Henderson Rory C, Gao Feng, Jayanthi Srinivas, Kight Alicia, Sharma Priyanka, Goforth Robyn L, Heyes Colin D, Henry Ralph L, Suresh Kumar Thallapuranam Krishnaswamy
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas.
Department of Biological Sciences, University of Arkansas, Fayetteville, Arkansas.
Biophys J. 2016 Sep 20;111(6):1151-1162. doi: 10.1016/j.bpj.2016.08.004.
Chloroplast signal recognition particle (cpSRP) is a heterodimer composed of an evolutionarily conserved 54-kDa GTPase (cpSRP54) and a unique 43-kDa subunit (cpSRP43) responsible for delivering light-harvesting chlorophyll binding protein to the thylakoid membrane. While a nearly complete three-dimensional structure of cpSRP43 has been determined, no high-resolution structure is yet available for cpSRP54. In this study, we developed and examined an in silico three-dimensional model of the structure of cpSRP54 by homology modeling using cytosolic homologs. Model selection was guided by single-molecule Förster resonance energy transfer experiments, which revealed the presence of at least two distinct conformations. Small angle x-ray scattering showed that the linking region among the GTPase (G-domain) and methionine-rich (M-domain) domains, an M-domain loop, and the cpSRP43 binding C-terminal extension of cpSRP54 are predominantly disordered. Interestingly, the linker and loop segments were observed to play an important role in organizing the domain arrangement of cpSRP54. Further, deletion of the finger loop abolished loading of the cpSRP cargo, light-harvesting chlorophyll binding protein. These data highlight important structural dynamics relevant to cpSRP54's role in the post- and cotranslational signaling processes.
叶绿体信号识别颗粒（cpSRP）是一种异源二聚体，由一个进化上保守的54 kDa GTP酶（cpSRP54）和一个独特的43 kDa亚基（cpSRP43）组成，负责将捕光叶绿素结合蛋白输送到类囊体膜。虽然cpSRP43的三维结构已基本确定，但cpSRP54的高分辨率结构尚未获得。在本研究中，我们通过使用胞质同源物进行同源建模，开发并检验了cpSRP54结构的计算机三维模型。模型选择以单分子Förster共振能量转移实验为指导，该实验揭示了至少两种不同构象的存在。小角X射线散射表明，GTP酶（G结构域）和富含甲硫氨酸（M结构域）结构域之间的连接区域、一个M结构域环以及cpSRP54与cpSRP43结合的C末端延伸主要是无序的。有趣的是，观察到连接子和环段在组织cpSRP54的结构域排列中起重要作用。此外，指状环的缺失消除了cpSRP货物（捕光叶绿素结合蛋白）的装载。这些数据突出了与cpSRP54在翻译后和共翻译信号传导过程中的作用相关的重要结构动力学。
