Direct interaction of gbetagamma with a C-terminal gbetagamma-binding domain of the Ca2+ channel alpha1 subunit is responsible for channel inhibition by G protein-coupled receptors

Several classes of voltage-gated Ca2+ 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 Ca2+ channels the executing arm of the activated G protein is its beta gamma dimer (G beta gamma). We report below the existence of two G beta gamma-binding sites on the A-, B-, and E-type alpha(1) subunits that form non-L-type Ca2+ channels. One, reported previously, is in loop 1 connecting transmembrane domains I and Il, The second is located approximately in the middle of the ca, 600-aa-long C-terminal tails, Both G beta gamma-binding regions also bind the Ca2+ channel beta subunit (CC beta), which, when overexpressed, interferes with inhibition by activated G proteins, Replacement in alpha(1E) Of loop 1 with that of the G protein-insensitive and G beta gamma-binding-negative loop 1 of alpha(1C) did not abolish inhibition by G proteins, but the exchange of the alpha(1E) C terminus with that of alpha(1C) did, This and properties of alpha(1E) C-terminal truncations indicated that the G beta gamma-binding site mediating the inhibition of Ca2+ channel activity is the one in the C terminus, Binding of G beta gamma to this site mas inhibited by an alpha(1)-binding domain of CC beta, thus providing an explanation for the functional antagonism existing between CC beta and G protein inhibition. The data do not support proposals that G beta gamma inhibits alpha(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.