Translocatable voltage-gated Ca channel β subunits in α1-β complexes reveal competitive replacement yet no spontaneous dissociation.

β subunits of high voltage-gated Ca (Ca) channels promote cell-surface expression of pore-forming α1 subunits and regulate channel gating through binding to the α-interaction domain (AID) in the first intracellular loop. We addressed the stability of Ca α1B-β interactions by rapamycin-translocatable Ca β subunits that allow drug-induced sequestration and uncoupling of the β subunit from Ca2.2 channel complexes in intact cells. Without Ca α1B/α2δ1, all modified β subunits, except membrane-tethered β2a and β2e, are in the cytosol and rapidly translocate upon rapamycin addition to anchors on target organelles: plasma membrane, mitochondria, or endoplasmic reticulum. In cells coexpressing Ca α1B/α2δ1 subunits, the translocatable β subunits colocalize at the plasma membrane with α1B and stay there after rapamycin application, indicating that interactions between α1B and bound β subunits are very stable. However, the interaction becomes dynamic when other competing β isoforms are coexpressed. Addition of rapamycin, then, switches channel gating and regulation by phosphatidylinositol 4,5-bisphosphate [PI(4,5)P] lipid. Thus, expression of free β isoforms around the channel reveals a dynamic aspect to the α1B-β interaction. On the other hand, translocatable β subunits with AID-binding site mutations are easily dissociated from Ca α1B on the addition of rapamycin, decreasing current amplitude and PI(4,5)P sensitivity. Furthermore, the mutations slow Ca2.2 current inactivation and shift the voltage dependence of activation to more positive potentials. Mutated translocatable β subunits work similarly in Ca2.3 channels. In sum, the strong interaction of Ca α1B-β subunits can be overcome by other free β isoforms, permitting dynamic changes in channel properties in intact cells.