Multiple modulation pathways of calcium channel activity by a beta subunit. Direct evidence of beta subunit participation in membrane trafficking of the alpha1C subunit.

In order to study the precise mechanisms of alpha1 subunit modulation by an auxiliary beta subunit of voltage-dependent calcium channels, a recombinant beta3 subunit fusion protein was produced and introduced into oocytes that express the human alpha1C subunit. Injection of the beta3 subunit protein rapidly modulated the current kinetics and voltage dependence of activation, whereas massive augmentation of peak current amplitude occurred over a longer time scale. Consistent with the latter, a severalfold increase in the amount of the alpha1C subunit in the plasma membrane was detected by quantitative confocal laser-scanning microscopy after beta3 subunit injection. Pretreatment of oocytes with bafilomycin A1, a vacuolar type H+-ATPase inhibitor, abolished the increase of the alpha1C subunit in the plasma membrane, attenuated current increase, but did not affect the modulation of current kinetics and voltage dependence by the beta3 subunit. These results provide clear evidence that the beta subunit modifies the calcium channel complex in a binary fashion; one is an allosteric modulation of the alpha1 subunit function and the other is a chaperoning of the alpha1 subunit to the plasma membrane.