Identification of Intracellular β-Barrel Residues Involved in Ion Selectivity in the Mechanosensitive Channel of Thermoanaerobacter tengcongensis .

The mechanosensitive channel of small conductance (MscS) is a bacterial membrane pore that senses membrane tension and protects cells from lysis by releasing osmolytes. MscS is a homoheptameric channel with a cytoplasmic domain with seven portals and a β-barrel opening to the cytoplasm. Tt MscS, an MscS channel from Thermoanaerobacter tengcongensis , is an anion-selective channel. A previous study from our laboratory has defined the crucial role of β-barrel in the anion selectivity of Tt MscS (Zhang et al., 2012). However, the mechanistic details by which the β-barrel determines anion selectivity remain unclear. Here, using mutagenesis and patch-clamp recordings, we investigated the function and structural correlations between β-barrels and the anion selectivity of Tt MscS at the atomic level. Our results indicated that mutation of V274, a residue at the center of the inner wall of the β-barrel in Tt MscS, caused the anion selectivity of Tt MscS reverse to cation selectivity. Moreover, the electrostatic potential (T272) and physical size (L276) of residues in the inner wall of β-barrel also determine the anion selectivity of Tt MscS. In summary, the present study confirmed that the β-barrel region of Tt MscS acts as a "selective filter" that renders Tt MscS anion selectivity.