Measuring the Infl uence of the BK Ca β 1 Subunit on Ca 2+ binding to the BK Ca channel

Abstract

The large-conductance Ca 2++ -activated potassium (BK Ca ) channel of smooth muscle is unusually sensitive to Ca 2++ as compared with the BK Ca channels of brain and skeletal muscle. This is due to the tissue-specifi c expression of the BK Ca auxiliary subunit β 1, whose presence dramatically increases both the potency and effi cacy of Ca 2++ in promoting channel opening. β 1 contains no Ca 2++ binding sites of its own, and thus the mechanism by which it increases the BK Ca channel ' s Ca 2++ sensitivity has been of some interest. Previously, we demonstrated that β 1 stabilizes voltage sensor activation, such that activation occurs at more negative voltages with β 1 present. This decreases the work that Ca 2++ must do to open the channel and thereby increases the channel ' s apparent Ca 2++ affi nity without altering the real affi nities of the channel ' s Ca 2++ binding sites. To explain the full effect of β 1 on the channel ' s Ca 2++ sensitivity, however, we also proposed that there must be effects of β 1 on Ca 2++ binding. Here, to test this hypothesis, we have used high-resolution Ca 2++ dose - response curves together with binding site - specifi c mutations to measure the effects of β 1 on Ca 2++ binding. We fi nd that coexpression of β 1 alters Ca 2++ binding at both of the BK Ca channel ' s two types of high-affi nity Ca 2++ binding sites, primarily increasing the affi nity of the RCK1 sites when the channel is open and decreasing the affi nity of the Ca 2++ bowl sites when the channel is closed. Both of these modifi cations increase the difference in affi nity between open and closed, such that Ca 2++ binding at either site has a larger effect on channel opening when β 1 is present. © 2009 Sweet and Cox.