Cabib E, Drgonová J, Drgon T
National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
Annu Rev Biochem. 1998;67:307-33. doi: 10.1146/annurev.biochem.67.1.307.
In the vegetative (mitotic) cycle and during sexual conjugation, yeast cells display polarized growth, giving rise to a bud or to a mating projection, respectively. In both cases one can distinguish three steps in these processes: choice of a growth site, organization of the growth site, and actual growth and morphogenesis. In all three steps, small GTP-binding proteins (G proteins) and their regulators play essential signaling functions. For the choice of a bud site, Bud1, a small G protein, Bud2, a negative regulator of Bud1, and Bud5, an activator, are all required. If any of them is defective, the cell loses its ability to select a proper bud position and buds randomly. In the organization of the bud site or of the site in which a mating projection appears, Cdc42, its activator Cdc24, and its negative regulators play a fundamental role. In the absence of Cdc42 or Cdc24, the actin cytoskeleton does not become organized and budding does not take place. Finally, another small G protein, Rho1, is required for activity of beta (1-->3)glucan synthase, the enzyme that catalyzes the synthesis of the major structural component of the yeast cell wall. In all of the above processes, G proteins can work as molecular switches because of their ability to shift between an active GTP-bound state and an inactive GDP-bound state.
在营养（有丝分裂）周期和有性接合过程中，酵母细胞呈现出极性生长，分别形成芽或交配突起。在这两种情况下，都可以区分出这些过程中的三个步骤：生长位点的选择、生长位点的组织以及实际的生长和形态发生。在所有这三个步骤中，小GTP结合蛋白（G蛋白）及其调节因子发挥着重要的信号传导功能。对于芽位点的选择，小G蛋白Bud1、Bud1的负调节因子Bud2以及激活剂Bud5都是必需的。如果其中任何一个有缺陷，细胞就会失去选择合适芽位置的能力，从而随机出芽。在芽位点或出现交配突起的位点的组织过程中，Cdc42、其激活剂Cdc24及其负调节因子发挥着重要作用。在没有Cdc42或Cdc24的情况下，肌动蛋白细胞骨架无法组织起来，芽也无法形成。最后，另一个小G蛋白Rho1是β（1→3）葡聚糖合酶活性所必需的，该酶催化酵母细胞壁主要结构成分的合成。在上述所有过程中，G蛋白因其能够在活性GTP结合状态和非活性GDP结合状态之间转换而可作为分子开关发挥作用。
