Backbone resonance assignments for G protein αi3 subunit in the GDP-bound state

Guanine-nucleotide binding proteins (G proteins) serve as molecular switches in signaling pathways, by coupling the activation of G protein-coupled receptors (GPCRs) at the cell surface to intracellular responses. In the resting state, G protein forms a heterotrimer, consisting of the G protein alpha subunit with GDP (G alpha center dot GDP) and the G protein beta gamma subunit (G beta gamma). Ligand binding to GPCRs promotes the GDP-GTP exchange on G alpha, leading to the dissociation of the GTP-bound form of G alpha (G alpha center dot GTP) and G beta gamma. Then, G alpha center dot GTP and G beta gamma bind to their downstream effector enzymes or ion channels and regulate their activities, leading to a variety of cellular responses. Finally, G alpha hydrolyzes the bound GTP to GDP and returns to the resting state by re-associating with G beta gamma. The G proteins are classified with four major families based on the amino acid sequences of G alpha: i/o, s, q/11, and 12/13. Here, we established the backbone resonance assignments of human G alpha(i3), a member of the i/o family with a molecular weight of 41 K, in complex with GDP. The chemical shifts were compared with those of G alpha(i3) in complex with a GTP-analogue, GTP gamma S, which we recently reported, indicating that the residues with significant chemical shift differences are mostly consistent with the regions with the structural differences between the GDP- and GTP gamma S-bound states, as indicated in the crystal structures. The assignments of G alpha(i3)center dot GDP would be useful for the analyses of the dynamics of G alpha(i3) and its interactions with various target molecules.