The assembly of light-harvesting chlorophyll-binding proteins (LHCPs) is coordinated with chlorophyll biosynthesis during chloroplast development. The ATP-independent chaperone known as chloroplast signal recognition particle 43 (cpSRP43) mediates post-translational LHCP targeting to the thylakoid membrane and also participates in tetrapyrrole biosynthesis (TBS). How these distinct actions of cpSRP43 are controlled has remained unclear. Here, we demonstrate that cpSRP43 effectively protects several TBS proteins from heat-induced aggregation and enhances their stability during leaf greening and heat shock. While the substrate-binding domain of cpSRP43 is sufficient for chaperoning LHCPs, the stabilization of TBS clients requires the chromodomain 2 of the protein. Strikingly, cpSRP54—which activates cpSRP43’s LHCP-targeted function—inhibits the chaperone activity of cpSRP43 towards TBS proteins. High temperature weakens the interaction of cpSRP54 with cpSRP43, thus freeing cpSRP43 to interact with and protect the integrity of TBS proteins. Our data indicate that the temperature sensitivity of the cpSRP43–cpSRP54 complex enables cpSRP43 to serve as an autonomous chaperone for the thermoprotection of TBS proteins.

光捕获叶绿素结合蛋白 (LHCP) 的组装与叶绿体发育过程中的叶绿素生物合成相协调。称为叶绿体信号识别粒子 43 (cpSRP43) 的 ATP 独立伴侣介导翻译后 LHCP 靶向类囊体膜，并且还参与四吡咯生物合成 (TBS)。cpSRP43 的这些不同行为是如何被控制的仍不清楚。在这里，我们证明 cpSRP43 可有效保护多种 TBS 蛋白免受热诱导聚集，并增强其在叶绿化和热休克期间的稳定性。虽然 cpSRP43 的底物结合域足以陪伴 LHCP，但 TBS 客户的稳定需要蛋白质的染色质域 2。引人注目的是，cpSRP54——激活 cpSRP43 的 LHCP 靶向功能——抑制 cpSRP43 对 TBS 蛋白的伴侣活性。高温削弱了 cpSRP54 与 cpSRP43 的相互作用，从而释放 cpSRP43 与 TBS 蛋白相互作用并保护其完整性。我们的数据表明 cpSRP43–cpSRP54 复合物的温度敏感性使 cpSRP43 能够作为 TBS 蛋白热保护的自主伴侣。