A Critical Role of the S6 Transmembrane Helix in BK Channel Modulation by Auxiliary γ Subunits

Abstract

BK channels consist of the pore-forming, voltage- and Ca2+-sensing α-subunits (BKα or Slo1) and the regulatory auxiliary β or γ subunits. BK channels are potently modulated by the auxiliary γ subunits, which shift the voltage-dependence of channel activation by up to 140 mV in the hyperpolarizing direction. We have recently identified the γ subunits’ transmembrane (TM) segment and its neighboring intracellular charged residues as key determinants for their modulatory functions on BK channels. However, the molecular mechanisms underlying BK channel modulation by auxiliary γ subunits remain largely unknown. The proton-gated Slo3 channel has a high sequence similarity to Slo1 but showed nearly no sensitivity to the BKγ1 (LRRC26) subunit. To identify the key protein regions of Slo1 that are involved in modulation by auxiliary γ subunits, we generated a series of Slo1/Slo3 chimeras in the N-terminal TM (S0-S6) domain and analyzed their responses to the auxiliary γ1 subunit in voltage-dependence of channel activation. We found that the Slo3 channel's lack of sensitivity to BKγ1 is largely caused by its S6 TM segment, whose inclusion in the Slo1/Slo3 chimera greatly attenuated the channel's response to BKγ1. Our further mutational analyses of the differential residues on the S6 TM segment identified several key residues that are largely responsible for the difference between Slo1 and Slo3 channels in their responses to BKγ1. Therefore, we concluded that the S6 TM helix is critical in BK channel modulation by auxiliary γ subunit.