Evidence for Two Concentration-Dependent Processes for β-Subunit Effects on α1B Calcium Channels

Abstractβ-Subunits of voltage-dependent Ca2+ channels regulate both their expression and biophysical properties. We have injected a range of concentrations of β3-cDNA into Xenopus oocytes, with a fixed concentration of α1B (CaV2.2) cDNA, and have quantified the corresponding linear increase of β3 protein. The concentration dependence of a number of β3-dependent processes has been studied. First, the dependence of the a1B maximum conductance on β3-protein occurs with a midpoint around the endogenous concentration of β3 (∼17nM). This may represent the interaction of the β-subunit, responsible for trafficking, with the I-II linker of the nascent channel. Second, the effect of β3-subunits on the voltage dependence of steady-state inactivation provides evidence for two channel populations, interpreted as representing α1B without or with a β3-subunit, bound with a lower affinity of 120nM. Third, the effect of β3 on the facilitation rate of G-protein-modulated α1B currents during a depolarizing prepulse to +100mV provides evidence for the same two populations, with the rapid facilitation rate being attributed to Gβγ dissociation from the β-subunit-bound α1B channels. The data are discussed in terms of two hypotheses, either binding of two β-subunits to the α1B channel or a state-dependent alteration in affinity of the channel for the β-subunit