Department of Molecular, Cellular and Developmental Biology, University of Michigan, Ann Arbor, MI, USA.
J Exp Bot. 2022 Jun 24;73(12):3929-3945. doi: 10.1093/jxb/erac131.
In plant cells, linkage between the cytoskeleton, plasma membrane, and cell wall is crucial for maintaining cell shape. In highly polarized pollen tubes, this coordination is especially important to allow rapid tip growth and successful fertilization. Class I formins contain cytoplasmic actin-nucleating formin homology domains as well as a proline-rich extracellular domain and are candidate coordination factors. Here, using Arabidopsis, we investigated the functional significance of the extracellular domain of two pollen-expressed class I formins: AtFH3, which does not have a polar localization, and AtFH5, which is limited to the growing tip region. We show that the extracellular domain of both is necessary for their function, and identify distinct O-glycans attached to these sequences, AtFH5 being hydroxyproline-arabinosylated and AtFH3 carrying arabinogalactan chains. Loss of hydroxyproline arabinosylation altered the plasma membrane localization of AtFH5 and disrupted actin cytoskeleton organization. Moreover, we show that O-glycans differentially affect lateral mobility in the plasma membrane. Together, our results support a model of protein sub-functionalization in which AtFH5 and AtFH3, restricted to specific plasma membrane domains by their extracellular domains and the glycans attached to them, organize distinct subarrays of actin during pollen tube elongation.
在植物细胞中，细胞骨架、质膜和细胞壁之间的连接对于维持细胞形状至关重要。在高度极化的花粉管中，这种协调对于允许快速顶端生长和成功受精尤为重要。I 类formin 包含细胞质肌动蛋白成核 formin 同源结构域以及富含脯氨酸的细胞外结构域，是候选的协调因子。在这里，我们使用拟南芥研究了两种花粉表达的 I 类 formin 的细胞外结构域的功能意义：AtFH3 没有极性定位，而 AtFH5 仅限于生长尖端区域。我们表明，两者的细胞外结构域对于它们的功能都是必需的，并鉴定了附着在这些序列上的不同 O-聚糖，AtFH5 被羟脯氨酸阿拉伯化，而 AtFH3 携带阿拉伯半乳聚糖链。羟脯氨酸阿拉伯化的丧失改变了 AtFH5 的质膜定位并破坏了肌动蛋白细胞骨架组织。此外，我们表明 O-聚糖在质膜中以不同的方式影响侧向流动性。总之，我们的结果支持蛋白质亚功能化的模型，其中 AtFH5 和 AtFH3 通过其细胞外结构域和附着的聚糖限制在特定的质膜区域，在花粉管伸长过程中组织不同的肌动蛋白亚数组。
