Gating properties conferred on BK channels by the beta 3b auxiliary subunit in the absence of its NH2- and COOH termini

Both beta1 and beta2 auxiliary subunits of the BK-type K+ channel family pr ofoundly regulate the apparent Ca2+ sensitivity of BK-type Ca2+-activated K + channels. Each produces a pronounced leftward shift in the voltage of hal f-activation (V-0.5) at a given C2+ concentration, particularly at Ca2+ abo ve 1 muM. In contrast, the rapidly inactivating beta 3b auxiliary produces a leftward shift in activation at Ca2+ below 1 muM. In the companion work ( Lingle, C.J., X.-H. Zeng,J.-P. Ding, and X.-M. Xia. 2001.J: Gen. Physiol. 1 17:583-605, this issue), we have shown that some of the apparent PSb-mediat ed shift in activation at low Ca2+ arises from rapid unblocking of inactiva ted channels, unlike the actions of the beta1 and beta2 subunits. Here, we compare effects of the beta 3b subunit that arise from inactivation, per se , versus those that may arise from other functional effects of the subunit. In particular, we examine gating properties of the beta 3b subunit and com pare it to beta 3b constructs lacking either the NH2- or COOH terminus or b oth. The results demonstrate that, although the NH2 terminus appears to be the primary determinant of the beta 2h-mediated shift in V-0.5 at low Ca2+, removal of the NH2 terminus reveals two other interesting aspects of the a ction of the beta 3b subunit. First, the conductance-voltage curves for act ivation of channels containing the beta 3b subunit are best described by a double Boltzmann shape, which is proposed to arise from two independent vol tage-dependent activation steps. Second, the presence of the beta 3b subuni t results in channels that exhibit an anomalous instantaneous outward curre nt rectification that is correlated with a voltage dependence in the time-a veraged single-channel current. The two effects appear to be unrelated, but indicative of the variety of ways that interactions between beta and alpha subunits can affect BK channel function. The COOH terminus of the beta 3b subunit produces no discernible functional effects.