The Extreme C-Terminal Region of Gαs Differentially Couples to the Luteinizing Hormone and β2-Adrenergic Receptors

The mechanisms of G protein coupling to G protein-coupled receptors (GPCR) share general characteristics but may exhibit specific interactions unique for each GPCR/G protein partnership. The extreme C terminus (CT) of G protein alpha-subunits has been shown to be important for association with GPCR. Hypothesizing that the extreme CT of G alpha(s) is an essential component of the molecular landscape of the GPCR, human LH receptor (LHR), and beta(2)-adrenergic receptor (beta(2)-AR), a model cell system was created for the expression and manipulation of G alpha(s) subunits in LHR+ s49 ck cells that lack endogenous G alpha(s). On the basis of studies involving truncations, mutations, and chain extensions of G alpha(s), the CT was found to be necessary for LHR and beta(2)-AR signaling. Some general similarities were found for the responses of the two receptors, but significant differences were also noted. Computational modeling was performed with a combination of comparative modeling, molecular dynamics simulations, and rigid body docking. The resulting models, focused on the G alpha(s) CT, are supported by the experimental observations and are characterized by the interaction of the four extreme CT amino acid residues of G alpha(s) with residues in LHR and beta(2)-AR helix 3, (including R of the DRY motif), helix 6, and intracellular loop 2. This portion of G alpha(s) recognizes the same regions of the two GPCR, although with differences in the details of selected interactions. The predicted longer cytosolic extensions of helices 5 and 6 of beta(2)-AR are expected to contribute significantly to differences in G alpha(s) recognition by the two receptors. (Molecular Endocrinology 25: 1416-1430, 2011)