Phenotypic Alteration of a Human BK (hSlo) Channel byhSloβSubunit Coexpression: Changes in Blocker Sensitivity, Activation/Relaxation and Inactivation Kinetics, and Protein Kinase A Modulation

Abstract

A human homolog of the large-conductance calcium-activated potassium (BK) channel beta subunit (hSlobeta) was cloned, and its effects on a human BK channel (hSlo) phenotype are reported. Coexpression of hSlo and hSlobeta, in both oocytes and human embryonic kidney 293 cells, resulted in increased Ca2+ sensitivity, marked slowing of BK channel activation and relaxation, and significant reduction in slow inactivation. In addition, coexpression changed the pharmacology of the BK channel phenotype: hSlo-mediated currents in oocytes were more sensitive to the peptide toxin iberiotoxin than were hSlo + hSlobeta currents, and the potency of blockade by the alkaloid BK blocker tetrandrine was much greater on hSlo + hSlobeta- mediated currents compared with hSlo currents alone. No significant differences in the response to charybdotoxin or the BK channel opener NS1619 were observed. Modulation of BK channel activity by phosphorylation was also affected by the presence of the hSlobeta subunit. Application of cAMP-dependent protein kinase increased P(OPEN) of hSlo channels, but decreased P(OPEN)of most hSlo + hSlobeta channels. Taken together, these altered characteristics may explain some of the wide diversity of BK channel phenotypes observed in native tissues.

Molecular Basis for the Inactivation of Ca2+- and Voltage-Dependent BK Channels in Adrenal Chromaffin Cells and Rat Insulinoma Tumor Cells

“…The ␤3 subunit shifts gating of Slo subunits A major functional role of the previously described ␤1 and ␤2 subunits is to produce a shift in activation by Ca 2ϩ to more negative membrane potentials (McManus et al, 1995;Dworetzky et al, 1996;Wallner et al, 1996;Saito et al, 1997;Ramanathan et al, 1999). Thus, smooth muscle BK channels or other BK channels containing a ␤1 subunit can be activated by elevations of submembrane Ca 2ϩ even at membrane potentials near rest.…”

Alternative Splicing Switches Potassium Channel Sensitivity to Protein Phosphorylation

“…3, a and b). Importantly, the inhibitory effect of cAMP on STREX-S869A channels is dependent upon endogenous PKA activity as cAMP had no effect on the activity of STREX-S869A channels in the absence of ATP (n ϭ 5, not shown) or in the presence of PKI [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] (n ϭ 6, Fig. 3b).…”

“…Importantly, alternative splicing of the ␣-subunit is dynamically regulated in adults, for example during stress or pregnancy (15,16). Thus the diversity and plasticity of responses to PKA-dependent protein phosphorylation observed between BK channels in native tissues may result either from differential modulation of alternatively spliced BK channel ␣-subunits (12)(13)(14)(15) or through their interaction with different signaling complexes and ␤-subunits (17)(18)(19).…”

The Large Conductance, Voltage-dependent, and Calcium-sensitive K+ Channel, Hslo, Is a Target of cGMP-dependent Protein Kinase Phosphorylation in Vivo

“…This difference may be due to the level of expression of the modulatory ␤-subunit (25,42) or of splice variants of Hslo in native cells. The regulatory ␤-subunit is known to modify the biophysical, pharmacological, and modulatory properties of Hslo (31,41,(43)(44)(45). Future studies are necessary to address the question of whether the ␤-subunit affects the PKG-induced activation of the ␣-subunit (Hslo) of maxi-K channels.…”