In Drosophila and higher animals the large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel is involved in regulating the membrane potential of both excitable and non-excitable cells. In mammalian central neurons, BK_Ca channels co-localise with N-type calcium channels (Marrion & Tavalin, 1998), and are mainly targeted to axons and nerve terminals (Knaus et al. 1996). The mechanisms by which this potassium channel is targeted to discrete sub-cellular regions are not understood. We have identified a human cDNA derived from human brain that encodes a protein similar to Drosophila dSLIP1 (Xia et al. 1998), a protein that interacts with the dSlo BK_Ca channel. The human isoform, hSLIP1, is a cytoplasmic protein of 1098 amino acids, possesses two PDZ domains, and interacts directly with the human BK_Ca channel subunit hSloα1, which we determined by the co-immunoprecipitation of subunits co-expressed in HEK cells. Like its Drosophila counterpart hSLIP1 significantly (P < 0.01, t test) reduced macroscopic BK_Ca currents when co-expressed with hSloα1 in Xenopus oocytes. The current evoked by a depolarising pulse to +120 mV was (mean ± S.E.M.) 9.03 ± 0.86µA (n = 7) in oocytes expressing hSloα1 alone and 4.37 ± 0.56µA (n = 6) in oocytes co-expressing hSloα1 and hSLIP1. This property of hSLIP1 was prevented by co-injecting mRNA encoding the fragment F_C, the last 142 amino acids of the hSloα1 intracellular C-terminus (9.99 ± 1.56µA, n = 5), implicating this domain in interacting with hSLIP1. Co-expressing hSLIP1 or F_C with hSloα1 in oocytes had no significant effect on channel gating or unitary conductance: in inside-out patches the activation V_1/2 was 10.0 ± 5.2 mV, 20.2 ± 5.4 mV, and 17.5 ± 2.6 mV in 10µM [Ca²⁺]_i (n = 5) and conductance was 279 ± 5 pS, 265 ± 7 pS, and 264 ± 5 pS (n = 3) in symmetrical 140 mM K⁺ for hSloα1, hSloα1 + hSLIP1, and hSloα1 + F_C, respectively. We therefore hypothesise that hSLIP1 is involved in regulating the surface expression and sub-cellular distribution of BK_Ca channels.

We thanks the Kazusa DNA Research Institute (Chiba, Japan) for the KIAA1095 cDNA, and Ellina Michailova and Janet Storm for help with plasmid preparation. This work was supported by The Wellcome Trust and The Royal Society. JDL holds an EPA Cephalosporin Fellowship at Linacre College and FMA is the Royal Society GlaxoSmithKline Research Professor.