The C-terminal tail preceding the CAAX box of a yeast G protein gamma subunit is dispensable for receptor-mediated G protein activation in vivo

Structural determinants involved in the formation and activation of G protein betagamma dimers

Heterotrimeric G proteins, composed of an alpha, beta and gamma subunit, represent one of the most important and dynamic families of signaling proteins. As a testament to the significance of G protein signaling, the hundreds of seven-transmembrane-spanning receptors that interact with G proteins are estimated to occupy 1-2% of the human genome. This broad diversity of receptors is echoed in the number of potential heterotrimer combinations that can arise from the 23 alpha subunit, 7 beta subunit and 12 gamma subunit isoforms that have been identified. The potential for such vast complexity implies that the receptor G protein interface is the site of much regulation. The historical model for the activation of a G protein holds that activated receptor catalyzes the exchange of GDP for GTP on the alpha subunit, inducing a conformational change that substantially lowers the affinity of alpha for betagamma. This decreased affinity enables dissociation of betagamma from alpha and receptor. The free form of betagamma is thought to activate effectors, until the hydrolysis of GTP by G alpha (aided by RGS proteins) allows the subunits to re-associate, effectively deactivating the G protein until another interaction with activated receptor.

Dopamine Receptor-interacting Protein 78 Acts as a Molecular Chaperone for Gγ Subunits before Assembly with Gβ

Heterotrimeric G proteins play a central role in intracellular communication mediated by extracellular signals, and both Galpha and Gbetagamma subunits regulate effectors downstream of activated receptors. The particular constituents of the G protein heterotrimer affect both specificity and efficiency of signal transduction. However, little is known about mechanistic aspects of G protein assembly in the cell that would certainly contribute to formation of heterotrimers of specific composition. It was recently shown that phosducin-like protein (PhLP) modulated both Gbetagamma expression and subsequent signaling by chaperoning nascent Gbeta and facilitating heterodimer formation with Ggamma subunits (Lukov, G. L., Hu, T., McLaughlin, J. N., Hamm, H. E., and Willardson, B. M. (2005) EMBO J. 24, 1965-1975; Humrich, J., Bermel, C., Bunemann, M., Harmark, L., Frost, R., Quitterer, U., and Lohse, M. J. (2005) J. Biol. Chem. 280, 20042-20050). Here we demonstrate using a variety of techniques that DRiP78, an endoplasmic reticulum resident protein known to regulate the trafficking of several seven transmembrane receptors, interacts specifically with the Ggamma subunit but not Gbeta or Galpha subunits. Furthermore, we demonstrate that DRiP78 and the Gbeta subunit can compete for the Ggamma subunit. DRiP78 also protects Ggamma from degradation until a stable partner such as Gbeta is provided. Furthermore, DRiP78 interaction may represent a mechanism for assembly of specific Gbetagamma heterodimers, as selectivity was observed among Ggamma isoforms for interaction with DRiP78 depending on the presence of particular Gbeta subunits. Interestingly, we could detect an interaction between DRiP78 and PhLP, suggesting a role of DRiP78 in the assembly of Gbetagamma by linking Ggamma to PhLP.Gbeta complexes. Our results, therefore, suggest a role of DRiP78 as a chaperone in the assembly of Gbetagamma subunits of the G protein.

