Kida Yuichiro, Ishihara Yudai, Fujita Hidenobu, Onishi Yukiko, Sakaguchi Masao
Graduate School of Life Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan
Graduate School of Life Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan.
Mol Biol Cell. 2016 Mar 15;27(6):930-40. doi: 10.1091/mbc.E15-09-0672. Epub 2016 Jan 28.
Many membrane proteins are integrated into the endoplasmic reticulum membrane through the protein-conducting channel, the translocon. Transmembrane segments with insufficient hydrophobicity for membrane integration are frequently found in multispanning membrane proteins, and such marginally hydrophobic (mH) segments should be accommodated, at least transiently, at the membrane. Here we investigated how mH-segments stall at the membrane and their stability. Our findings show that mH-segments can be retained at the membrane without moving into the lipid phase and that such segments flank Sec61α, the core channel of the translocon, in the translational intermediate state. The mH-segments are gradually transferred from the Sec61 channel to the lipid environment in a hydrophobicity-dependent manner, and this lateral movement may be affected by the ribosome. In addition, stalling mH-segments allow for insertion of the following transmembrane segment, forming an Ncytosol/Clumen orientation, suggesting that mH-segments can move laterally to accommodate the next transmembrane segment. These findings suggest that mH-segments may be accommodated at the ER membrane with lateral fluctuation between the Sec61 channel and the lipid phase.
许多膜蛋白通过蛋白质传导通道（转位子）整合到内质网膜中。跨膜片段若疏水性不足则无法整合入膜，这种情况在多次跨膜蛋白中很常见，这类疏水性稍弱（mH）的片段至少在膜上会有短暂停留。在此，我们研究了mH片段是如何在膜上滞留及其稳定性。我们的研究结果表明，mH片段可停留在膜上而不进入脂质相，且在翻译中间状态下，这些片段位于转位子的核心通道Sec61α两侧。mH片段以疏水性依赖的方式逐渐从Sec61通道转移至脂质环境，这种横向移动可能受核糖体影响。此外，滞留的mH片段允许后续跨膜片段插入，形成胞质溶胶/腔室的取向，这表明mH片段可横向移动以容纳下一个跨膜片段。这些研究结果表明，mH片段可能通过在Sec61通道和脂质相之间的横向波动而存在于内质网膜上。
