Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, MD 20742, USA.
Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, MD 20742, USA.
Curr Biol. 2019 Sep 9;29(17):2942-2947.e2. doi: 10.1016/j.cub.2019.07.022. Epub 2019 Aug 15.
Cilia are ancient organelles used by unicellular and multicellular organisms not only for motility but also to receive and respond to multiple environmental cues, including light, odorants, morphogens, growth factors, and contact with cilia of other cells. Much is known about the cellular mechanisms that deliver membrane proteins to cilia during ciliogenesis. Execution of a ciliary signaling pathway, however, can critically depend on rapid alterations in the receptor composition of the cilium itself, and our understanding of the mechanisms that underlie these rapid, regulated alterations remains limited [1-6]. In the bi-ciliated, unicellular alga Chlamydomonas reinhardtii, interactions between cilia of mating type plus and mating type minus gametes mediated by adhesion receptors SAG1 and SAD1 activate a ciliary signaling pathway [7]. In response, a large, inactive pool of SAG1 on the plasma membrane of plus gametes rapidly becomes enriched in the peri-ciliary membrane and enters the cilia to become active and maintain and enhance ciliary adhesion and signaling [8-14]. Ciliary entry per se of SAG1 is independent of anterograde intraflagellar transport (IFT) [13], but the rapid apical enrichment requires cytoplasmic microtubules and the retrograde IFT motor, dynein 1b [14]. Whether the receptors move laterally within the plasma membrane or transit internally during redistribution is unknown. Here, in coupled immunolocalization/biochemical studies on SAG1, we show that, within minutes after gamete activation is initiated, cell-surface SAG1 is internalized, associates with an apico-basally polarized array of cytoplasmic microtubules, and returns to the cell surface at a peri-ciliary staging area for entry into cilia.
纤毛是单细胞和多细胞生物用来运动的古老细胞器，但也用来接收和响应多种环境线索，包括光、气味分子、形态发生素、生长因子以及与其他细胞纤毛的接触。在纤毛发生过程中，将膜蛋白递送到纤毛的细胞机制已经有很多了解。然而，纤毛信号通路的执行可以严重依赖于纤毛自身受体组成的快速改变，我们对这些快速、调节改变的机制的理解仍然有限[1-6]。在双纤毛、单细胞绿藻衣藻中，由黏附受体 SAG1 和 SAD1 介导的交配型+和交配型-配子之间的纤毛相互作用激活了一个纤毛信号通路[7]。作为回应，+配子质膜上大量无活性的 SAG1 迅速在纤毛旁膜中富集，并进入纤毛，变得活跃，维持和增强纤毛黏附和信号传递[8-14]。SAG1 的纤毛内进入本身独立于正向纤毛内运输（IFT）[13]，但快速的顶端富集需要细胞质微管和逆行 IFT 马达，动力蛋白 1b[14]。受体在质膜内是否侧向移动或在重新分布过程中是否内部转运尚不清楚。在这里，在 SAG1 的偶联免疫定位/生化研究中，我们表明，在配子激活开始后的几分钟内，细胞表面的 SAG1 被内化，与一个顶端到基底极化的细胞质微管阵列相关联，并返回质膜上的一个纤毛旁分期区域，以便进入纤毛。
