Functional Effects of Auxiliary β4-Subunit on Rat Large-Conductance Ca2+-Activated K+ Channel

Abstract

Large-conductance calcium-activated potassium (BK(Ca)) channels are composed of the pore-forming alpha-subunit and the auxiliary beta-subunits. The beta4-subunit is dominantly expressed in the mammalian central nervous system. To understand the physiological roles of the beta4-subunit on the BK(Ca) channel alpha-subunit (Slo), we isolated a full-length complementary DNA of rat beta4-subunit (rbeta4), expressed heterolgously in Xenopus oocytes, and investigated the detailed functional effects using electrophysiological means. When expressed together with rat Slo (rSlo), rbeta4 profoundly altered the gating characteristics of the Slo channel. At a given concentration of intracellular Ca(2+), rSlo/rbeta4 channels were more sensitive to transmembrane voltage changes. The activation and deactivation rates of macroscopic currents were decreased in a Ca(2+)-dependent manner. The channel activation by Ca(2+) became more cooperative by the coexpression of rbeta4. Single-channel recordings showed that the increased Hill coefficient for Ca(2+) was due to the changes in the open probability of the rSlo/rbeta4 channel. Single BK(Ca) channels composed of rSlo and rbeta4 also exhibited slower kinetics for steady-state gating compared with rSlo channels. Dwell times of both open and closed events were significantly increased. Because BK(Ca) channels are known to modulate neuroexcitability and the expression of the beta4-subunit is highly concentrated in certain subregions of brain, the electrophysiological properties of individual neurons should be affected profoundly by the expression of this second subunit.

Down-regulation of BK channel expression in the pilocarpine model of temporal lobe epilepsy

“…BK channel complexes, especially formed by β4 subunits, are widely distributed throughout the mammalian brain (Behrens et al, 2000;Brenner et al, 2000;Ha et al, 2004). The precise distribution and preferential trafficking of BK channels to axon terminals and somatodendritic regions can ultimately determine the function of BK channels in neurons and circuits.…”

Electrophysiological Characterization of Benzofuroindole-Induced Potentiation of Large-Conductance Ca2+-Activated K+Channels

“…The cDNAs of rat BK Ca channel ␣ subunit (rSlo), human ␤1 subunit (h␤1), and rat ␤4 subunit (r␤4) were subcloned into pGH vector for expression in X. laevis oocytes. The sequence information of rSlo, h␤1, and r␤4 used in this study are listed with GenBank under the accession numbers AF135265 (Ha et al, 2000), NM004137 (Meera et al, 1996), and AY028605 (Ha et al, 2004), respectively. Each cDNA was subcloned into pGH expression vector containing the 5Ј-and 3Ј-untranslated regions of X. laevis ␤-globin gene, because it is known to enhance the protein expression of certain mammalian messages in X. laevis oocytes (Liman et al, 1992).…”

“…BK Ca channels are composed of two different subunits: the poreforming ␣ subunit and the auxiliary ␤ subunits. Although channels formed only by four ␣ subunits can be functional, ␤ subunits alter the biophysical and pharmacological properties of homomeric channels, including Ca 2ϩ and voltage sensitivity, and gating kinetics (Valverde et al, 1999;Wallner et al, 1999;Xia et al, 1999;Qian et al, 2002;Ha et al, 2004). Several compounds have been developed and reported to be BK Ca channel openers (e.g., dehydrosoyasaponin-I, maxikdiol, NS-1619, BMS-204352, 17␤-estradiol, ethylbromide tamoxifen, pimaric acid, and epoxyeicosatrienoic acids) (Vergara et al, 1998;Valverde et al, 1999;Coghlan et al, 2001;Dick et al, 2002;Imaizumi et al, 2002).…”

Identification and Functional Characterization of Ankyrin-Repeat Family Protein ANKRA as a Protein Interacting with BKCaChannel

“…Signals were filtered at 1 kHz by using a four-pole low-pass Bassel filter, digitized at a rate of 20 samples/ms by using Digidata 1200 (Axon Instruments), and stored in a personal computer. pClamp8 (Axon Instruments) software was used to control the amplifier and to acquire and analyze the data as described previously (Ha et al, 2004).…”