Defining the physiological role of SRP in protein-targeting efficiency and specificity

The signal recognition particle (SRP) enables cotranslational delivery of proteins for translocation into the endoplasmic reticulum (ER), but its full in vivo role remains incompletely explored. We combined rapid auxin-induced SRP degradation with proximity-specific ribosome profiling to define SRP’s in vivo function in yeast. Despite the classic view that SRP recognizes amino-terminal signal sequences, we show that SRP was generally essential for targeting transmembrane domains regardless of their position relative to the amino-terminus. By contrast, many proteins containing cleavable amino-terminal signal peptides were efficiently cotranslationally targeted in SRP’s absence. We also reveal an unanticipated consequence of SRP loss: Transcripts normally targeted to the ER were mistargeted to mitochondria, leading to mitochondrial defects. These results elucidate SRP’s essential roles in maintaining the efficiency and specificity of protein targeting.信号识别颗粒（signal recognition particle, SRP）可介导蛋白质共翻译递送至内质网（endoplasmic reticulum, ER）进行转运，但其在体内的完整功能仍未被完全探究。我们结合快速生长素诱导的SRP降解技术与邻近特异性核糖体谱分析，以解析酵母中SRP的体内功能。尽管经典观点认为SRP可识别氨基端信号序列，但我们的研究表明，SRP普遍对于靶向跨膜结构域至关重要，无论其相对于氨基端的位置如何。相比之下，许多携带可裂解氨基端信号肽的蛋白质在SRP缺失的情况下仍可被高效共翻译靶向。我们还揭示了SRP缺失带来的意外后果：原本应靶向内质网的转录本被错误靶向至线粒体，进而引发线粒体功能缺陷。这些结果阐明了SRP在维持蛋白质靶向效率与特异性方面的核心作用。