Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605-2324, USA.
Mol Biol Cell. 2011 Sep;22(17):2983-93. doi: 10.1091/mbc.E11-01-0070. Epub 2011 Jul 7.
The transition between the closed and open conformations of the Sec61 complex permits nascent protein insertion into the translocation channel. A critical event in this structural transition is the opening of the lateral translocon gate that is formed by four transmembrane (TM) spans (TM2, TM3, TM7, and TM8 in Sec61p) to expose the signal sequence-binding site. To gain mechanistic insight into lateral gate opening, mutations were introduced into a lumenal loop (L7) that connects TM7 and TM8. The sec61 L7 mutants were found to have defects in both the posttranslational and cotranslational translocation pathways due to a kinetic delay in channel gating. The translocation defect caused by L7 mutations could be suppressed by the prl class of sec61 alleles, which reduce the fidelity of signal sequence recognition. The prl mutants are proposed to act by destabilizing the closed conformation of the translocation channel. Our results indicate that the equilibrium between the open and closed conformations of the protein translocation channel maintains a balance between translocation activity and signal sequence recognition fidelity.
Sec61 复合物的关闭和开放构象之间的转变允许新生蛋白插入易位通道。这种结构转变的一个关键事件是打开由四个跨膜（TM）跨度（Sec61p 中的 TM2、TM3、TM7 和 TM8）形成的侧向转位门，以暴露信号序列结合位点。为了深入了解侧向门的开启机制，在连接 TM7 和 TM8 的腔内环（L7）中引入了突变。由于通道门控的动力学延迟，sec61 L7 突变体在后翻译和共翻译易位途径中都存在缺陷。由 L7 突变引起的易位缺陷可以被 prl 类 sec61 等位基因抑制，该等位基因降低了信号序列识别的保真度。prl 突变体被认为通过破坏易位通道的关闭构象起作用。我们的结果表明，蛋白质易位通道的开放和关闭构象之间的平衡在易位活性和信号序列识别保真度之间保持平衡。
