Abstract

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 α-subunit of G_i1 was reduced after a single-point mutation (Cys³⁵¹Gly) in the G protein. Although UK14304 was close to a full agonist at the fusion construct containing wild-type (Cys³⁵¹)G_i1α, it was a partial agonist at that containing Gly³⁵¹G_i1α. Moreover, although clonidine functioned as a good partial agonist to activate the fusion protein containing Cys³⁵¹G_i1α, it was essentially an antagonist at the Gly³⁵¹G_i1α-containing fusion protein. By contrast, incorporation of Ile³⁵¹G_i1α 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 α-subunit interaction interface.