Program of Life and Environmental Sciences, Graduate School of Integral Science for Life, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8521, Japan.
Mol Biol Cell. 2019 Nov 1;30(23):2890-2900. doi: 10.1091/mbc.E19-08-0434. Epub 2019 Sep 25.
Most membrane proteins are synthesized on and inserted into the membrane of the endoplasmic reticulum (ER), in eukaryote. The widely conserved ER membrane protein complex (EMC) facilitates the biogenesis of a wide range of membrane proteins. In this study, we investigated the EMC function using photoreceptor as a model system. We found that the EMC was necessary only for the biogenesis of a subset of multipass membrane proteins such as rhodopsin (Rh1), TRP, TRPL, Csat, Cni, SERCA, and NaKATPase α, but not for that of secretory or single-pass membrane proteins. Additionally, in EMC-deficient cells, Rh1 was translated to its C terminus but degraded independently from ER-associated degradation. Thus, EMC exerted its effect after translation but before or during the membrane integration of transmembrane domains (TMDs). Finally, we found that EMC was not required for the stable expression of the first three TMDs of Rh1 but was required for that of the fourth and fifth TMDs. Our results suggested that EMC is required for the ER membrane insertion of succeeding TMDs of multipass membrane proteins.
大多数膜蛋白都是在真核生物的内质网膜（ER）上合成并插入其中的。广泛保守的 ER 膜蛋白复合物（EMC）有助于多种膜蛋白的生物发生。在这项研究中，我们使用感光器作为模型系统来研究 EMC 的功能。我们发现，EMC 仅对于一组多跨膜蛋白（如视紫红质（Rh1）、TRP、TRPL、Csat、Cni、SERCA 和 NaKATPaseα）的生物发生是必需的，但对于分泌或单次跨膜蛋白则不是必需的。此外，在 EMC 缺陷细胞中，Rh1 被翻译到其 C 末端，但独立于 ER 相关降解而降解。因此，EMC 在翻译后但在跨膜结构域（TMD）的膜整合之前或期间发挥作用。最后，我们发现 EMC 对于 Rh1 的前三个 TMD 的稳定表达不是必需的，但对于第四个和第五个 TMD 的稳定表达是必需的。我们的结果表明，EMC 对于多跨膜蛋白的后续 TMD 的 ER 膜插入是必需的。
