On the role of individual subunits in MscL gating: "all for one, one for all?"

The mechanosensitive channel of large conductance (MscL) is a homopentameric membrane protein that protects bacteria from hypoosmotic stress. Its mechanics are coupled to structural changes in the membrane, yet the molecular mechanism of the transition from closed to open states and the cooperation between subunits are poorly understood. To determine the early stages of channel activation, we have created a chemically addressable heteropentameric MscL, which allows us to selectively trigger only one subunit in the pentameric protein assembly. By employing a liposome leakage assay developed in house, we measured the size-exclusion limits of MscL (G22C homopentamer and WTG22C heteropentamer). Patch-clamp, single-channel conductance recordings were used to electrically characterize the various channel substates. We show that a decrease in the hydrophobicity of a pore residue in only one subunit breaks the energy barrier for gating and increases the pore diameter up to 10 Å. A further decrease on the hydrophobicity of the same pore residue in other subunits opens the channel further, up to a diameter of 25 Å. However, it is not sufficient for full opening of the channel. This suggests the presence of supplementary mechanisms other than only the hydrophobic gate for MscL opening and closing and/or insufficient expansion of the channel by hydrophobic gating in the absence of applied membrane tension.