Yamamoto-Negi Yoshiko, Higa Takeshi, Komatsu Aino, Sasaki Kanta, Ishizaki Kimitsune, Nishihama Ryuichi, Gotoh Eiji, Kohchi Takayuki, Suetsugu Noriyuki
Faculty of Agriculture, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395 Japan.
Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo, 153-8902 Japan.
Plant Cell Physiol. 2024 Dec 6;65(11):1787-1800. doi: 10.1093/pcp/pcae101.
Chloroplasts accumulate on the cell surface under weak light conditions to efficiently capture light but avoid strong light to minimize photodamage. The blue light receptor phototropin regulates the chloroplast movement in various plant species. In Arabidopsis thaliana, phototropin mediates the light-induced chloroplast movement and positioning via specialized actin filaments on the chloroplasts, chloroplast-actin filaments. KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT (KAC) and CHLOROPLAST UNUSUAL POSITIONING 1 (CHUP1) are pivotal for actin-based chloroplast movement and positioning in land plants. However, the mechanisms by which KAC and CHUP1 regulate chloroplast movement and positioning remain unclear. In this study, we characterized KAC and CHUP1 orthologs in the liverwort Marchantia polymorpha, MpKAC and MpCHUP1, respectively. Their knockout mutants, Mpkacko and Mpchup1ko, impaired the light-induced chloroplast movement. Although Mpchup1ko showed mild chloroplast aggregation, Mpkacko displayed severe chloroplast aggregation, suggesting the greater contribution of MpKAC to the chloroplast anchorage to the plasma membrane. Analysis of the subcellular localization of the functional MpKAC-Citrine indicated that MpKAC-Citrine formed a punctate structure on the plasma membrane. Structure-function analysis of MpKAC revealed that the deletion of the conserved C-terminal domain abrogates its targeting to the plasma membrane and its function. The deletion of the N-terminal motor domain retains the plasma membrane targeting but abrogates the formation of punctate structure and shows a severe defect in the light-induced chloroplast movement. Our findings suggest that the formation of the punctate structure on the plasma membrane of MpKAC is essential for chloroplast movement.
在弱光条件下，叶绿体聚集在细胞表面以有效捕获光线，但避免强光以将光损伤降至最低。蓝光受体向光素调节多种植物物种中的叶绿体运动。在拟南芥中，向光素通过叶绿体上的特殊肌动蛋白丝（叶绿体 - 肌动蛋白丝）介导光诱导的叶绿体运动和定位。基于肌动蛋白的叶绿体运动和定位蛋白（KAC）和叶绿体异常定位1（CHUP1）对于陆地植物中基于肌动蛋白的叶绿体运动和定位至关重要。然而，KAC和CHUP1调节叶绿体运动和定位的机制仍不清楚。在本研究中，我们分别鉴定了地钱多歧藻中的KAC和CHUP1直系同源物，即MpKAC和MpCHUP1。它们的敲除突变体Mpkacko和Mpchup1ko损害了光诱导的叶绿体运动。虽然Mpchup1ko表现出轻度的叶绿体聚集，但Mpkacko表现出严重的叶绿体聚集，这表明MpKAC对叶绿体锚定到质膜的贡献更大。对功能性MpKAC - 柠檬黄的亚细胞定位分析表明，MpKAC - 柠檬黄在质膜上形成点状结构。MpKAC的结构 - 功能分析表明，保守C末端结构域的缺失消除了其靶向质膜的能力及其功能。N末端运动结构域的缺失保留了质膜靶向，但消除了点状结构的形成，并在光诱导的叶绿体运动中表现出严重缺陷。我们的研究结果表明，MpKAC质膜上点状结构的形成对于叶绿体运动至关重要。
