National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Chinese Academy of Sciences, Shanghai 200032, China.
Plant Cell. 2011 Feb;23(2):661-80. doi: 10.1105/tpc.110.081802. Epub 2011 Feb 9.
The actin cytoskeleton is an important regulator of cell expansion and morphogenesis in plants. However, the molecular mechanisms linking the actin cytoskeleton to these processes remain largely unknown. Here, we report the functional analysis of rice (Oryza sativa) FH5/BENT UPPERMOST INTERNODE1 (BUI1), which encodes a formin-type actin nucleation factor and affects cell expansion and plant morphogenesis in rice. The bui1 mutant displayed pleiotropic phenotypes, including bent uppermost internode, dwarfism, wavy panicle rachis, and enhanced gravitropic response. Cytological observation indicated that the growth defects of bui1 were caused mainly by inhibition of cell expansion. Map-based cloning revealed that BUI1 encodes the class II formin FH5. FH5 contains a phosphatase tensin-like domain at its amino terminus and two highly conserved formin-homology domains, FH1 and FH2. In vitro biochemical analyses indicated that FH5 is capable of nucleating actin assembly from free or profilin-bound monomeric actin. FH5 also interacts with the barbed end of actin filaments and prevents the addition and loss of actin subunits from the same end. Interestingly, the FH2 domain of FH5 could bundle actin filaments directly and stabilize actin filaments in vitro. Consistent with these in vitro biochemical activities of FH5/BUI1, the amount of filamentous actin decreased, and the longitudinal actin cables almost disappeared in bui1 cells. The FH2 or FH1FH2 domains of FH5 could also bind to and bundle microtubules in vitro. Thus, our study identified a rice formin protein that regulates de novo actin nucleation and spatial organization of the actin filaments, which are important for proper cell expansion and rice morphogenesis.
肌动蛋白细胞骨架是植物细胞扩展和形态发生的重要调节剂。然而，将肌动蛋白细胞骨架与这些过程联系起来的分子机制在很大程度上仍然未知。在这里，我们报告了水稻（Oryza sativa）FH5/BENT UPPERMOST INTERNODE1（BUI1）的功能分析，该基因编码一种形成素型肌动蛋白成核因子，影响水稻细胞扩展和植物形态发生。bui1 突变体表现出多种表型，包括弯曲的最上节间、矮化、波浪状的穗轴和增强的向地性反应。细胞学观察表明，bui1 的生长缺陷主要是由于细胞扩展的抑制。基于图谱的克隆表明，BUI1 编码 II 类形成素 FH5。FH5 在其氨基末端含有一个磷酸酶张力蛋白样结构域和两个高度保守的形成素同源结构域 FH1 和 FH2。体外生化分析表明，FH5 能够从游离或结合前肌动蛋白的单体肌动蛋白中引发肌动蛋白组装。FH5 还与肌动蛋白丝的帽状末端相互作用，并阻止同一末端肌动蛋白亚基的添加和损失。有趣的是，FH5 的 FH2 结构域可以直接束状肌动蛋白丝并在体外稳定肌动蛋白丝。与 FH5/BUI1 的这些体外生化活性一致，丝状肌动蛋白的数量减少，并且在 bui1 细胞中几乎没有纵向肌动蛋白电缆。FH5 的 FH2 或 FH1FH2 结构域也可以在体外结合和束状微管。因此，我们的研究鉴定了一种水稻形成素蛋白，它调节新形成的肌动蛋白成核和肌动蛋白丝的空间组织，这对适当的细胞扩展和水稻形态发生很重要。
