α Subunit Histidine 55 at the Interface between Extracellular and Transmembrane Domains Affects Preactivation and Desensitization of the GABA Receptor.

The GABA receptor is a member of the Cys-loop family and plays a crucial role in the adult mammalian brain inhibition. Although the static structure of this receptor is emerging, the molecular mechanisms underlying its conformational transitions remain elusive. It is known that in the Cys-loop receptors, the interface between extracellular and transmembrane domains plays a key role in transmitting the "activation wave" down to the channel gate in the pore. It has been previously reported that histidine 55 (H55), located centrally at the interfacial β1-β2 loop of the α subunit, is important in the receptor activation, but it is unknown which specific gating steps it is affecting. In the present study, we addressed this issue by taking advantage of the state-of-the-art macroscopic and single-channel recordings together with extensive modeling. Considering that H55 is known to affect the local electrostatic landscape and because it is neighbored by two negatively charged aspartates, a well conserved feature in the α subunits, we considered substitution with negative (E) and positive (K) residues. We found that these mutations markedly affected the receptor gating, altering primarily preactivation and desensitization transitions. Importantly, opposite effects were observed for these two mutations strongly suggesting involvement of electrostatic interactions. Single-channel recordings suggested also a minor effect on opening/closing transitions which did not depend on the electric charge of the substituting amino acid. Altogether, we demonstrate that H55 mutations affect primarily preactivation and desensitization most likely by influencing local electrostatic interactions at the receptor interface.