Evidence for two concentration-dependent processes for β-subunit effects on α1B calcium channels

beta -Subunits of voltage-dependent Ca(2+) channels regulate both their expression and biophysical properties. We have injected a range of concentrations of beta3-cDNA into Xenopus oocytes, with a fixed concentration of alpha 1B (Ca(v)2.2) cDNA, and have quantified the corresponding linear increase of beta3 protein. The concentration dependence of a number of beta3-dependent processes has been studied. First, the dependence of the a1B maximum conductance on beta3-protein occurs with a midpoint around the endogenous concentration of beta3 (similar to 17 nM). This may represent the interaction of the beta -subunit, responsible for trafficking, with the I-II linker of the nascent channel. Second, the effect of beta3-subunits on the voltage dependence of steady-state inactivation provides evidence for two channel populations, interpreted as representing alpha1 beta without or with a beta3-subunit, bound with a lower affinity of 120 nM. Third, the effect of beta3 on the facilitation rate of G-protein-modulated alpha 1B currents during a depolarizing prepulse to +100 mV provides evidence for the same two populations, with the rapid facilitation rate being attributed to G beta gamma dissociation from the beta -subunit-bound alpha 1B channels. The data are discussed in terms of two hypotheses, either binding of two beta -subunits to the alpha1 beta channel or a state-dependent alteration in affinity of the channel for the beta -subunit.