Pruyne D W, Schott D H, Bretscher A
Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, New York 14853, USA.
J Cell Biol. 1998 Dec 28;143(7):1931-45. doi: 10.1083/jcb.143.7.1931.
The actin cytoskeleton in budding yeast consists of cortical patches and cables, both of which polarize toward regions of cell growth. Tropomyosin localizes specifically to actin cables and not cortical patches. Upon shifting cells with conditionally defective tropomyosin to restrictive temperatures, actin cables disappear within 1 min and both the unconventional class V myosin Myo2p and the secretory vesicle-associated Rab GTPase Sec4p depolarize rapidly. Bud growth ceases and the mother cell grows isotropically. When returned to permissive temperatures, tropomyosin-containing cables reform within 1 min in polarized arrays. Cable reassembly permits rapid enrichment of Myo2p at the focus of nascent cables as well as the Myo2p- dependent recruitment of Sec4p and the exocyst protein Sec8p, and the initiation of bud emergence. With the loss of actin cables, cortical patches slowly assume an isotropic distribution within the cell and will repolarize only after restoration of cables. Therefore, actin cables respond to polarity cues independently of the overall distribution of cortical patches and are able to directly target the Myo2p-dependent delivery of secretory vesicles and polarization of growth.
出芽酵母中的肌动蛋白细胞骨架由皮质斑和肌动蛋白丝束组成，二者均会向细胞生长区域极化。原肌球蛋白特异性定位于肌动蛋白丝束而非皮质斑。将条件性缺陷原肌球蛋白的细胞转移至限制温度后，肌动蛋白丝束在1分钟内消失，非常规的V类肌球蛋白Myo2p和与分泌囊泡相关的Rab GTP酶Sec4p均迅速去极化。芽生长停止，母细胞各向同性生长。当恢复到允许温度时，含原肌球蛋白的丝束在1分钟内在极化阵列中重新形成。丝束重新组装允许Myo2p在新生丝束的焦点处迅速富集，以及Myo2p依赖的Sec4p和外排复合体蛋白Sec8p的募集，从而启动芽的出现。随着肌动蛋白丝束的丧失，皮质斑在细胞内缓慢呈现各向同性分布，并且仅在丝束恢复后才会重新极化。因此，肌动蛋白丝束独立于皮质斑的整体分布对极性线索作出反应，并且能够直接靶向依赖Myo2p的分泌囊泡运输和生长极化。
