Chartron Justin W, Hunt Katherine C L, Frydman Judith
Nature. 2016 Aug 11;536(7615):224-8. doi: 10.1038/nature19309. Epub 2016 Aug 3.
Ribosome-associated factors must properly decode the limited information available in nascent polypeptides to direct them to their correct cellular fate. It is unclear how the low complexity information exposed by the nascent chain suffices for accurate recognition by the many factors competing for the limited surface near the ribosomal exit site. Questions remain even for the well-studied cotranslational targeting cycle to the endoplasmic reticulum, involving recognition of linear hydrophobic signal sequences or transmembrane domains by the signal recognition particle (SRP). Notably, the SRP has low abundance relative to the large number of ribosome-nascent-chain complexes (RNCs), yet it accurately selects those destined for the endoplasmic reticulum. Despite their overlapping specificities, the SRP and the cotranslationally acting Hsp70 display precise mutually exclusive selectivity in vivo for their cognate RNCs. To understand cotranslational nascent chain recognition in vivo, here we investigate the cotranslational membrane-targeting cycle using ribosome profiling in yeast cells coupled with biochemical fractionation of ribosome populations. We show that the SRP preferentially binds secretory RNCs before their targeting signals are translated. Non-coding mRNA elements can promote this signal-independent pre-recruitment of SRP. Our study defines the complex kinetic interaction between elongation in the cytosol and determinants in the polypeptide and mRNA that modulate SRP–substrate selection and membrane targeting.
核糖体相关因子必须正确解读新生多肽中有限的信息，以引导它们走向正确的细胞命运。目前尚不清楚新生链暴露的低复杂性信息如何足以被众多竞争核糖体出口位点附近有限表面的因子准确识别。即使对于研究充分的共翻译靶向内质网的循环过程，即信号识别颗粒（SRP）识别线性疏水信号序列或跨膜结构域的过程，也仍然存在问题。值得注意的是，相对于大量的核糖体-新生链复合物（RNC）而言，SRP的丰度较低，但它却能准确地选择那些注定要进入内质网的复合物。尽管SRP和共翻译作用的Hsp70具有重叠的特异性，但它们在体内对其同源RNC显示出精确的相互排斥的选择性。为了了解体内共翻译新生链的识别过程，我们在此利用酵母细胞中的核糖体分析技术结合核糖体群体的生化分级分离，研究共翻译膜靶向循环。我们发现，SRP在其靶向信号被翻译之前优先结合分泌型RNC。非编码mRNA元件可以促进SRP这种不依赖信号的预招募。我们的研究确定了细胞质中延伸过程与多肽和mRNA中的决定因素之间复杂的动力学相互作用，这些决定因素调节SRP-底物的选择和膜靶向。
