Structure, Function, and Dynamics of the Gα Binding Domain of Ric-8A

Ric-8A is a 530-amino acid cytoplasmic molecular chaperone and guanine nucleotide exchange factor (GEF) for i, q, and 12/13 classes of heterortrimeric G protein alpha subunits (Gα). We report the 2.2-Å crystal structure of the Ric-8A Gα-binding domain with GEF activity, residues 1-452, and is phosphorylated at Ser435 and Thr440. Residues 1-429 adopt a superhelical fold comprised of Armadillo (ARM) and HEAT repeats, and the C terminus is disordered. One of the phosphorylated residues potentially binds to a basic cluster in an ARM motif. Amino acid sequence conservation and published hydrogen-deuterium exchange data indicate repeats 3 through 6 to be a putative Gα-binding surface. Normal mode modeling of small-angle X-ray scattering data indicates that phosphorylation induces relative rotation between repeats 1-4, 5-6, and 7-9. 2D ¹H-¹⁵N-TROSY spectra of [²H,¹⁵N]-labeled Gαi1 in the presence of R452 reveals chemical shift perturbations of the C terminus and Gαi1 residues involved in nucleotide binding.