Valdez-Taubas Javier, Pelham Hugh R B
MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, United Kingdom.
Curr Biol. 2003 Sep 16;13(18):1636-40. doi: 10.1016/j.cub.2003.09.001.
Many cells show a polarized distribution of some plasma membrane proteins, which may be maintained either by a diffusion barrier or kinetically: as first demonstrated in fibroblasts, locally exocytosed proteins will remain polarized if they are endocytosed and recycled before they can diffuse to equilibrium. In yeast, actin cables direct exocytosis to the bud and to the tips of polarized mating intermediates termed shmoos. A septin ring at the bud neck retains some proteins, but shmoos lack this. Here, we show that the exocytic SNARE Snc1 is kinetically polarized. It is concentrated at bud and shmoo tips, and this requires its endocytosis. Kinetic polarization is possible in these small cells because proteins diffuse much more slowly in the yeast plasma membrane than would be expected from measurements in animal cells. Slow diffusion requires neither the cell wall nor polymerized actin, but it is affected in the ergosterol synthesis mutant erg6. Other proteins also require endocytosis for efficient polarization, and the plasma membrane SNARE Sso1 can be polarized merely by appending an endocytic signal. Thus, despite their small size, yeast cells can use localized exocytosis and endocytic recycling as a simple mechanism to maintain polarity.
许多细胞呈现出一些质膜蛋白的极化分布，这种分布可以通过扩散屏障或动力学方式维持：正如在成纤维细胞中首次证明的那样，如果局部胞吐的蛋白质在扩散至平衡之前被内吞并循环利用，它们将保持极化状态。在酵母中，肌动蛋白丝将胞吐作用导向芽以及称为“shmoo”的极化交配中间体的尖端。芽颈处的隔膜环保留了一些蛋白质，但“shmoo”缺乏此结构。在这里，我们表明胞吐SNARE蛋白Snc1在动力学上是极化的。它集中在芽和“shmoo”的尖端，这需要其进行内吞作用。在这些小细胞中，动力学极化是可能的，因为蛋白质在酵母质膜中的扩散速度比在动物细胞中的测量预期要慢得多。缓慢扩散既不需要细胞壁也不需要聚合的肌动蛋白，但在麦角固醇合成突变体erg6中会受到影响。其他蛋白质也需要内吞作用来实现有效的极化，并且质膜SNARE蛋白Sso1仅通过附加一个内吞信号就可以实现极化。因此，尽管酵母细胞体积小，但它们可以利用局部胞吐作用和内吞循环作为维持极性的简单机制。
