Department of Fundamental Microbiology, Faculty of Biology and Medicine, University of Lausanne, 1015 Lausanne, Switzerland.
Department of Fundamental Microbiology, Faculty of Biology and Medicine, University of Lausanne, 1015 Lausanne, Switzerland.
Curr Biol. 2019 Oct 7;29(19):3165-3176.e6. doi: 10.1016/j.cub.2019.07.088. Epub 2019 Sep 5.
How actin structures of distinct identities and functions coexist within the same environment is a critical self-organization question. Fission yeast cells have a simple actin cytoskeleton made of four structures: Arp2/3 assembles actin patches around endocytic pits, and the formins For3, Cdc12, and Fus1 assemble actin cables, the cytokinetic ring during division, and the fusion focus during sexual reproduction, respectively. The focus concentrates the delivery of hydrolases by myosin V to digest the cell wall for cell fusion. We discovered that cells lacking capping protein (CP), a heterodimer that blocks barbed-end dynamics and associates with actin patches, exhibit a delay in fusion. Consistent with CP-formin competition for barbed-end binding, Fus1, F-actin, and the linear filament marker tropomyosin hyper-accumulate at the fusion focus in cells lacking CP. CP deletion also rescues the fusion defect of a mutation in the Fus1 knob region. However, myosin V and exocytic cargoes are reduced at the fusion focus and diverted to ectopic foci, which underlies the fusion defect. Remarkably, the ectopic foci coincide with Arp2/3-assembled actin patches, which now contain low levels of Fus1. We further show that CP localization to actin patches is required to prevent the formation of ectopic foci and promote efficient cell fusion. During mitotic growth, actin patches lacking CP similarly display a dual identity, as they accumulate the formins For3 and Cdc12, normally absent from patches, and are co-decorated by the linear filament-binding protein tropomyosin and the patch marker fimbrin. Thus, CP serves to protect Arp2/3-nucleated structures from formin activity.
不同身份和功能的肌动蛋白结构如何在同一环境中共存是一个关键的自组织问题。裂殖酵母细胞具有一个简单的肌动蛋白细胞骨架，由四种结构组成：Arp2/3 在胞吞陷窝周围组装肌动蛋白斑点，而formin 家族成员 For3、Cdc12 和 Fus1 分别组装肌动蛋白纤维、有丝分裂时的细胞分裂环和有性生殖时的融合焦点。焦点集中肌球蛋白 V 水解酶的输送，以消化细胞壁进行细胞融合。我们发现，缺乏帽蛋白 (CP) 的细胞融合会延迟，CP 是一种阻止突刺末端动力学并与肌动蛋白斑点结合的异二聚体。与 CP-formin 对突刺末端结合的竞争一致，Fus1、F-actin 和线性丝蛋白标记 tropomyosin 在缺乏 CP 的细胞中在融合焦点处过度积累。CP 缺失还挽救了 Fus1 旋钮区域突变的融合缺陷。然而，肌球蛋白 V 和胞吐货物在融合焦点处减少，并转移到异位焦点，这是融合缺陷的基础。值得注意的是，异位焦点与由 Arp2/3 组装的肌动蛋白斑点重合，这些斑点现在含有低水平的 Fus1。我们进一步表明，CP 定位于肌动蛋白斑点是防止异位焦点形成和促进有效细胞融合所必需的。在有丝分裂生长过程中，缺乏 CP 的肌动蛋白斑点也表现出双重身份，因为它们积累了通常不存在于斑点中的 formin For3 和 Cdc12，并且被线性丝蛋白结合蛋白 tropomyosin 和斑点标记 fimbrin 共同装饰。因此，CP 可防止 Arp2/3 引发的结构被formin 活性影响。
