Chloroplast-encoded multi-span thylakoid membrane proteins are crucial for photosynthetic complexes, yet the coordination of their biogenesis remains poorly understood. To identify factors that specifically support the cotranslational biogenesis of the reaction center protein D1 of photosystem (PS) II, we generated and affinity-purified stalled ribosome-nascent chain complexes (RNCs) bearing D1 nascent chains. Stalled RNCs translating the soluble ribosomal subunit uS2c were used for comparison. Quantitative tandem-mass spectrometry of the purified RNCs identified around 140 proteins specifically associated with D1 RNCs, mainly involved in protein and cofactor biogenesis, including chlorophyll biosynthesis, and other metabolic pathways. Functional analysis of STIC2, a newly identified D1 RNC interactor, revealed its cooperation with chloroplast protein SRP54 in the de novo biogenesis and repair of D1, and potentially other cotranslationally-targeted reaction center subunits of PSII and PSI. The primary binding interface between STIC2 and the thylakoid insertase Alb3 and its homolog Alb4 was mapped to STIC2's β-sheet region, and the conserved Motif III in the C-terminal regions of Alb3/4.

中文翻译：

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STIC2 在拟南芥类囊体蛋白的共翻译分选中选择性结合核糖体新生链复合物。


叶绿体编码的多跨度类囊体膜蛋白对于光合复合物至关重要，但对其生物发生的协调仍然知之甚少。为了确定专门支持光系统 （PS） II 反应中心蛋白 D1 的共翻译生物发生的因子，我们生成并亲和纯化了带有 D1 新生链的停滞核糖体-新生链复合物 （RNC）。翻译可溶性核糖体亚基 uS2c 的停滞 RNC 用于比较。纯化 RNC 的定量串联质谱法鉴定了大约 140 种与 D1 RNC 特异性相关的蛋白质，主要参与蛋白质和辅因子生物发生，包括叶绿素生物合成和其他代谢途径。STIC2 是一种新发现的 D1 RNC 相互作用子，其功能分析揭示了它与叶绿体蛋白 SRP54 在 D1 的从头生物发生和修复中的合作，以及 PSII 和 PSI 的潜在其他共翻译靶向反应中心亚基。STIC2 与类囊体插入酶 Alb3 及其同源物 Alb4 之间的初级结合界面被定位到 STIC2 的 β 折叠区域，而保守的基序 III 位于 Alb3/4 的 C 端区域。