Modulation of relative intrinsic activity of agonists at the alpha-2A adrenoceptor by mutation of residue 351 of G protein Gi1α

Compared with epinephrine, the relative intrinsic activity of a series of partial agonists to activate fusion proteins between the porcine alpha-2A adrenoceptor and the cr-subunit of Gi, was reduced after a single-point mutation (Cys(351)Gly) in the G protein. Although UK14304 was close to a full agonist at the fusion construct containing wild-type (Cys(351))G(i1 alpha), it was a partial agonist at that containing Gly(351)G(i1 alpha). Moreover, although clonidine functioned as a good partial agonist to activate the fusion protein containing Cys(351)G(i1 alpha), it was essentially an antagonist at the Gly(351)G(i1 alpha)-containing fusion protein. By contrast, incorporation of Ile(351)G(i1 alpha) into the fusion protein resulted in all partial agonists displaying higher intrinsic activity relative to epinephrine to activate this fusion protein than the one containing the wild-type G protein sequence. This is the first demonstration that the relative intrinsic activity of a series of agonists can be modified by a point mutation in a G protein rather than a receptor and indicates that the nature of a key contact site between a G protein and a receptor can selectively regulate partial agonist function. We provide a model for this based on the hydrophobicity of a key receptor-G protein alpha-subunit interaction interface.