17β-Estradiol Binds and Modulates BK Channel through its β1 Auxiliary Subunit

BK channels are ubiquitously expressed and are involved in the regulation of vascular smooth muscle contractility. Changes in the expression of the BK channel or its auxiliary β subunits are associated with several physiological abnormalities including hypertension. It is known that 17β-Estradiol (E2) activates the BK channel, causing a membrane hyperpolarization and in vascular tissue, through its action on BK channels, decreases vasoconstriction. Although the mechanism by means of which E2 modulates BK channels has not been completely described and the binding site of E2 in the BK channel has not been identified, it was demonstrated that the presence of the accessory β1 subunit is necessary for this modulatory effect (Valverde et al., 1999). Our results indicate that the β1 subunit can be expressed in a heterologous system alone, and by performing binding assays using the fluorescent complex of E2 (E2-BSA-FITC), we show that β1 is the target of E2. Using molecular dynamics, we identified the W163 residue of the β1 subunit as a possible structural component of the binding site for E2. Consistent with this result the point mutation W163I completely abolishes E2 binding. We propose that the binding site for E2 is located in the β1 subunit of the BK channel and that the W163 residue is directly involved in the binding between the BK channel and E2.