Subunit-selective contribution to channel gating of the M4 domain of the nicotinic receptor

The muscle nicotinic receptor (AChR) is a pentamer of four different subunits, each of which contains four transmembrane domains (M1-M4). We recently showed that channel opening and closing rates of the AChR depend on a hydrogen bond involving a threonine at position 14′ of the M4 domain in the α-subunit. To determine whether residues in equivalent positions in non-α-subunits contribute to channel gating, we mutated δT14′, βT14′, and εS14′ and evaluated changes in the kinetics of acetylcholine-activated currents. The mutation εS14′A profoundly slows the rate of channel closing, an effect opposite to that produced by mutation of αT14′. Unlike mutations of αT14′, εS14′A does not affect the rate of channel opening. Mutations in δT14′ and βT14′ do not affect channel opening or closing kinetics, showing that conserved residues are not functionally equivalent in all subunits. Whereas αT14′A and εS14′A subunits contribute additively to the closing rate, they contribute nonadditively to the opening rate. Substitution of residues preserving the hydrogen bonding ability at position 14′ produce nearly normal gating kinetics. Thus, we identify subunit-specific contributions to channel gating of equivalent residues in M4 and elucidate the underlying mechanistic and structural bases.