Functionally significant mobile regions of Escherichia coli SecA ATPase identified by NMR.

SecA, a 204-kDa homodimeric protein, is a major component of the cellular machinery that mediates the translocation of proteins across the Escherichia coli plasma membrane. SecA promotes translocation by nucleotide-modulated insertion and deinsertion into the cytoplasmic membrane once bound to both the signal sequence and portions of the mature domain of the preprotein. SecA is proposed to undergo major conformational changes during translocation. These conformational changes are accompanied by major rearrangements of SecA structural domains. To understand the interdomain rearrangements, we have examined SecA by NMR and identified regions that display narrow resonances indicating high mobility. The mobile regions of SecA have been assigned to a sequence from the second of two domains with nucleotide-binding folds (NBF-II; residues 564-579) and to the extreme C-terminal segment of SecA (residues 864-901), both of which are essential for preprotein translocation activity. Interactions with ligands suggest that the mobile regions are involved in functionally critical regulatory steps in SecA.