A MUTATION IN AUTOSOMAL-DOMINANT MYOTONIA-CONGENITA AFFECTS PORE PROPERTIES OF THE MUSCLE CHLORIDE CHANNEL

Autosomal dominant myotonia congenita is an inherited disorder of skel etal muscle caused by mutations in a voltage-gated Cl- channel gene (C LCN1, 7q35). Here, we report that a mutation predicting the substituti on of Gly 230 by glutamic acid (G230E) between segments D3 and D4 dram atically alters the pore properties of a recombinant human muscle Cl- channel (hCIC-1) expressed in a mammalian cell line (tsA201),The G230E mutation causes substantial changes in anion and cation selectivity a s well as a fundamental change in rectification of the current-voltage relationship, Whereas wild-type channels are characterized by pronoun ced inward rectification and a Cl > thiocyanate > Br > NO3 > I > CH3SO 3 selectivity, G230E exhibits outward rectification at positive potent ials and a thiocyanate > NO3 > I > Br > Cl > CH3SO3 selectivity. Furth ermore, the cation-to-anion permeability ratio of the mutant is much g reater than that of the wild-type channel, Voltage-dependent blocks by intracellular and extracellular iodide help to distinguish two distin ct ion binding sites within the hCIC-1 conduction pathway, Both bindin g sites are preserved in the mutant but have decreased affinities for iodide. These findings suggest that Gly 230 is critical for normal ion conductance in hCIC-1 and that this residue resides within the channe l pore.