Consequences of the stoichiometry of Slo1, α and auxiliary β subunits on functional properties of large-conductance Ca²⁺-activated K⁺ channels

Auxiliary β subunits play a major role in defining the functional properties of large-conductance, Ca²⁺-dependent BK-type K⁺ channels. In particular, both the β1 and β2 subunits produce strong shifts in the voltage dependence of channel activation at a given Ca²⁺. β subunits are thought to coassemble with α subunits in a 1:1 stoichiometry, such that a full ion channel complex may contain up to four β subunits per channel. However, previous results raise the possibility that ion channels with less than a full complement of β subunits may also occur. The functional consequence of channels with differing stoichiometries remains unknown. Here, using expression of α and β subunits in Xenopus oocytes, we show explicitly that functional BK channels can arise with less than four β subunits. Furthermore, the results show that, for both β1 and β2 subunits, each individual β subunit produces an essentially identical, incremental effect on the voltage dependence of gating. For channels arising from α + β2 subunits, the number of β2 subunits per channel also has a substantial impact on properties of steady-state inactivation and recovery from inactivation. Thus, the stoichiometry of α:β subunit assembly can play a major functional role in defining the apparent Ca²⁺ dependence of activation of BK channels and in influencing the availability of BK channels for activation.