It is well known that in addition to its classical role in protein turnover, ubiquitylation is required for a variety of membrane protein sorting events. However, and despite substantial progress in the field, a long-standing question remains: given that all ubiquitin units are identical, how do different elements of the sorting machinery recognize their specific cargoes? Our results indicate that the yeast Na⁺ pump Ena1 is an epsin (Ent1 and Ent2 in yeast)-specific cargo and that its internalization requires K¹⁰⁹⁰, which likely undergoes Art3-dependent ubiquitylation. In addition, an Ena1 serine and threonine (ST)-rich patch, proposed to be targeted for phosphorylation by casein kinases, was also required for its uptake. Interestingly, our data suggest that this phosphorylation was not needed for cargo ubiquitylation. Furthermore, epsin-mediated internalization of Ena1 required a specific spatial organization of the ST patch with respect to K¹⁰⁹⁰ within the cytoplasmic tail of the pump. We hypothesize that ubiquitylation and phosphorylation of Ena1 are required for epsin-mediated internalization.

众所周知，除了在蛋白质周转中的经典作用外，泛素化对于各种膜蛋白分选事件也是必需的。然而，尽管该领域取得了实质性进展，但一个长期存在的问题仍然存在：鉴于所有泛素单元都是相同的，分拣机器的不同元件如何识别其特定的货物？我们的结果表明，酵母 Na ⁺泵 Ena1 是一种 epsin（酵母中的 Ent1 和 Ent2）特异性货物，其内化需要 K ¹⁰⁹⁰，而 K 1090 可能会经历 Art3 依赖性泛素化。此外，其摄取还需要富含 Ena1 丝氨酸和苏氨酸 (ST) 的斑块，该斑块被提议作为酪蛋白激酶磷酸化的目标。有趣的是，我们的数据表明，货物泛素化不需要这种磷酸化。此外，epsin 介导的 Ena1 内化需要相对于泵的细胞质尾部内的K ^{1090的 ST 斑块的特定空间组织。}我们假设 Ena1 的泛素化和磷酸化是 epsin 介导的内化所必需的。