Crystal and cryo-EM structures of the cytosolic G protein alpha chaperone and guanine nucleotide exchange factor Ric-8A bound to Gαi1

Ric-8A is a cytosolic Guanine Nucleotide exchange Factor (GEF) that activates heterotrimeric G protein alpha subunits (Gα). Ric-8A is essential to life in multicellular eukaryotes by virtue of its chaperone activity that is required for Gα biogenesis and membrane localization. Ric-8A adopts an armadillo (ARM)/HEAT repeat domain architecture and is structurally unrelated to G Protein-Coupled Receptors (GPCR). Both GEF and chaperone activities are stimulated by Casein Kinase II phosphorylation. The mechanisms by which Ric-8A catalyzes GDP release and GTP binding to Gα, or exerts chaperone activity are unknown. Here, we report the structure of the nanobody-stabilized complex of nucleotide-free Gαi1 (isoform 1 of Gα family i) and phosphorylated Ric-8A at near atomic resolution by cryo-electron microscopy and X-ray crystallography. We find that Ric-8A envelops the GTPase domain of Gα, disrupting all three switch regions that convey Gα nucleotide-binding and signaling activity, and displaces the C-terminal helix and helical domain of Gα. These cooperative interactions dismantle the GDP binding site and promote GDP release, while protecting structural elements of Gα that are dynamic in the nucleotide-free state. The structures also show how phosphorylation stabilizes Gα-binding elements of Ric-8A, thereby enhancing its GEF and chaperone activities.