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The BK channel, a voltage-and Ca2+-gated large-conductance potassium channel with many important functions, is often localized at specific subcellular domains.Although proper subcellular localization is likely a prerequisite for the channel to perform its physiological functions, little is known about the molecular basis of localization.Here, we show that CTN-1, a homologue of mammalian α-catulin, is required for subcellular localization of SLO-1, the Caenorhabditis elegans BK channel α-subunit, in body-wall muscle cells and neurons.CTN-1 was identified in a genetic screen for mutants that suppressed a lethargic phenotype caused by expressing a gain-of-function (gf) isoform of SLO-1.CTN-1 coclusters with SLO-1 at dense body regions of body-wall muscle cells and presynaptic sites of neurons.In ctn-1 loss-of-function (lf) mutants, SLO-1 was mislocalized but its transcription and protein level were unchanged.Evoked neurotransmitter release was increased at the neuromuscular junction in ctn-l(lf) mutant as well as slo-l(lf) mutant.These results suggest that CTN-1 plays an important role in BK channel function by mediating channel subcellular localization.