Differential effects of beta 1 and beta 2 subunits on BK channel activity

High conductance, calcium- and voltage-activated potassium (BK) channels are widely expressed in mammals. In some tissues, the biophysical properties of BK channels are highly affected by coexpression of regulatory (beta) subunits. beta 1 and beta 2 subunits increase apparent channel calcium sensitivity. The beta 1 subunit also decreases the voltage sensitivity of the channel and the beta 2 subunit produces an N-type inactivation of BK currents. We further characterized the effects of the beta 1 and beta 2 subunits on the calcium and voltage sensitivity of the channel, analyzing the data in the context of an allosteric model for BK channel activation by calcium and voltage (Horrigan and Aldrich, 2002). In this study, we used a beta 2 subunit without its N-type inactivation domain (beta 2IR). The results indicate that the beta 2IR subunit, like the beta 1 subunit, has a small effect on the calcium binding affinity of the channel. Unlike the beta 1 subunit, the beta 2IR subunit also has no effect on the voltage sensitivity of the channel. The limiting voltage dependence for steady-state channel activation, unrelated to voltage sensor movements, is unaffected by any of the studied beta subunits. The same is observed for the limiting voltage dependence of the deactivation time constant. Thus, the beta 1 subunit must affect the voltage sensitivity by altering the function of the voltage sensors of the channel. Both beta subunits reduce the intrinsic equilibrium constant for channel opening (L-0). In the allosteric activation model, the reduction of the voltage dependence for the activation of the voltage sensors accounts for most of the macroscopic steady-state effects of the beta 1 subunit, including the increase of the apparent calcium sensitivity of the BK channel. All allosteric coupling factors need to be increased in order to explain the observed effects when the alpha subunit is coexpressed with the beta 2IR subunit