Translational regulation in the ribosome tunnel

我们最近一直在研究SecA的表达调控，这是一种细胞质ATPase，在大肠杆菌中通过SecYEG复合体转运蛋白质起着重要作用。编码SecA的基因位于编码SECM的基因的下游，这两个基因形成了一个操纵子。当分泌受损时，SECM C末端附近的17个氨基酸基序(150FXXXXWIXXXXGIRAGP166)会导致翻译停滞。这个基序在核糖体隧道内被识别，但识别机制尚不清楚。虽然基序中的单个突变会削弱识别能力，但我们发现，通过SECM C-末端的重塑可以产生新的停滞诱导肽。我们发现R163是必不可少的，但侧翼残基在翻译抑制中起着重要的辅助作用，这些残基在数量、位置和侧链化学上都不同。观察到单个SECM变体显示出与核糖体蛋白不同的交联模式，这表明每个肽在隧道内都采用了一种独特的构象。根据我们的结果，我们认为当侧翼残基指定的多肽构象将R163移动到准确的隧道内位置时，翻译停滞发生。我们的数据表明，翻译停滞是SECM和核糖体隧道之间广泛沟通的结果，并有助于解释在细菌、真菌和高等真核生物中发现的停滞诱导肽的惊人多样性。

We have recently been investigating the regulation of the expression of SecA, a cytoplasmic ATPase that plays a major role in the translocation of proteins through the SecYEG complex in E. coli. The gene that encodes SecA resides immediately downstream of the gene that encodes SecM, and the two genes form a single operon. When secretion is impaired, a 17 amino acid motif (150FXXXXWIXXXXGIRAGP166) near the C-terminus of SecM induces a translation arrest. This motif is recognized inside the ribosome tunnel, but the mechanism of recognition is unknown. While single mutations in the motif impair recognition, we found that novel arrest-inducing peptides can be created through remodeling of the SecM C-terminus. We found that R163 is indispensable, but that flanking residues that vary in number, position, and side chain chemistry play an important secondary role in translation arrest. The observation that individual SecM variants show a distinct pattern of crosslinking to ribosomal proteins suggests that each peptide adopts a unique conformation inside the tunnel. Based on our results, we propose that translation arrest occurs when the peptide conformation specified by flanking residues moves R163 into a precise intra-tunnel location. Our data indicate that translation arrest results from extensive communication between SecM and the ribosome tunnel and help explain the striking diversity of arrest-inducing peptides found in bacteria, fungi and higher eukaryotes.