Sensing of individual stalled 80S ribosomes by Fap1 for non-functional rRNA turnover. Li et al

Cells can respond to stalled ribosomes by sensing ribosome collisions and employing quality control pathways. How ribosome stalling is resolved without collisions, however, has remained elusive. Here, focusing on non-colliding stalling exhibited by decoding-defective ribosomes, we identified Fap1 as a stalling sensor triggering 18S non-functional rRNA decay via poly-ubiquitination of uS3. Ribosome profiling revealed an enrichment of Fap1 at the translation initiation site but also association with elongating individual ribosomes. Cryo-EM structures of Fap1-bound ribosomes elucidated Fap1 probing the mRNA simultaneously at both the entry and exit channels suggesting a mRNA stasis sensing activity, and Fap1 sterically hinders formation of canonical collided di-ribosomes. Our findings indicate that individual stalled ribosomes are the potential signal for ribosome dysfunction, leading to accelerated turnover of the ribosome itself.细胞可通过感知核糖体碰撞并启动质量控制通路，对停滞核糖体作出应答。然而，不依赖碰撞的核糖体停滞是如何被解决的，这一问题迄今仍不明晰。本研究聚焦解码缺陷核糖体所呈现的非碰撞性停滞现象，鉴定出Fap1作为停滞传感器，可通过对核糖体蛋白uS3进行多泛素化修饰，触发18S无功能rRNA降解通路。核糖体谱分析结果显示，Fap1不仅在翻译起始位点富集，还可与单个正在延伸的核糖体结合。Fap1结合核糖体的冷冻电镜（Cryo-EM）结构解析表明，Fap1可同时在核糖体的mRNA进入与离开通道处探查mRNA，提示其具备mRNA停滞感知活性；同时Fap1可通过空间位阻阻碍典型碰撞双核糖体的形成。本研究结果证实，单个停滞核糖体可作为核糖体功能异常的潜在信号，进而加速核糖体自身的周转更新。