Abstract
Specific domains of the G-protein a subunit have been shown to control coupling to heptahelical receptors, The extreme N and C termini and a region between alpha4 and alpha5 helices of the G-protein alpha subunit are known to determine selective interaction with the receptors, The metabotropic glutamate receptor 2 activated both mouse G alpha (15) and its human homologue G alpha (16), whereas metabotropic glutamate receptor 8 activated G alpha (15) only. The extreme C-terminal 20 amino acid residues are identical between the G alpha (15) and G alpha (16) and are therefore unlikely to be involved in coupling selectivity, Our data reveal two regions on G alpha (16) that inhibit its coupling to metabotropic glutamate receptor 8, On a three-dimensional model, both regions are found in a close proximity to the extreme C terminus of G alpha (16).
One module comprises alpha4 helix, alpha4-beta6 loop (L9 Loop), beta6 sheet, and alpha5 helix. The other, not described previously, is located within the loop that links the N-terminal alpha helix to the beta1 strand of the Res-like domain of the alpha subunit, Coupling of G alpha (16) protein to the metabotropic glutamate receptor 8 is partially modulated by each module alone, whereas both modules are needed to eliminate the coupling fully