Department of Biology, Norwegian University of Science and Technology, Trondheim N-7491, Norway.
Department of Fisheries and New Biomarine Industry, SINTEF Ocean, Trondheim 7010, Norway.
Plant Cell Physiol. 2023 Jun 14;64(6):583-603. doi: 10.1093/pcp/pcad014.
The chloroplast signal recognition particle (CpSRP) receptor (CpFTSY) is a component of the CpSRP pathway that post-translationally targets light-harvesting complex proteins (LHCPs) to the thylakoid membranes in plants and green algae containing chloroplasts derived from primary endosymbiosis. In plants, CpFTSY also plays a major role in the co-translational incorporation of chloroplast-encoded subunits of photosynthetic complexes into the thylakoids. This role has not been demonstrated in green algae. So far, its function in organisms with chloroplasts derived from secondary endosymbiotic events has not been elucidated. Here, we report the generation and characterization of mutants lacking CpFTSY in the diatom Phaeodactylum tricornutum. We found that this protein is not involved in inserting LHCPs into thylakoid membranes, indicating that the post-translational part of the CpSRP pathway is not active in this group of microalgae. The lack of CpFTSY caused an increased level of photoprotection, low electron transport rates, inefficient repair of photosystem II (PSII), reduced growth, a strong decline in the PSI subunit PsaC and upregulation of proteins that might compensate for a non-functional co-translational CpSRP pathway during light stress conditions. The phenotype was highly similar to the one described for diatoms lacking another component of the co-translational CpSRP pathway, the CpSRP54 protein. However, in contrast to cpsrp54 mutants, only one thylakoid membrane protein, PetD of the Cytb6f complex, was downregulated in cpftsy. Our results point to a minor role for CpFTSY in the co-translational CpSRP pathway, suggesting that other mechanisms may partially compensate for the effect of a disrupted CpSRP pathway.
叶绿体信号识别颗粒（CpSRP）受体（CpFTSY）是 CpSRP 途径的一个组成部分，该途径在后翻译水平上将捕光复合物蛋白（LHCPs）靶向到植物和含有叶绿体的绿藻的类囊体膜中，这些叶绿体源自最初的内共生事件。在植物中，CpFTSY 还在叶绿体编码的光合复合物亚基的共翻译整合到类囊体中发挥主要作用。这一作用在绿藻中尚未得到证明。到目前为止，其在源自二次内共生事件的叶绿体生物中的功能尚未阐明。在这里，我们报告了在硅藻三角褐指藻中缺乏 CpFTSY 的突变体的产生和特征。我们发现，该蛋白不参与将 LHCPs 插入类囊体膜中，这表明 CpSRP 途径的后翻译部分在这组微藻中没有活性。缺乏 CpFTSY 导致光保护水平增加、电子传递速率降低、光系统 II（PSII）修复效率降低、生长受阻、PSI 亚基 PsaC 水平大幅下降以及可能补偿非功能性共翻译 CpSRP 途径的蛋白质上调。这种表型与缺乏共翻译 CpSRP 途径另一个组成部分 CpSRP54 蛋白的硅藻非常相似。然而，与 cpsrp54 突变体不同的是，只有一个类囊体膜蛋白，Cytb6f 复合物的 PetD，在 cpftsy 中下调。我们的结果表明 CpFTSY 在共翻译 CpSRP 途径中作用较小，表明其他机制可能部分补偿了 CpSRP 途径中断的影响。
