Multiple modulation pathways of calcium channel activity by a β subunit -: Direct evidence of β subunit participation in membrane trafficking of the α1c subunit

In order to study the precise mechanisms of alpha(1) subunit modulation by an auxiliary beta subunit of voltage-dependent calcium channels, a recombinant beta(3) subunit fusion protein was produced and introduced into oocytes that express the human alpha(1C) subunit. Injection of the beta 3 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 alpha(1C) subunit in the plasma membrane was detected by quantitative confocal laser-scanning microscopy after beta(3) subunit injection, Pretreatment of oocytes with bafilomycin A(1), a vacuolar type H+-ATPase inhibitor, abolished the increase of the alpha(1C) subunit in the plasma membrane, attenuated current increase, but did not affect the modulation of current kinetics and voltage dependence by the beta(3) 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 alpha(1) subunit function and the other is a chaperoning of the alpha(1) subunit to the plasma membrane.