The β1-Subunit of the MaxiK Channel Associates with the Thromboxane A2 Receptor and Reduces Thromboxane A2 Functional Effects

The large conductance voltage-and Ca2+-activated K+ channel (MaxiK, BKCa, BK) is composed of four pore-forming alpha-subunits and can be associated with regulatory beta-subunits. One of the functional roles of MaxiK is to regulate vascular tone. We recently found that the MaxiK channel from coronary smooth muscle is trans-inhibited by activation of the vasoconstricting thromboxane A(2) prostanoid receptor (TP), a mechanism supported by MaxiK alpha-subunit (MaxiK alpha)-TP physical interaction. Here, we examined the role of the MaxiK beta 1-subunit in TP-MaxiK association. We found that the beta 1-subunit can by itself interact with TP and that this association can occur independently of MaxiK alpha. Subcellular localization analysis revealed that beta 1 and TP are closely associated at the cell periphery. The molecular mechanism of beta 1-TP interaction involves predominantly the beta 1 extracellular loop. As reported previously, TP activation by the thromboxane A(2) analog U46619 caused inhibition of MaxiK alpha macroscopic conductance or fractional open probability (FPo) as a function of voltage. However, the positive shift of the FPo versus voltage curve by U46619 relative to the control was less prominent when beta 1 was coexpressed with TP and MaxiK alpha proteins (20 +/- 6 mV, n = 7) than in cells expressing TP and MaxiK alpha alone (51 +/- 7 mV, n = 7). Finally, beta 1 gene ablation reduced the EC50 of the U46619 agonist in mediating aortic contraction from 18 +/- 1 nM (n = 12) to 9 +/- 1 nM (n = 12). The results indicate that the beta 1-subunit can form a tripartite complex with TP and MaxiK alpha, has the ability to associate with each protein independently, and diminishes U46619-induced MaxiK channel trans-inhibition as well as vasoconstriction.