Disentangling the mechanistic role of loop-C capping in Cys-loop receptor activation.

A comparison of atomic models of Cys-loop receptors reveals that the largest rearrangement upon agonist binding and gating is the contraction ("capping") of the neurotransmitter binding-site loop C. The capping of this loop has often been suggested to act as the mechanical link that couples the binding of extracellular ligands to the gating of the transmembrane pore. However, because binding and gating are inextricably linked, testing this idea experimentally has proved challenging. Here, we disentangle binding and gating using mutagenesis, chimeric constructs, and functional assays. Our results point to the notion that loop-C capping is not required for the pore of Cys-loop receptors to open/desensitize in response to agonist binding to the extracellular domain. Instead, the functional impact of this marked rearrangement seems to be confined to local changes at the level of the neurotransmitter-binding sites, such as the transition from the unliganded state to the low-affinity agonist-bound conformation and/or the transition from the latter to the high-affinity bound state.