Otte Stefan, Barlowe Charles
Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03755, USA.
Nat Cell Biol. 2004 Dec;6(12):1189-94. doi: 10.1038/ncb1195. Epub 2004 Oct 31.
Soluble secretory proteins are first translocated across endoplasmic reticulum (ER) membranes and folded in a specialized ER luminal environment. Fully folded and assembled secretory cargo are then segregated from ER-resident proteins into COPII-derived vesicles or tubular elements for anterograde transport. Mechanisms of bulk-flow, ER-retention and receptor-mediated export have been suggested to operate during this transport step, although these mechanisms are poorly understood. In yeast, there is evidence to suggest that Erv29p functions as a transmembrane receptor for the export of certain soluble cargo proteins including glycopro-alpha-factor (gpalphaf), the precursor of alpha-factor mating pheromone. Here we identify a hydrophobic signal within the pro-region of gpalphaf that is necessary for efficient packaging into COPII vesicles and for binding to Erv29p. When fused to Kar2p, an ER-resident protein, the pro-region sorting signal was sufficient to direct Erv29p-dependent export of the fusion protein into COPII vesicles. These findings indicate that specific motifs within soluble secretory proteins function in receptor-mediated export from the ER. Moreover, positive sorting signals seem to predominate over potential ER-retention mechanisms that may operate in localizing ER-resident proteins such as Kar2p.
可溶性分泌蛋白首先穿过内质网（ER）膜进行转运，并在特殊的内质网腔环境中折叠。然后，完全折叠并组装好的分泌货物从内质网驻留蛋白中分离出来，进入源自COPII的囊泡或管状结构，进行顺向运输。尽管对这些机制了解甚少，但已有研究表明，在这一运输步骤中存在整体流动、内质网滞留和受体介导的输出等机制。在酵母中，有证据表明Erv29p作为一种跨膜受体，参与某些可溶性货物蛋白的输出，包括糖蛋白α因子（gpalphaf），即α因子交配信息素的前体。在这里，我们在gpalphaf的前体区域中鉴定出一个疏水信号，该信号对于有效包装到COPII囊泡以及与Erv29p结合是必需的。当与内质网驻留蛋白Kar2p融合时，前体区域分选信号足以指导融合蛋白在Erv29p依赖下输出到COPII囊泡中。这些发现表明，可溶性分泌蛋白中的特定基序在受体介导的内质网输出中发挥作用。此外，在定位内质网驻留蛋白（如Kar2p）时可能起作用的潜在内质网滞留机制中，正向分选信号似乎占主导地位。
