Mapping the conformational wave of acetylcholine receptor channel gating

Allosteric transitions allow fast regulation of protein function in living systems. Even though the end points of such conformational changes are know n for many proteins, the characteristics of the paths connecting these stat es remain largely unexplored. Rate-equilibrium linear free-energy relations hips (LFERs) provide information about such pathways by relating changes in the free energy of the transition state to those of the ground states upon systematic perturbation of the system(1). Here we present an LFER analysis of the gating reaction pathway of the muscle acetylcholine receptor. We st udied the closed reversible arrow open conformational change at the single- molecule level following perturbation by series of single-site mutations, a gonists and membrane voltages. This method provided a snapshot of several r egions of the receptor at the transition state in terms of their approximat e positions along the reaction coordinate, on a scale from 0 (closed-like) to 1 (open-like). The resulting map reveals a spatial gradient of positiona l values, which suggests that the conformational change proceeds in a wave- like manner, with the low-to-high affinity change at the transmitter-bindin g sites preceding the complete opening of the pore.