SecA is a helicase-like motor that couples ATP hydrolysis with the translocation of extracytoplasmic protein substrates. As in most helicases, this process is thought to occur through nucleotide-regulated rigid-body movement of the motor domains. NMR, thermodynamic and biochemical data show that SecA uses a novel mechanism wherein conserved regions lining the nucleotide cleft undergo cycles of disorder-order transitions while switching among functional catalytic states. The transitions are regulated by interdomain interactions mediated by crucial 'arginine finger' residues located on helicase motifs. Furthermore, we show that the nucleotide cleft allosterically communicates with the preprotein substrate-binding domain and the regulatory, membrane-inserting C domain, thereby allowing for the coupling of the ATPase cycle to the translocation activity. The intrinsic plasticity and functional disorder-order folding transitions coupled to ligand binding seem to provide a precise control of the catalytic activation process and simple regulation of allosteric mechanisms.
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We wish to thank L. Gierasch for helpful discussions, F. Jordan for the use of the VP-ITC, Y. Papanikolau and K. Petratos for sharing data before publication, S. Papanikou and B. Pozidis for help with chromatography and R. Monteiro, A. Khan and M. Vougioukalaki for experiments performed during their undergraduate training. Initial experiments were supported by an EU large-scale facility grant to SON-NMR Utrecht, The Netherlands. Research was supported by a Scientist Development Grant from the American Heart Association (to C.G.K.), a Johnson & Johnson Discovery Award (to C.G.K.), a Rutgers Busch grant (to C.G.K.), grants from the EU (RTN1-1999-00149, QLK3-CT-2000-00082 and QLK3-CT-2000-00082; to A.E), Pfizer, Inc. and grants from the Greek General Secretariat of Research (01AKMON46 and PENED03ED623; to A.E).