Molecular dynamic studies on the interaction of a TatA oligomer with Tat translocon substrates

The Tat translocon directly utilizes the Proton Motive Force to transport folded proteins from the n-side to the p-side of energized membranes. In most organisms the translocon consists of three subunits, TatA, TatB and TatC exhibiting a stoichiometry of ~20-50/1/1. While TatB/TatC recognize the canonical twin-arginine motif-containing signal sequence of substrate proteins, TatA has been hypothesized to interact with TatB/TatC and translocon substrates facilitating their transport across the membrane and appears to form higher-order oligomers in the biological membranes. In this communication we have used course-grained molecular dynamic simulations to examine the interactions between a membrane-associated E. coli TatA nonamer, alone, and in association with two Tat translocon substrate proteins, either OEE17 or TorA. In all simulations the TatA nonamer markedly thinned the lipid bilayer which may facilitate substrate translocation. In the absence of substrate, the TatA nonamer was unstable with its radial architecture collapsing in 200-300 ns while in the presence of substrate the radial geometry of the nonamer persists for at least 1000 ns. Interestingly, it appears that the molecularity of the TatA oligomer is adjusted to the size of the substrate. Specific residues of the TatA amphipathic helixes were found to interact with substrate molecules, forming stable charge-pair or hydrogen-bonding interactions. While the substrate proteins were initially placed adjacent to the amphipathic helixes of the nonamer, during the simulation trajectories the substrates moved to a more central position adjacent to, and partially entering, the oligomer pore. These latter observations may constitute a glimpse of the initial stages of protein translocation.