Solution NMR Structure of the TatA Component of the Twin-Arginine Protein Transport System from Gram-Positive Bacterium Bacillus subtilis

摘要

The twin-arginine transport (Tat) system translocates folded proteins across the bacterial cytoplasmic or chloroplast thylakoid membrane of plants. The Tat system in most Gram-positive bacteria consists of two essential components, the TatA and TatC proteins. TatA is considered to be a bifunctional subunit, which can form a protein-conducting channel by self-oligomerization and can also participate in substrate recognition. However, the molecular mechanism underlying protein translocation remains elusive. Herein, we report the solution structure of the TatA_d protein from Bacillus subtilis by NMR spectroscopy, the first structure of the Tat system at atomic resolution. TatA_d shows an L-shaped structure formed by a transmembrane helix and an amphipathic helix, while the C-terminal tail is largely unstructured. Our results strongly support the postulated topology of TatA_d in which the transmembrane helix is inserted into the lipid bilayer while the amphipathic helix lies at the membrane−water interface. Moreover, the structure of TatA_d revealed the structural importance of several conserved residues at the hinge region, thus shedding new light on further elucidation of the protein transport mechanism of the Tat system.