BK β1 Transmembrane Regions Critically Control the Characteristic Phenotype of β1-Containing Bk Channel Currents

Abstract

Voltage- and Ca2+-gated, large conductance K+ (BK) channels are ubiquitously expressed and play a key role in linking membrane excitability to Ca2+ signaling. In most tissues, BK channel complexes consist of channel-forming α and regulatory β subunits. Each β contains short intracellular N- and C-termini, and two transmembrane domains (TM1, TM2) joined by an extracellular loop. The smooth muscle-abundant BK β1 increases the channel's apparent Ca2+ sensitivity and is required for channel activation by cholane steroids (Brenner et al., 2000; Bukiya et al., 2009). The participation of different β1 regions in regulating channel function has not been fully settled. Here, we used BK channel-forming cbv1 and β1 subunits cloned from rat cerebral artery myocytes, neuron-abundant β4, and chimeras constructed by swapping regions between β1 and β4 to determine the contribution of specific β1 regions to the β1-containing BK phenotype. After co-expressing cbv1 with wt βs or chimeric βs in Xenopus oocytes, macroscopic currents were evoked by 200 ms-long, 10 mV depolarizing steps from −150 to +150 mV (VH=-80 mV). At Ca2+ levels found near the BK channel during smooth muscle cell contraction (10 μM), channel complexes that included chimeras combining both TMs from β1 with remaining regions (EC loop, IC ends) from β4 showed a phenotype (Vhalf, τact, τdeact) identical to that of channel complexes containing wt β1. However, this phenotype could not be evoked by channel complexes that included chimeras combining either β1 TM1 or β1 TM2 with the remaining β4 regions (TM2-EC loop-IC ends and TM1-EC loop-IC ends, respectively). We conclude that at physiological voltages and Ca2+, both TMs from β1 are necessary to confer the characteristic current phenotype of β1-containing BK channels. Support: R37-AA11560, R01-HL10463 (AMD).