Heterotrimeric G-Proteins are GTPases composed of α-, β-, and γ-subunits. These GTPases are classically associated with plasma membrane receptors containing seven transmembrane domains (G protein-coupled receptors [GPCRs]). Receptor activation activates the G-Protein by inducing the exchange of GTP for GDP at a binding site on Gα. Gα and/or Gβγ, then goes on to interact with effector proteins.
In mammals, about 20 Gα subunits (39 to 46 kD), about 5 distinct Gβ (ca. 35 kD) and at least 12 Gγ subunits (from 7 to 10 kD) have been identified.