Folding and Self-Assembly of the Pore-Forming Unit Tat-A of the Bacterial Twin-Arginine Translocase

The bacterial Twin-arginine translocation pathway is able to transport fully folded proteins across membranes. In B. subtilis it consists only of two components: TatCd, which serves as a receptor for the signal peptide, and the pore forming unit TatAd, which occurs in high stoichiometric excess. According to circular dichroism TatA contains a transmembrane segment, an amphiphilic helix, and an unstructured C-terminus [1]. Its detailed moelcular structure was resolved by solid-state NMR spectroscopy in oriented bilayers [2]. A striking pattern on the monomeric protein surface allowed us to assemble several units into protomers and into an open oligomeric pore. The stability of these complexes was supported by all-atom MD simulations and using structure-based modeling [3]. The observed interactions suggest that a novel motif for folding and self-assembly motif is present in this membrane-bound transport system, which allows reversible pore formation. Our comprehensive three-dimensional model thus reconciles for the first time TatA transport with a pore size of variable diameter, which can open and close by an energetically feasible mechanism. [1] Müller, S.D., A.A. De Angelis, T.H. Walther, S.L. Grage, C. Lange, S.J. Opella & A.S. Ulrich (2007) Biochim. Biophys. Acta 1768: 3071-3079 [2] Walther, T.H., S.L. Grage, N. Roth & A.S. Ulrich (2010) J. Am. Chem. Soc., in press [3] Grage, S.L., T.H. Walther, M. Wolf, A. Vargiu, M.J. Klein, P. Ruggerone, W. Wenzel, A.S. Ulrich (2010) submitted