母中，在 G1 期选择一个特定的生长位点。Rsr1 是一种 Ras GTPase，与 Cdc42 及其相关蛋白相互作用，以促进在适当的芽位点进行极化生长。然而，Rsr1 如何调节细胞极性尚不完全清楚。在这里，我们表明 Rsr1-GDP 在早期 G1 与支架蛋白 Bem1 相互作用，可能阻碍 Bem1 在 Cdc42 极化和极化分泌中的作用。与这些体内观察结果一致，数学模型预测，在存在 Rsr1-GDP 的情况下，Bem1 无法在早期 G1 促进 Cdc42 极化。我们发现，与外泌体成分 Exo70 相互作用的区域重叠的 Bem1 Phox 同源结构域的一部分对于 Bem1 与 Rsr1-GDP 的结合是必需的。GDP 锁定的 Rsr1 的过表达会干扰 Bem1 依赖的 Exo70 极化。因此，我们提出 Rsr1 通过与其 GTP 和 GDP 结合状态下的不同极性因子结合，在空间和时间上调节极性建立。
