Gene Center and Department of Biochemistry, University of Munich, Munich, Germany.
PLoS Biol. 2011 Jan 18;9(1):e1000581. doi: 10.1371/journal.pbio.1000581.
As nascent polypeptide chains are synthesized, they pass through a tunnel in the large ribosomal subunit. Interaction between specific nascent chains and the ribosomal tunnel is used to induce translational stalling for the regulation of gene expression. One well-characterized example is the Escherichia coli SecM (secretion monitor) gene product, which induces stalling to up-regulate translation initiation of the downstream secA gene, which is needed for protein export. Although many of the key components of SecM and the ribosomal tunnel have been identified, understanding of the mechanism by which the peptidyl transferase center of the ribosome is inactivated has been lacking. Here we present a cryo-electron microscopy reconstruction of a SecM-stalled ribosome nascent chain complex at 5.6 Å. While no cascade of rRNA conformational changes is evident, this structure reveals the direct interaction between critical residues of SecM and the ribosomal tunnel. Moreover, a shift in the position of the tRNA-nascent peptide linkage of the SecM-tRNA provides a rationale for peptidyl transferase center silencing, conditional on the simultaneous presence of a Pro-tRNA(Pro) in the ribosomal A-site. These results suggest a distinct allosteric mechanism of regulating translational elongation by the SecM stalling peptide.
新生多肽链在合成时，会穿过大亚基中的一个隧道。特定的新生链与核糖体隧道之间的相互作用可诱导翻译暂停，从而调节基因表达。大肠杆菌 SecM（分泌监控）基因产物就是一个很好的例子，它诱导翻译暂停，从而上调下游 secA 基因的翻译起始，该基因对于蛋白质输出是必需的。尽管已经鉴定出 SecM 和核糖体隧道的许多关键成分，但核糖体肽基转移酶中心失活的机制仍不清楚。在这里，我们以 5.6 Å 的分辨率呈现了一个 SecM 诱导的核糖体新生链复合物的冷冻电镜重建结构。虽然没有明显的 rRNA 构象变化级联，但这个结构揭示了 SecM 和核糖体隧道之间的直接相互作用。此外，SecM-tRNA 上的 tRNA-新生肽连接的位置发生了移动，为肽基转移酶中心沉默提供了依据，其条件是核糖体 A 位同时存在 Pro-tRNA(Pro)。这些结果表明，SecM 诱导的翻译暂停肽通过一种独特的变构机制来调节翻译延伸。
