BK Channels with β3a Subunits Generate Use-Dependent Slow Afterhyperpolarizing Currents by an Inactivation-Coupled Mechanism

Abstract

Large-conductance, Ca2ϩ -and voltage-activated K ϩ (BK) channels are broadly expressed proteins that respond to both cellular depolarization and elevations in cytosolic Ca 2ϩ . The characteristic functional properties of BK channels among different cells are determined, in part, by tissue-specific expression of auxiliary ␤ subunits. One important functional property conferred on BK channels by ␤ subunits is inactivation. Yet, the physiological role of BK channel inactivation remains poorly understood. Here we report that as a consequence of a specific mechanism of inactivation, BK channels containing the ␤3a auxiliary subunit exhibit an anomalous slowing of channel closing. This produces a net repolarizing current flux that markedly exceeds that expected if all open channels had simply closed. Because of the time dependence of inactivation, this behavior results in a Ca2ϩ -independent but time-dependent increase in a slow tail current, providing an unexpected mechanism by which use-dependent changes in slow afterhyperpolarizations might regulate electrical firing. The physiological significance of inactivation in BK channels mediated by different ␤ subunits may therefore arise not from inactivation itself, but from the differences in the amplitude and duration of repolarizing currents arising from the ␤-subunit-specific energetics of recovery from inactivation.

Species-specific Differences among KCNMB3 BK β3 Auxiliary Subunits: Some β3 N-terminal Variants May Be Primate-specific Subunits

“…Effects of the human ␤ 3 subunits on gating shifts appear to be minimal, except as a consequence of inactivation ( Xia et al, 2000 ). Two of the ␤ 3 inactivating isoforms, ␤ 3a, and ␤ 3b, have been studied in some detail ( Xia et al, 2000 ;Lingle et al, 2001;Zeng et al, 2007 ) and exhibit distinctive inactivation behaviors that presumably depend on the specifi c N-terminal sequences. Given that further exploration of the physiological roles of inactivating BK auxiliary subunits will certainly depend on investigations in mice or rats, here we have undertaken an examination of the identity and function of mouse BK ␤ 3 subunits.…”

“…The mouse (and rat) appear to share with humans only one true orthologue, ␤ 3a. Both human and rodent ␤ 3a subunits result in qualitatively similar behavior, an interesting inactivation-dependent development of a slow tail current ( Zeng et al, 2007 ). However, there are marked differences between the two in terms of the rate of onset of inactivation and also the extent of steadystate inactivation, which impact on the rate of development of the slow tail current.…”

“…Overall, we would therefore predict that BK channels containing either rodent or human ␤ 3a subunits would have quite different effects on cellular excitability. Yet, we think it is important to keep in mind that the strong conservation in mammals of the 20-residue ␤ 3a exon suggests that the slow, use-dependent tail current prolongation produced by this N terminus ( Zeng et al, 2007 ) likely plays some conserved physiological role in BK channel function, wherever it is expressed.…”

“…Such ␤ subunits can also contribute in an up to 1:1 stoichiometry ( Knaus et al, 1994 ;Wang et al, 2002 ) to the BK channel complex and play a critical role in defi ning many fundamental properties of BK channels. This includes the apparent Ca 2+ dependence of channel activation ( Cox and Aldrich, 2000 ), the tail current behaviors ( Xia et al, 2000 ;Zeng et al, 2007 ), and the ability to inactivate ( Wallner et al, 1999 ;Xia et al, 1999Xia et al, , 2000Zeng et al, 2007 ). Both ␤ 1 and ␤ 2 subunits are able to shift the apparent Ca 2+ dependence of gating ( McManus et al, 1995 ;Wallner et al, 1999 ;Xia et al, 1999 ;Cox and Aldrich, 2000 ), such that at a given voltage, less Ca 2+ is required for a given level of activation.…”

BK channel opening involves side-chain reorientation of multiple deep-pore residues

“…BK channel proteins are homotetramers formed by BK α-subunits, which then associate with different β-or γ-subunits in a tissue-specific manner (13)(14)(15)(16)(17)(18)(19)(20)(21)(22). The α-subunit of the BK (K Ca 1.1) channel is encoded by the KCNMA1 gene, first discovered in Drosophila as the slowpoke mutation (dSlo) (23, 24), and later identified in mouse (mSlo1) and human (hSlo1) (25, 26).…”