Translocatable voltage-gated Ca2+channel β subunits in α1–β complexes reveal competitive replacement yet no spontaneous dissociation

Significance Voltage-gated Ca2+ (CaV ) channels have an α1-α2δ core complexed with one of several alternative β subunits. Contradictory evidence says that, once bound, (i ) a β subunit is permanently associated with the α1-α2δ core or (ii ) that it is free to be exchanged for other β subunits. We designed rapamycin-translocatable CaV β subunits that allow drug-induced sequestration of free β subunits to several organelle anchors. Sequestering free subunits does not dissociate bound subunits from channels except when the binding site is mutated to weaken the interaction. Nevertheless, our rapamycin constructs show that, when nontranslocatable β subunits are coexpressed with a translocatable subunit, sequestering the translocatable subunit changes the channel properties, revealing a quick replacement by the nontranslocatable subunit in the channel complex.