BK channel activation by tungstate requires the ß1 subunit extracellular loop residues essential to modulate voltage sensor function and channel gating.

ABSTRACT

Tungstate, a compound with antidiabetic, antiobesity, and antihypertensive properties, activates the large-conductance voltage- and Ca(2+)-dependent K(+) (BK) channel containing either ß1 or ß4 subunits. The BK activation by tungstate is Mg(2+)-dependent and promotes arterial vasodilation, but only in precontracted mouse arteries expressing ß1. In this study, we further explored how the ß1 subunit participates in tungstate activation of BK channels. Activation of heterologously expressed human BKαß1 channels in inside-out patches is fully dependent on the Mg(2+) sensitivity of the BK α channel subunit even at high (10 µM) cytosolic Ca(2+) concentration. Alanine mutagenesis of ß1 extracellular residues Y74 or S104, which destabilize the active voltage sensor, greatly decreased the tungstate-induced left-shift of the BKαß1 G-V curves in either the absence or presence of physiologically relevant cytosolic Ca(2+) levels (10 µM). The weakened tungstate activation of the BKαß1Y74A and BKαß1S104A mutant channels was not related to decreased Mg(2+) sensitivity. These results, together with previously published reports, support the idea that the putative binding site for tungstate-mediated BK channel activation is located in the pore-forming α channel subunit, around the Mg(2+) binding site. The role of ß1 in tungstate-induced channel activation seems to rely on its interaction with the BK α subunit to modulate channel activity. Loop residues that are essential for the regulation of voltage sensor activation and gating of the BK channel are also relevant for BK activation by tungstate.