Extracellular interaction of the voltage-dependent Ca2+ channel alpha(2)delta and alpha(1) subunits

The role of the extracellular domain of the voltage-dependent Ca2+ channel alpha(2) delta subunit in assembly with the alpha(1C) subunit was investigated. Transiently transfected tsA201 cells processed the alpha(2) delta subunit properly as disulfide linkages and cleavage sites between the alpha(2) and delta subunits were shown to be similar to native channel protein. Coimmunoprecipitation experiments demonstrated that in the absense of delta subunits, alpha(2) subunits do not assemble with alpha(1) subunits. Furthermore, the transmembrane and cytoplasmic sequences in delta can be exchanged with those of an unrelated protein without any effect on the association between the alpha(2) delta and alpha(1) proteins. Extracellular domains of the alpha(2) delta subunits are also shown to be responsible for increasing the binding affinity of [H-3]PN200-11- (isopropyl-4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5-([H-3]methoxycarbonyl)-pyridine-3-carboxylate) for the alpha(1C) subunit. Investigation of the corresponding interaction site on the alpha(1) subunit revealed that although tryptic peptides containing repeat III of native alpha(1S) subunit remain in association with the alpha(2) delta subunit during wheat germ agglutinin chromatography, repeat III by itself is not sufficient for assembly with the alpha(2) delta subunit likely interacts with more than one extracellular loop of the alpha(1) subunit.