Inactivation of BK channels mediated by the NH2 terminus of the beta 3b auxiliary subunit involves a two-step mechanism: Possible separation of binding and blockade

A family of auxiliary beta subunits coassemble with Slo alpha subunit to fo rm Ca2+-regulated, voltage-activated BK-type K+ channels. The beta subunits play an important role in regulating the functional properties of the resu lting channel protein, including apparent Ca2+ dependence and inactivation. The beta 3b auxiliary subunit, when coexpressed with the Slo alpha subunit , results in a particularly rapid (similar to1 ms), but incomplete inactiva tion, mediated by the cytosolic NH2 terminus of the beta 3b subunit (Xia et al., 2000). Here, we evaluate whether a simple block of the open channel b y the NH2-terminal domain accounts for the inactivation mechanism. Analysis of the onset of block, recovery from block, time-dependent changes in the shape of instantaneous current-voltage curves, and properties of deactivati on tails suggest that a simple, one step blocking reaction is insufficient to explain the observed currents. Rather, blockade can be largely accounted for by a two-step blocking mechanism (C-n reversible arrow O-n reversible arrow O-n(.) reversible arrow I-n) in which preblocked open states (O-n*) p recede blocked states (I-n). The transitions between O* and I are exceeding ly rapid accounting for an almost instantaneous block or unblock of open ch annels observed with changes in potential. However, the macroscopic current relaxations are determined primarily by slower transitions between O and O *. We propose that the O to O" transition corresponds to binding of the NH, terminal inactivation domain to a receptor site. Blockade of current subseq uently reflects either additional movement of the NH2-terminal domain into a position that hinders ion permeation or a gating transition to a closed s tate induced by binding of the NH2 terminus.