Bachewich C, Heath I B
Department of Biology, York University, North York, Ontario, M3J 1P3, Canada.
J Cell Sci. 1998 Jul 30;111 ( Pt 14):2005-16. doi: 10.1242/jcs.111.14.2005.
The roles of cortical F-actin in initiating and regulating polarized cell expansion in the form of hyphal tip morphogenesis were investigated by analyzing long term effects of F-actin disruption by latrunculin B in the oomycete Saprolegnia ferax, and detecting localized changes in the cortical F-actin organization preceding hyphal formation. Tubular hyphal morphology was dependent on proper F-actin organization, since latrunculin induced dose-dependent actin disruption and corresponding changes in hyphal morphology and wall deposition. With long incubation times (1 to 3 hours), abundant subapical expansion occurred, the polar form of which was increasingly lost with increasing actin disruption, culminating in diffuse subapical expansion. These extreme effects were accompanied by disorganized cytoplasm, and novel reorganization of microtubules, characterized by star-burst asters. Upon removing latrunculin, hyperbranching produced abundant polar branches with normal F-actin organization throughout the colony. The results are consistent with F-actin regulating polar vesicle delivery and controlling vesicle fusion at the plasma membrane, and suggest that F-actin participates in establishing polar growth. To test this idea further, we utilized the hyperbranching growth form of Saprolegnia. Early during the recovery time, prior to multiple branch formation, radial arrays of filamentous F-actin were observed in regions with no detectable surface protrusion. Their locations were consistent with those of the numerous branches that formed with longer recovery times. Similar radial arrays preceded germ tube formation in asexual spores. The arrays were important for initiating polar growth since the spores lost their ability to polarize when the F-actin was disrupted with latrunculin, and increased isometrically in size rather than producing germ tubes. Therefore, F-actin participates in initiating tip formation in addition to its previously demonstrated participation in maintenance of hyphal tip growth. The cortical location and radial organization of the arrays suggest that they recruit and stabilize membrane-bound and cytosolic factors required to build a new tip.
通过分析拉春库林B对卵菌纲真菌绵霉F-肌动蛋白的长期破坏作用，并检测菌丝形成前皮层F-肌动蛋白组织的局部变化，研究了皮层F-肌动蛋白在以菌丝顶端形态发生形式启动和调节极化细胞扩张中的作用。管状菌丝形态依赖于适当的F-肌动蛋白组织，因为拉春库林诱导了剂量依赖性的肌动蛋白破坏以及菌丝形态和壁沉积的相应变化。长时间孵育（1至3小时）后，会发生大量亚顶端扩张，随着肌动蛋白破坏程度的增加，其极性形式逐渐丧失，最终导致亚顶端弥漫性扩张。这些极端效应伴随着细胞质紊乱以及微管的重新组织，其特征是呈星爆状星状体。去除拉春库林后，超分支产生了大量极性分支，整个菌落中F-肌动蛋白组织正常。结果表明F-肌动蛋白调节极性囊泡运输并控制质膜处的囊泡融合，这表明F-肌动蛋白参与建立极性生长。为了进一步验证这一观点，我们利用了绵霉的超分支生长形式。在恢复早期，在多个分支形成之前，在没有可检测到的表面突出的区域观察到丝状F-肌动蛋白的径向排列。它们的位置与恢复时间较长时形成的众多分支的位置一致。类似的径向排列在无性孢子的芽管形成之前出现。这些排列对于启动极性生长很重要，因为当用拉春库林破坏F-肌动蛋白时，孢子失去了极化能力，并且等比例增大而不是产生芽管。因此，F-肌动蛋白除了先前证明的参与维持菌丝顶端生长外，还参与启动顶端形成。这些排列的皮层位置和径向组织表明它们募集并稳定构建新顶端所需的膜结合和胞质因子。
