Chloroplast-mediated regulation of CO2-concentrating mechanism by Ca2+-binding protein CAS in the green alga Chlamydomonas reinhardtii
Chloroplast-mediated regulation of CO2-concentrating mechanism by Ca2+-binding protein CAS in the green alga Chlamydomonas reinhardtii
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DOI：
10.1073/pnas.1606519113
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发表时间：
2016-10
期刊：
影响因子：
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通讯作者：
Lian-li Wang;T. Yamano;Shunsuke Takane;Yuki Niikawa;Chihana Toyokawa;Shin-ichiro Ozawa;Ryutaro Tokutsu;Yuichiro Takahashi;J. Minagawa;Y. Kanesaki;H. Yoshikawa;H. Fukuzawa
中科院分区：
文献类型：
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作者：
Lian-li Wang;T. Yamano;Shunsuke Takane;Yuki Niikawa;Chihana Toyokawa;Shin-ichiro Ozawa;Ryutaro Tokutsu;Yuichiro Takahashi;J. Minagawa;Y. Kanesaki;H. Yoshikawa;H. Fukuzawa
Ca2+和CO2是微生物、动物和植物的基本生物信号分子。虽然Ca2+被认为是陆生植物保护细胞中CO2信号转导的第二信使，但在水生光合生物中Ca2+在CO2信号转导途径中的作用仍然很大程度上未知。我们在这里展示了叶绿体Ca2+结合蛋白CAS在环境CO2条件下改变其定位，并调节核编码的限制CO2诱导基因的表达，包括两个关键的碳酸氢盐转运蛋白。这些发现使我们提出了Ca2+信号参与水生光合生物叶绿体调节的CO2信号转导的模型，并帮助我们进一步了解Ca2+在真核生物CO2信号转导中的作用。包括莱茵衣藻（Chlamydomonas reinhardtii）在内的水生光合生物通过感知环境CO2和光的可用性，诱导CO2浓缩机制（CCM）来维持CO2限制条件下的光合活性。此前，一种新的高二氧化碳需求突变体H82在诱导CCM方面存在缺陷，被分离出来。最初在拟南芥中发现的钙（Ca2+）结合蛋白CAS的同源物在H82细胞中被破坏。虽然有报道称拟南芥CAS通过叶绿体介导的逆行信号与气孔关闭或免疫应答有关，但在水生环境中，Ca2+信号与CAS功能相关的CCM之间的关系尚不清楚。在本研究中，将完整的CAS基因导入H82细胞，恢复了对无机碳的光合亲和力，RNA-seq分析显示，CAS可以维持核编码的二氧化碳限制诱导基因的表达水平，包括HCO3 -转运体高光激活3 （HLA3）和低二氧化碳诱导基因A （LCIA）。CAS的定位从在高co2条件下或黑暗中分散在类囊体膜上转变为在co2限制条件下与类pyrenoid中的小管状结构相关，同时类pyrenoid中Ca2+指示剂的荧光信号显著增加。衣藻CAS具有Ca2+结合活性，Ca2+螯合剂或钙调素拮抗剂对细胞内Ca2+稳态的扰动破坏了HLA3和LCIA的积累。这些结果表明，衣藻CAS是一种Ca2+介导的ccm相关基因的调节剂，通过叶绿体中的类核蛋白逆行信号到达细胞核。
Significance Ca2+ and CO2 are fundamental biological signaling molecules in microbes, animals, and plants. Although Ca2+ was proposed to act as a second messenger in CO2 signaling in guard cells of terrestrial plants, the role of Ca2+ in CO2 signal transduction pathways in aquatic photosynthetic organisms remains largely unknown. We show here that a chloroplast Ca2+-binding protein, CAS, changes its localization in response to environmental CO2 conditions and regulates the expression of nuclear-encoded limiting-CO2–induced genes, including two key bicarbonate transporters. These findings led us to propose a model for the participation of Ca2+ signals in chloroplast-regulated CO2 signal transduction of aquatic photosynthetic organisms and help us to further understand the role of Ca2+ in CO2 signal transduction in eukaryotes. Aquatic photosynthetic organisms, including the green alga Chlamydomonas reinhardtii, induce a CO2-concentrating mechanism (CCM) to maintain photosynthetic activity in CO2-limiting conditions by sensing environmental CO2 and light availability. Previously, a novel high-CO2–requiring mutant, H82, defective in the induction of the CCM, was isolated. A homolog of calcium (Ca2+)-binding protein CAS, originally found in Arabidopsis thaliana, was disrupted in H82 cells. Although Arabidopsis CAS is reported to be associated with stomatal closure or immune responses via a chloroplast-mediated retrograde signal, the relationship between a Ca2+ signal and the CCM associated with the function of CAS in an aquatic environment is still unclear. In this study, the introduction of an intact CAS gene into H82 cells restored photosynthetic affinity for inorganic carbon, and RNA-seq analyses revealed that CAS could function in maintaining the expression levels of nuclear-encoded CO2-limiting–inducible genes, including the HCO3– transporters high-light activated 3 (HLA3) and low-CO2–inducible gene A (LCIA). CAS changed its localization from dispersed across the thylakoid membrane in high-CO2 conditions or in the dark to being associated with tubule-like structures in the pyrenoid in CO2-limiting conditions, along with a significant increase of the fluorescent signals of the Ca2+ indicator in the pyrenoid. Chlamydomonas CAS had Ca2+-binding activity, and the perturbation of intracellular Ca2+ homeostasis by a Ca2+-chelator or calmodulin antagonist impaired the accumulation of HLA3 and LCIA. These results suggest that Chlamydomonas CAS is a Ca2+-mediated regulator of CCM-related genes via a retrograde signal from the pyrenoid in the chloroplast to the nucleus.