Gating Properties Conferred on Bk Channels by the β3b Auxiliary Subunit in the Absence of Its Nh₂- and Cooh Termini

Both β1 and β2 auxiliary subunits of the BK-type K⁺ channel family profoundly regulate the apparent Ca²+ sensitivity of BK-type Ca²+-activated K⁺ channels. Each produces a pronounced leftward shift in the voltage of half-activation (V_0.5) at a given Ca²+ concentration, particularly at Ca²+ above 1 μM. In contrast, the rapidly inactivating β3b auxiliary produces a leftward shift in activation at Ca²+ below 1 μM. In the companion work (Lingle, C.J., X.-H. Zeng, J.-P. Ding, and X.-M. Xia. 2001. J. Gen. Physiol. 117:583–605, this issue), we have shown that some of the apparent β3b-mediated shift in activation at low Ca²+ arises from rapid unblocking of inactivated channels, unlike the actions of the β1 and β2 subunits. Here, we compare effects of the β3b 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 β3b subunit and compare it to β3b constructs lacking either the NH₂- or COOH terminus or both. The results demonstrate that, although the NH₂ terminus appears to be the primary determinant of the β3b-mediated shift in V_0.5 at low Ca²+, removal of the NH₂ terminus reveals two other interesting aspects of the action of the β3b subunit. First, the conductance-voltage curves for activation of channels containing the β3b subunit are best described by a double Boltzmann shape, which is proposed to arise from two independent voltage-dependent activation steps. Second, the presence of the β3b subunit results in channels that exhibit an anomalous instantaneous outward current rectification that is correlated with a voltage dependence in the time-averaged single-channel current. The two effects appear to be unrelated, but indicative of the variety of ways that interactions between β and α subunits can affect BK channel function. The COOH terminus of the β3b subunit produces no discernible functional effects.