Direct interaction of Gβγ with a C-terminal Gβγ-binding domain of the Ca(2+) channel α(1) subunit is responsible for channel inhibition by G protein-coupled receptors

Several classes of voltage-gated Ca(2+) channels (VGCCs) are inhibited by G proteins activated by receptors for neurotransmitters and neuromodulatory peptides. Evidence has accumulated to indicate that for non-L-type Ca(2+) channels the executing arm of the activated G protein is its βγ dimer (Gβγ). We report below the existence of two Gβγ-binding sites on the A-, B-, and E-type α(1) subunits that form non-L-type Ca(2+) channels. One, reported previously, is in loop 1 connecting transmembrane domains I and II. The second is located approximately in the middle of the ca. 600-aa-long C-terminal tails. Both Gβγ-binding regions also bind the Ca(2+) channel β subunit (CCβ), which, when overexpressed, interferes with inhibition by activated G proteins. Replacement in α(1E) of loop 1 with that of the G protein-insensitive and Gβγ-binding-negative loop 1 of α(1C) did not abolish inhibition by G proteins, but the exchange of the α(1E) C terminus with that of α(1C) did. This and properties of α(1E) C-terminal truncations indicated that the Gβγ-binding site mediating the inhibition of Ca(2+) channel activity is the one in the C terminus. Binding of Gβγ to this site was inhibited by an α(1)-binding domain of CCβ, thus providing an explanation for the functional antagonism existing between CCβ and G protein inhibition. The data do not support proposals that Gβγ inhibits α(1) function by interacting with the site located in the loop I–II linker. These results define the molecular mechanism by which presynaptic G protein-coupled receptors inhibit neurotransmission.