Role of the isoprenyl pocket of the G protein beta gamma subunit complex in the binding of phosducin and phosducin-like protein.

Phosducin (Pdc) and phosducin-like protein (PhLP) regulate G protein-mediated signaling by binding to the βγ subunit complex of heterotrimeric G proteins (Gβγ) and removing the dimer from cell membranes. The binding of Pdc induces a conformational change in the β-propeller structure of Gβγ, creating a pocket between blades 6 and 7. It has been proposed that the isoprenyl group of Gβγ inserts into this pocket, stabilizing the Pdc.Gβγ structure and decreasing the affinity of the complex for the lipid bilayer. To test this hypothesis, the binding of Pdc and PhLP to several Gβγ dimers containing variants of the β or y subunit was measured. These variants included modifications of the isoprenyl group (y), residues involved in the conformational change (β), and residues lining the proposed prenyl pocket (β). Switching prenyl groups from farnesyl to geranylgeranyl or vice versa had little effect on binding. However, alanine substitution of one residue in the β subunit involved in the conformational change (W332) decreased binding 5-fold. Alanine substitution of certain residues within the prenyl pocket caused only minor decreases in binding, while a lysine substitution of T329 within the pocket inhibited binding 10-fold. Molecular modeling of the binding energy of the Pdc.Gβ 1 γ 2 complex required insertion of the geranylgeranyl group into the prenyl pocket in order to accurately predict the effects of prenyl pocket amino acid substitutions. Finally, a dimer containing a y subunit with no prenyl group (γ 2 -C68S) decreased binding by nearly 20-fold. These results support the structural model in which the prenyl group escapes contact with the aqueous milieu by inserting into the prenyl pocket and stabilizing the Pdc-binding conformation of Gβγ.