Structural and molecular characterization of a preferred protein interaction surface on G protein βγ subunits

G protein beta gamma subunits associate with many binding partners in cellular signaling cascades. In previous work, we used random-peptide phage display screening to identify a diverse family of peptides that bound to a common surface on G beta gamma subunits and blocked a subset of G beta gamma effectors. Later studies showed that one of the peptides caused G protein activation through a novel G beta gamma-dependent, nucleotide exchange-independent mechanism. Here we report the X-ray crystal structure of G beta(1)gamma(2) bound to this peptide, SIGK (SIGKAFKILGYPDYD), at 2.7 angstrom resolution. SIGK forms a helical structure that binds the same face of Get as the switch II region of G alpha. The interaction interface can be subdivided into polar and nonpolar interfaces that together contain a mixture of binding determinants that may be responsible for the ability of this surface to recognize multiple protein partners. Systematic mutagenic analysis of the peptide-G alpha gamma interface indicates that distinct sets of amino acids within this interface are required for binding of different peptides. Among these unique amino acid interactions, specific electrostatic binding contacts within the polar interface are required for peptide-mediated subunit dissociation. The data provide a mechanistic basis for multiple target recognition by G alpha gamma subunits with diverse functional interactions within a common interface and suggest that pharmacological targeting of distinct regions within this interface could allow for selective manipulation of G beta gamma-dependent signaling pathways.