Summary: Many chaperones promote nascent polypeptide folding followed by substrate release through ATP-dependent conformational changes. Here we show cryoEM structures of Gα subunit folding intermediates in complex with full-length Ric-8A, a unique chaperone-client system in which substrate release is facilitated by guanine nucleotide binding to the client G protein. The structures of Ric-8A-Gαi and Ric-8A-Gαq complexes reveal that the chaperone employs its extended C-terminal region to cradle the Ras-like domain of Gα, positioning the Ras core in contact with the Ric-8A core while engaging its switch2 nucleotide binding region. The C-terminal α5 helix of Gα is held away from the Ras-like domain through Ric-8A core domain interactions, which critically depend on recognition of the Gα C terminus by the chaperone. The structures, complemented with biochemical and cellular chaperoning data, support a folding quality control mechanism that ensures proper formation of the C-terminal α5 helix before allowing GTP-gated release of Gα from Ric-8A. : Seven et al. present cryoEM structures of Gα subunit folding intermediates in complex with their universal chaperone, Ric-8. Ric-8 forms key interactions with the Ras domain to prepare GTP-gated release of Gα subunits from their chaperone. The structures, complemented by biochemical and cellular experiments, reveal a folding quality control mechanism. Keywords: G protein alpha subunit, Ric-8, molecular chaperone, Gα, folding, guanine nucleotide exchange factor, GEF, cryoEM structure, protein complex, G protein-coupled receptor, GPCR, phosphorylation, quality control
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