A single coiled-coil domain mutation in hIKCa channel subunits disrupts preferential formation of heteromeric hSK1:hIKCa channels

The expression of IKCa (SK4) channel subunits overlaps with that of SK channel subunits, and it has been proposed that the two related subunits prefer to co-assemble to form heteromeric hSK1:hIKCa channels. This implicates hSK1:hIKCa heteromers in physiological roles that might have been attributed to activation of SK channels. We have used a mutation approach to confirm formation of heterometric hSK1:hIKCa channels. Introduction of residues within hSK1 that were predicted to impart sensitivity to the hIKCa current blocker TRAM-34 changed the pharmacology of functional heteromers. Heteromeric channels formed between wildtype hIKCa and mutant hSK1 subunits displayed a significantly higher sensitivity and maximum block to addition of TRAM-34 than heteromers formed between wildtype subunits. Heteromer formation was disrupted by a single point mutation within one COOH-terminal coiled-coil domain of the hIKCa channel subunit. This mutation only disrupted the formation of hSK1:hIKCa heteromeric channels, without affecting the formation of homomeric hIKCa channels. Finally, the Ca2+ gating sensitivity of heteromeric hSK1:hIKCa channels was found to be significantly lower than the Ca2+ gating sensitivity of homomeric hIKCa channels. These data confirmed the preferred formation of heteromeric channels that results from COOH-terminal interactions between subunits. The distinct sensitivity of the heteromer to activation by Ca2+ suggests that heteromeric channels fulfil a distinct function within those neurons that express both subunits. A single population of heteromeric channels with a distinct Ca2+ gating sensitivity was produced when wild-type hSK1 and hIKCa subunits were co-expressed in tsA201 cells. This was confirmed using a mutational approach. A single point mutation within the C-terminal coiled-coil domain of hIKCa subunits disrupted but did not prevent the formation of heteromeric channels, suggesting that heteromeric assembly is mediated by this region.image