The startle disease mutation Q266H, in the second transmembrane domain of the human glycine receptor, impairs channel gating

Hyperekplexia (startle disease) results from mutations in the glycine recep tor chloride channel that disrupt inhibitory synaptic transmission. The Q26 6H missense mutation is the only hyperekplexia mutation located in the tran smembrane domains of the receptor. Using recombinant expression and patch-c lamping techniques, we have investigated the functional properties of this mutation. The ability of glycine and taurine to open the channel was reduce d in the mutated channel, as shown by a 6-fold shift in the concentration-r esponse curve for both agonists. This was not accompanied by similar change s in agonist displacement of strychnine binding, suggesting that the mutati on affects functions subsequent to ligand binding. Taurine was also convert ed to a weak partial agonist and antagonized the actions of glycine, consis tent with changes in its channel gating efficacy. Because the Q266H mutatio n is within the pore-forming second transmembrane domain, we tested for a d irect interaction with permeating ions. No change in either the cation/anio n selectivity ratio or in single channel conductance levels was observed. N o differential effects of Zn++, pH, and diethylpyrocarbonate were observed, implying that the histidine side chain is not exposed to the channel lumen . Single-channel recordings revealed a significant reduction in open times in the mutant receptors, at both high and low agonist concentrations, consi stent with the open state of the channel being less stable. This study demo nstrates that residues within the second transmembrane domain of ligand-gat ed ion channel receptors, even those whose side chains do not directly inte ract with permeating ions, can affect the kinetics of channel gating.