Cross-talk between catalytic and regulatory elements in a DEAD motor domain is essential for SecA function

SecA, the motor subunit of bacterial polypeptide translocase, is an RNA hel icase, SecA comprises a dimerization C-terminal domain fused to an ATPase N -terminal domain containing conserved DEAD helicase motifs, We show that th e N-terminal domain is organized like the motor core of DEAD proteins, enco mpassing two subdomains, NBD1 and IRA2, NBD1, a rigid nucleotide-binding do main, contains the minimal ATPase catalytic machinery, IRA2 binds to NBD1 a nd acts as an intramolecular regulator of ATP hydrolysis by controling ADP release and optimal ATP catalysis at NBD1, IRA2 is flexible and can undergo changes in its alpha -helical content, The C-terminal domain associates wi th NBD1 and IRA2 and restricts IRA2 activator function, Thus, cytoplasmic S ecA is maintained in the thermally stabilized ADP-bound state and unnecessa ry ATP hydrolysis cycles are prevented, Two DEAD family motifs in IRA2 are essential for IRA2-NBD1 binding, optimal nucleotide turnover and polypeptid e translocation, We propose that translocation ligands alleviate C-terminal domain suppression, allowing IRA2 to stimulate nucleotide turnover at NBD1 , DEAD motors may employ similar mechanisms to translocate different enzyme s along chemically unrelated biopolymers.