Role of the prenyl group on the G protein γ subunit in coupling trimeric G proteins to A1 adenosine receptors

Abstract

The coupling of receptors to heterotrimeric G proteins is determined by interactions between the receptor and the G protein α subunits and by the composition of the βγ dimers. To determine the role of the γ subunit prenyl modification in this interaction, the CaaX motifs in the γ1 and γ2 subunits were altered to direct modification with different prenyl groups, recombinant βγ dimers expressed in the baculovirus/Sf9 insect cell system, and the dimers purified. The activity of the βγ dimers was compared in two assays: formation of the high affinity agonist binding conformation of the A1 adenosine receptor and receptor-catalyzed exchange of GDP for GTP on the a subunit. The β1γ1 dimer (modified with farnesyl) was significantly less effective than β1γ2 (modified with geranylgeranyl) in either assay. The β1γ1-S74L dimer (modified with geranylgeranyl) was nearly as effective as β1γ2 in either assay. The β1γ2-L71S dimer (modified with farnesyl) was significantly less active than β1γ2. Using 125I-labeled βγ subunits, it was determined that native and altered βγ dimers reconstituted equally well into Sf9 membranes containing A1 adenosine receptors. These data suggest that the prenyl group on the γ subunit is an important determinant of the interaction between receptors and G protein γ subunits.