The Extreme C-Terminal Region of G{alpha}s Differentially Couples to the Luteinizing Hormone and {beta}2-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 Galpha(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 Galpha(s) subunits in LHR(+) s49 ck cells that lack endogenous Galpha(s). On the basis of studies involving truncations, mutations, and chain extensions of Galpha(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 Galpha(s) CT, are supported by the experimental observations and are characterized by the interaction of the four extreme CT amino acid residues of Galpha(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 Galpha(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 Galpha(s) recognition by the two receptors.