Rhodopsin-interacting surface of the transducin gamma subunit

The visual signaling pathway is initiated by photoactivation of the GPCR rhodopsin, which activates nucleotide exchange on the heterotrimeric G-protein transducin (Gt). Domains on both Gtalpha and Gtbetagamma subunits participate in coupling to rhodopsin. Previously, we have shown by high-resolution NMR that the farnesylated C-terminal peptide of Gtgamma(60-71), DKNPFKELKGGC, assumes an amphipathic helical conformation during interaction with metarhodopsin II [Kisselev, O. G., and Downs, M. A. (2003) Structure 11, 367-373]. This conformation was docked to the structure of holo-Gt to create a model of rhodopsin-Gt interaction. Here we test this model by mutational analysis of Gt. To evaluate the contribution of specific amino acids of the Gtgamma C-terminal region involved in binding and GTP-dependent release of transducin from native rhodopsin membranes, we have systematically substituted each of the amino acids in the C-terminal region of Gtgamma for alanine. The mutants were co-expressed with six-histidine-tagged Gtbeta subunits in Sf9 insect cells. The Gtbeta-6-His-gamma mutant proteins were purified and assayed in the presence of Gtalpha for the GTP-dependent interactions with light-activated rhodopsin. Several of the alanine mutants, N62A, P63A, and F64A, exhibited significant functional defects at the level of R*-Gt complex formation. These data show that the conserved N-terminal end of the helical domain in the Gtgamma(60-71) region has the most significant effect on rhodopsin-Gt interactions, which places important constraints on the model of the rhodopsin-Gt complex.

Regions in the G Protein γ Subunit Important for Interaction with Receptors and Effectors

G betagamma dimers containing the gamma11 or gamma1 subunits are often less potent and effective in their ability to regulate effectors compared with dimers containing the gamma2 subunit. To explore the regions of the gamma subunit that affect the activity of the betagamma dimer, we constructed eight chimeric gamma subunits from the gamma1 and gamma2 subunits. Two chimeras were made in which the N-terminal regions of gamma1 and gamma2 were exchanged and two in which the C-terminal regions were transposed. Another set of chimeras was made in which the CAAX motifs of the chimeras were altered to direct modification with different prenyl groups. All eight gamma chimeras were expressed in Sf9 cells with the beta1 subunit, G betagamma dimers were purified, and then they were assayed in vitro for their ability to bind to the G alpha(i1) subunit, to couple G alpha(i1) to the A1 adenosine receptor, to stimulate phospholipase C-beta, and to regulate type I or type II adenyl cyclases. Dimers containing the C-terminal sequence of the gamma2 subunit modified with the geranylgeranyl lipid had the highest affinity for G(i1)alpha (range, 0.5-1.2 nM) and were most effective at coupling the G(i1)alpha subunit to receptor. These dimers were most effective at stimulating the phosphatidylinositol-specific phospholipase C-beta isoform and inhibiting type I adenyl cyclase. In contrast, betagamma dimers containing the N-terminal sequence of the gamma2 subunit and a geranylgeranyl group are most effective at activating type II adenyl cyclase. The results indicate that both the N- and C-terminal regions of the gamma subunit impart specificity to receptor and effector interactions.

Multiple roles of a heterotrimeric G-protein gamma-subunit in governing growth and development of Aspergillus nidulans

Vegetative growth signaling in the filamentous fungus Aspergillus nidulans is primarily mediated by the heterotrimeric G-protein composed of FadA (G alpha), SfaD (G beta), and a presumed G gamma. Analysis of the A. nidulans genome identified a single gene named gpgA encoding a putative G gamma-subunit. The predicted GpgA protein consists of 90 amino acids showing 72% similarity with yeast Ste18p. Deletion (delta) of gpgA resulted in restricted vegetative growth and lowered asexual sporulation. Moreover, similar to the delta sfaD mutant, the delta gpgA mutant was unable to produce sexual fruiting bodies (cleistothecia) in self-fertilization and was severely impaired with cleistothecial development in outcross, indicating that both SfaD and GpgA are required for fruiting body formation. Developmental and morphological defects caused by deletion of flbA encoding an RGS protein negatively controlling FadA-mediated vegetative growth signaling were suppressed by delta gpgA, indicating that GpgA functions in FadA-SfaD-mediated vegetative growth signaling. However, deletion of gpgA could not bypass the need for the early developmental activator FluG in asexual sporulation, suggesting that GpgA functions in a separate signaling pathway. We propose that GpgA is the only A. nidulans G gamma-subunit and is required for normal vegetative growth as well as proper asexual and sexual developmental progression.