IDENTIFICATION OF A 3-AMINO-ACID REGION IN G-PROTEIN GAMMA(1) AS A DETERMINANT OF SELECTIVE BETA-GAMMA HETERODIMERIZATION

Guanine-nucleotide-binding protein beta and gamma subunits belong to l arge protein families encompassing at least five and ten members, resp ectively, from mammalian cells. The formation of the stable beta gamma heterodimers is a selective process determined by the primary sequenc es of both the beta and the gamma subunit. For example, gamma(2) dimer izes with both beta(1) and beta(2), gamma(1) with beta(1), but not wit h beta(2). To identify the structural elements of gamma subunits relev ant to the selectivity of beta gamma dimerization, we have used the ba culovirus-insect cell-expression system to produce chimeric beta and g amma subunits and have studied their dimerization using an assay based on the ability of isoprenylation-resistant gamma subunit mutants to d raw beta subunits into the cytosol and including sucrose density gradi ent analysis of soluble recombinant beta gamma dimers. The results sho w that replacement of three consecutive residues of gamma(1), Cys36-Cy s37-Glu38, by the corresponding residues of gamma(2), Ala33-Ala34-Ala3 5, suffices to render the mutant gamma(1) subunit capable of forming h eterodimers with beta(2). The ability of mutant gamma(1) subunits to d imerize with beta(2) does not correlate with the probability of the mu tated region to participate in coiled-coil structures. The tripeptide region identified here as a critical determinant of the selectivity of beta gamma dimer formation is distinct from, but partially overlaps w ith, the region reported by Lee et al. [Lee, C., Murakami, T. & Simond s, W. F. (1995) J. Biol. Chem. 270, 8779-8784]. The results of this st udy, therefore, not only extend the region of gamma(1) selecting betwe en beta(1) and beta(2) to the five-residue sequence between Cys36 and Phe40, but also argue against the notion that the hydrophobic terminal residue of this motif represents the key determinant of selective bet a gamma interaction.