Residues within transmembrane segment M2 determine chloride conductance of glycine receptor homo- and hetero-oligomers.

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Abstract
We have expressed glycine receptor (GlyR) alpha and beta subunit cDNAs in HEK‐293 cells to study the functional properties of homo‐ versus hetero‐oligomeric GlyR channels. Dose‐response curves of whole‐cell currents in cells expressing alpha 1 subunits revealed an average Hill coefficient of h = 4.2. Co‐expression with the beta subunit markedly increased glycine‐gated whole‐cell currents, which now exhibited a mean Hill coefficient of only h = 2.5. For alpha 1, alpha 2 and alpha 3 homo‐oligomers, the main‐state single‐channel conductances were 86, 111 and 105 pS, respectively, recorded at symmetrical Cl‐ concentrations of 145 mM. The mutant alpha 1 G221A gave rise to a main‐state of 107 pS. This indicates that the main‐state of alpha homo‐oligomers depends on residue 221 which is located within transmembrane segment M2. Importantly, the main‐state conductances of alpha 1/beta, alpha 2/beta and alpha 3/beta hetero‐oligomers were only 44, 54 and 48 pS, respectively. The latter values are similar to those found in spinal neurons, suggesting that native GlyRs are predominantly alpha/beta hetero‐oligomers. Co‐expression of alpha 1 with mutant beta subunits revealed that residues within and close to segment M2 of the beta subunit determine the conductance differences between homo‐ and hetero‐oligomers.