When ribosomes fail to complete normal translation, all cells have mechanisms to ensure degradation of the resulting partial proteins to safeguard proteome integrity. In E. coli and other eubacteria, the tmRNA system rescues stalled ribosomes and adds an ssrA tag or degron to the C-terminus of the incomplete protein, which directs degradation by the AAA+ ClpXP protease. Here, we present cryo-EM structures of ClpXP bound to the ssrA degron. C-terminal residues of the ssrA degron initially bind in the top of an otherwise closed ClpX axial channel and subsequently move deeper into an open channel. For short-degron protein substrates, we show that unfolding can occur directly from the initial closed-channel complex. For longer-degron substrates, our studies illuminate how ClpXP transitions from specific recognition into a nonspecific unfolding and translocation machine. Many AAA+ proteases and protein-remodeling motors are likely to employ similar multistep recognition and engagement strategies.

中文翻译：

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ClpXP识别和ssrA标签底物展开的结构基础

当核糖体无法完成正常翻译时，所有细胞都有确保所产生的部分蛋白质降解以保护蛋白质组完整性的机制。在大肠杆菌和其他真细菌中，tmRNA系统可以拯救失速的核糖体，并在不完全蛋白质的C末端添加ssrA标签或degron，从而指导AAA + ClpXP蛋白酶的降解。在这里，我们介绍绑定到ssrA degron的ClpXP的低温EMC结构。ssrA degron的C末端残基最初结合在原本封闭的ClpX轴向通道的顶部，然后更深地进入开放通道。对于短德格隆蛋白底物，我们表明展开可以直接从最初的封闭通道复合体发生。对于更长德龙的基材，我们的研究阐明了ClpXP如何从特定的识别过渡到非特定的展开和移位机器。许多AAA +蛋白酶和蛋白质重塑马达可能会采用类似的多步识别和参与策略。