Evidence for two concentration-dependent processes for beta-subunit effects on alpha 1B calcium channels

beta -Subunits of voltage-dependent Ca2+ channels regulate both their expre ssion and biophysical properties. We have injected a range of concentration s of beta3-cDNA into Xenopus oocytes, with a fixed concentration of alpha 1 B (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 condu ctance on beta3-protein occurs with a midpoint around the endogenous concen tration 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 depen dence of steady-state inactivation provides evidence for two channel popula tions, interpreted as representing alpha1 beta without or with a beta3-subu nit, bound with a lower affinity of 120 nM. Third, the effect of beta3 on t he facilitation rate of G-protein-modulated alpha 1B currents during a depo larizing prepulse to +100 mV provides evidence for the same two populations , with the rapid facilitation rate being attributed to G beta gamma dissoci ation from the beta -subunit-bound alpha 1B channels. The data are discusse d in terms of two hypotheses, either binding of two beta -subunits to the a lpha1 beta channel or a state-dependent alteration in affinity of the chann el for the beta -subunit.