Medical Research Council Laboratory of Molecular Biology, Cambridge, UK.
J Cell Biol. 2020 Jul 6;219(7). doi: 10.1083/jcb.201806038.
Accurate maintenance of organelle identity in the secretory pathway relies on retention and retrieval of resident proteins. In the endoplasmic reticulum (ER), secretory proteins are packaged into COPII vesicles that largely exclude ER residents and misfolded proteins by mechanisms that remain unresolved. Here we combined biochemistry and genetics with correlative light and electron microscopy (CLEM) to explore how selectivity is achieved. Our data suggest that vesicle occupancy contributes to ER retention: in the absence of abundant cargo, nonspecific bulk flow increases. We demonstrate that ER leakage is influenced by vesicle size and cargo occupancy: overexpressing an inert cargo protein or reducing vesicle size restores sorting stringency. We propose that cargo recruitment into vesicles creates a crowded lumen that drives selectivity. Retention of ER residents thus derives in part from the biophysical process of cargo enrichment into a constrained spherical membrane-bound carrier.
准确维持分泌途径中的细胞器身份依赖于驻留蛋白的保留和回收。在内质网（ER）中，分泌蛋白被包装到 COPII 小泡中，这些小泡通过仍未解决的机制，主要排除 ER 驻留蛋白和错误折叠的蛋白质。在这里，我们将生物化学和遗传学与相关的光和电子显微镜（CLEM）相结合，探索了如何实现选择性。我们的数据表明，囊泡占据有助于内质网保留：在没有大量货物的情况下，非特异性的批量流动增加。我们证明 ER 泄漏受囊泡大小和货物占据的影响：过表达惰性货物蛋白或减小囊泡大小会恢复分拣严格性。我们提出，货物被招募到囊泡中会形成一个拥挤的腔室，从而产生选择性。因此，ER 驻留蛋白的保留部分源于货物富集到受限的球形膜结合载体中的物理过程。
