Location of the β4 transmembrane helices in the BK potassium channel

Abstract

Large-conductance, voltage-and Ca2+-gated potassium (BK) channels control excitability in a number of cell types. BK channels are composed of α subunits, which contain the voltage-sensor domains and the Ca2+-sensor domains and form the pore, and often one of four types of β subunits, which modulate the channel in a cell-specific manner. β4 is expressed in neurons throughout the brain. Deletion of β4 in mice causes temporal lobe epilepsy. Compared with channels composed of α alone, channels composed of α and β4 activate and deactivate more slowly. We inferred the locations of the two β4 transmembrane (TM) helices TM1 and TM2 relative to the seven α TM helices, S0-S6, from the extent of disulfide bond formation between cysteines substituted in the extracellular flanks of these TM helices. We found that β4 TM2 is close to α S0 and that β4 TM1 is close to both α S1 and S2. At least at their extracellular ends, TM1 and TM2 are not close to S3-S6. In six of eight of the most highly crosslinked cysteine pairs, four crosslinks from TM2 to S0 and one each from TM1 to S1 and S2 had small effects on the V50 and on the rates of activation and deactivation. That disulfide crosslinking caused only small functional perturbations is consistent with the proximity of the extracellular ends of TM2 to S0 and of TM1 to S1 and to S2, in both the open and closed states. Copyright © 2009 Society for Neuroscience.