Differential effects of Ca2+ channel β(1a) and β(2a) subunits on complex formation with α(1S) and on current expression in tsA201 cells

Abstract

To study the interactions of the α(1S) subunit of the skeletal muscle L-type Ca2+ channel with the skeletal β(1a) and the cardiac β(2a), these subunits were expressed alone or in combination in tsA201 cells. Immunofluorescence- and green fluorescent protein-labeling showed that, when expressed alone, β(1a) was diffusely distributed throughout the cytoplasm, β(2a) was localized in the plasma membrane, and α(1S) was concentrated in a tubular/reticular membrane system, presumably the endoplasmic reticulum (ER). Upon coexpression with α(1S), β(1a) became colocalized with α(1S) in the ER. Upon coexpression with β(2a), α(1S) redistributed to the plasma membrane, where it aggregated in large cluster. Coexpression of α(1S) with β(1a), but not with β(2a) increased the frequency at which cells expressed L-type currents. A point mutation (α(1S)-Y366S) or deletion (α(1S)-Δ351- 380) in the β interaction domain of α(1S) blocked both translocation β(1a) the ER and β(2a)-induced translocation of the α(1S) mutants to the plasma membrane. However, the point mutation did not interfere with β(1a)-induced current stimulation. Thus, β(1a) and β(2a) are differentially distributed in tsA201 cells and upon coexpression with α(1S), form α(1S)·β complexes in different cellular compartments. Complex formation but not current stimulation requires the intact β interaction domain in the I-II cytoplasmic loop of α(1S).