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  • Title: Diverse pore loops of the AAA+ ClpX machine mediate unassisted and adaptor-dependent recognition of ssrA-tagged substrates.
    Author: Martin A, Baker TA, Sauer RT.
    Journal: Mol Cell; 2008 Feb 29; 29(4):441-50. PubMed ID: 18313382.
    Abstract:
    ClpX, an archetypal proteolytic AAA+ unfoldase, must engage the ssrA tags of appropriate substrates prior to ATP-dependent unfolding and translocation of the denatured polypeptide into ClpP for degradation. Here, specificity-transplant and disulfide-crosslinking experiments reveal that the ssrA tag interacts with different loops that form the top, middle, and lower portions of the central channel of the ClpX hexamer. Our results support a two-step binding mechanism, in which the top loop serves as a specificity filter and the remaining loops form a binding site for the peptide tag relatively deep within the pore. Crosslinking experiments suggest a staggered arrangement of pore loops in the hexamer and nucleotide-dependent changes in pore-loop conformations. This mechanism of initial tag binding would allow ATP-dependent conformational changes in one or more pore loops to drive peptide translocation, force unfolding, and mediate threading of the denatured protein through the ClpX pore.
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