A novel signal recognition particle targets light-harvesting proteins to the thylakoid membranes

The mechanisms involved in the posttranslational targeting of membrane proteins are not well understood. The light-harvesting chlorophyll proteins (LHCP) of the thylakoid membrane are a large family of hydrophobic proteins that are targeted in this manner. They are synthesized in the cytoplasm, translocated across the chloroplast envelope membranes into the stroma, bound by a stromal factor to form a soluble intermediate, “transit complex”, and then integrated into the thylakoid membrane by a GTP dependent reaction. Signal recognition particle (SRP), a cytoplasmic ribonucleoprotein, is known to mediate the GTP dependent cotranslational targeting of proteins to the endoplasmic reticulum. We show that chloroplasts contain an SRP consisting of, cpSRP54, a homologue of SRP54 and a previously undescribed 43-kDa polypeptide (cpSRP43) instead of an RNA. We demonstrate that both subunits of cpSRP are required for the formation of the transit complex with LHCP. Furthermore, cpSRP54, cpSRP43, and LHCP are sufficient to form a complex that appears to be identical to authentic transit complex. We also show that the complex formed between LHCP and cpSRP, together with an additional soluble factor(s) are required for the proper integration of LHCP into the thylakoid membrane. It appears that the expanded role of cpSRP in posttranslational targeting of LHCP has arisen through the evolution of the 43-kDa protein.

膜蛋白翻译后靶向所涉及的分子机制目前尚未被充分阐明。叶绿体类囊体膜的捕光叶绿素蛋白（light-harvesting chlorophyll proteins, LHCP）是一类通过该途径靶向的疏水蛋白大家族。这类蛋白在细胞质中合成，随后穿越叶绿体被膜转运至叶绿体基质，结合基质因子形成可溶性中间产物——“转运复合物（transit complex）”，最终通过GTP依赖的反应整合进入类囊体膜。信号识别颗粒（Signal recognition particle, SRP）是一种细胞质核糖核蛋白，已知其可介导蛋白质向内质网（endoplasmic reticulum）的GTP依赖共翻译靶向过程。本研究证实，叶绿体中存在一类SRP，其组成成分为SRP54的同源物cpSRP54，以及此前未被报道的43 kDa多肽（cpSRP43），而非RNA分子。我们证明，cpSRP的两个亚基均为与LHCP形成转运复合物所必需的组分。进一步研究表明，仅cpSRP54、cpSRP43与LHCP三者即可组装出与天然转运复合物完全一致的复合体。此外，我们还发现，LHCP与cpSRP形成的复合物，再结合额外的可溶性因子，是LHCP正确整合进入类囊体膜的必要条件。由此可见，cpSRP在LHCP翻译后靶向过程中拓展的功能，是通过43 kDa蛋白的演化而获得的。