Hydrophobic Gate of Mechanosensitive Channel of Large Conductance in Lipid Bilayers Revealed by Solid-State NMR Spectroscopy.

The bacterial mechanosensitive channel of large conductance (MscL) functions as a pressure-relief safety valve to prevent cells from lysing during sudden hypo-osmotic shock. The hydrophobic gate of MscL in the closed state forms a barrier to the permeation of ions and water molecules and can be switched to the open state for releasing solutions and ions. Currently, the gate-constituting residues and the functional role of these residues in the hydrophobic gate of MscL remain elusive and controversial. Here, we employ magic angle spinning solid-state nuclear magnetic resonance (ssNMR) techniques and functional assays to investigate the hydrophobic gate of MscL from Methanosarcina acetivorans (Ma-MscL) in lipid bilayers. We obtain chemical shift assignments of ∼70% residues of Ma-MscL and predict its 3D structure. Based on the structural characterization, we identify that the residues I21-T30 in the transmembrane helix 1 constitute the hydrophobic gate by detecting water distributions in the transmembrane pore using ssNMR H/D exchange and water-edited experiments. By using ssNMR structural characterization and functional assays, we reveal that the packing of aromatic rings of F23 in each subunit of Ma-MscL is critical to the hydrophobic gate, and hydrophilic substitutions of the other functionally important residues A22 and G26 modulate channel gating by attenuating hydrophobicity of constriction of F23.