Lipid mediated–phase separation of AGO proteins on ER controls nascent peptide ubiquitination

AGO/miRNA-mediated gene silencing and ubiquitin-mediated protein quality control represent two fundamental mechanisms that control proper gene expression. Here we unexpectedly discover that fly and human AGO proteins, key components in the miRNA pathway, undergo lipid-mediated phase separation and condense into RNP-granules on the endoplasmic reticulum (ER) membrane to control protein production. Phase separation on the ER is mediated by electrostatic interactions between a conserved lipid-binding motif within the AGOs and the lipid PI(4,5)P2. The ER-localized AGO condensates recruit the E3 ubiquitin ligase Ltn1 to catalyze nascent peptide ubiquitination and coordinate with the VCP-Ufd1-Npl4 complex to process unwanted protein products for proteasomal degradation. Collectively, our study provides insight into the understanding of post-transcription-translation coupling controlled by AGOs via lipid-mediated phase separation.

AGO/miRNA介导的基因沉默（AGO/miRNA-mediated gene silencing）与泛素介导的蛋白质质量控制（ubiquitin-mediated protein quality control）是调控基因正确表达的两大基础机制。本研究意外发现，果蝇与人类的AGO蛋白——miRNA通路中的关键组分——可通过脂介导的相分离（lipid-mediated phase separation）过程，在内质网（endoplasmic reticulum, ER）膜上凝聚形成核糖核蛋白（RNP）颗粒，进而调控蛋白质合成。内质网膜上的相分离由AGO蛋白保守的脂结合基序与脂质PI(4,5)P₂之间的静电相互作用所介导。定位于内质网的AGO凝聚体可招募E3泛素连接酶Ltn1，以催化新生肽链的泛素化，并协同VCP-Ufd1-Npl4复合物处理冗余蛋白产物，使其经蛋白酶体降解。综上，本研究为理解AGO蛋白通过脂介导的相分离调控转录后-翻译偶联的过程提供了新的见解。