Abstract：
In the chloroplast, the 54 kDa subunit of the signal recognition particle (cpSRP54) is involved in the posttranslational transport of the light-harvesting chlorophyll a/b-binding proteins (LHCPs) and the cotranslational transport of plastid-encoded subunits of the photosynthetic complexes to the thylakoid membrane. It forms a high-affinity complex with plastid-specific cpSRP43 for posttranslational transport, while a ribosome-associated pool coordinates its cotranslational function. CpSRP54 constitutes a conserved multidomain protein, comprising a GTPase (NG) and a methionine-rich (M) domain linked by a flexible region. It is further characterized by a plastid-specific C-terminal tail region containing the cpSRP43-binding motif. To characterize the physiological role of the various regions of cpSRP54 in thylakoid membrane protein transport, we generated Arabidopsis thaliana cpSRP54 knockout (ffc1-2) lines producing truncated cpSRP54 variants or a GTPase point mutation variant. Phenotypic characterization of the complementation lines demonstrated that the C-terminal tail region of cpSRP54 plays an important role exclusively in posttranslational LHCP transport. Furthermore, we show that the GTPase activity of cpSRP54 plays an essential role in the transport pathways for both nuclear- as well as plastid-encoded proteins. In addition, our data revealed that plants expressing cpSRP54 without the C-terminal region exhibit a strongly increased accumulation of a photosystem I assembly intermediate.
摘要：
在叶绿体中，信号识别颗粒（cpSRP54）的54kDa亚基参与捕光叶绿素a/b结合蛋白（LHCP）的翻译后转运以及光合复合物的质体编码亚基向类囊体膜的共翻译转运。它与质体特异性cpSRP43形成高亲和力复合物，用于翻译后转运，而核糖体相关池协调其共翻译功能。CpSRP54构成了一个保守的多域蛋白，包含通过柔性区连接的GTP酶(NG)和富含甲硫氨酸(M)结构域。其进一步的特征在于含有cpSRP43结合基序的质体特异性C末端尾区。为了表征cpSRP54的各个区域在类囊体膜蛋白转运中的生理作用，我们产生了拟南芥cpSRP54敲除（ffc1-2）株系，产生截短的cpSRP54变体或GTPase点突变变体。互补系的表型表征表明，cpSRP54的C末端尾部区域仅在翻译后LHCP转运中起重要作用。此外，我们表明，cpSRP54的GTP酶活性在核编码蛋白和质体编码蛋白的转运途径中起着至关重要的作用。此外,我们的数据显示，表达没有C末端区域的cpSRP54的植物表现出强烈增加的光系统I组装中间体的积累。
