Phosducin is a soluble phosphoprotein found in retinal photoreceptor cells and in the pineal gland. It binds to the βγ subunits of guanine nucleotide-binding proteins (G proteins) (Gβγ) and may regulate G-protein function. In this study, the ability of specific regions of phosducin to bind Gβγ was characterized. A series of deletion mutants were made in bovine phosducin. They were tested in cotransfection assays for their ability to inhibit Gβγ-mediated phospholipase C β_2 isoform activation. Overexpression of the N-terminal half of phosducin showed inhibition, whereas overexpression of the C-terminal half did not. The first 63 amino acid residues were required for inhibition. A tryptophan-to-valine substitution at residue 29, which is part of a well conserved 11-amino acid sequence, severely impaired phosducin inhibitory function. Glutathione S-transferase-phosducin fusion proteins were expressed in Escherichia coli to study phosducin-Gβγ interaction in vitro. The N-terminal 63-amino acid fragment was able to bind to Gβγ. In contrast, the C-terminal half failed to bind to Gβγ. The substitution mutants showed little or no binding. Furthermore, direct measurements of interaction between Gβγ and fragments of phosducin, using surface plasmon resonance technology, confirmed the assignment of binding activity to the 63-amino acid fragment and the importance of the tryptophan residu
