Two groups have uncovered new wrinkles in reactions involving heterotrimeric guanosine triphosphate (GTP)-binding proteins (G proteins). Ligand binding to G protein-coupled receptors (GPCRs) activates G proteins by catalyzing guanine nucleotide exchange and dissociation of the GTP-bound α subunit (Gα) from the βγ heterodimer (Gβγ). Both subunits are then free to activate various effectors. Ribas et al. partially purified a ligand-independent G-protein activator, NG108-15 G-protein activator (NG-GPA), from the neuroblastoma-glioma hybrid NG108-15. NG-GPA stimulated guanosine 5'-O-thiotriphosphate (GTPγS) binding to purified brain G protein, as well as to recombinant Gαo and Gαi, and inhibited adenylyl cyclase activity in DDT1-MF-2 cell membranes. NG-GPA's stimulation of GTPγS binding, unlike GPCR-stimulated GTPγS binding, was unaffected by treatment with pertussis toxin, which prevents agonist-induced receptor interactions with the Gi family of G proteins, suggesting that NG-GPA interacts with the G protein through a previously unidentified regulatory domain. Dell et al. used a yeast two-hybrid screen to identify new Gβγ effectors and isolated three proteins containing WD40 motifs (which Gβγ also contains) that bound to Gβγ. Gβγ interaction with two of these proteins, receptor for activated C kinase 1 (RACK1) and dynein intermediate chain (DIC), was confirmed by Western analysis of immunoprecipitates. A RACK1 fusion protein bound to both Gβ1γ1 and the heterotrimeric Gαtβ1γ1 transducin heterotrimer but not to Gαt alone; Gαtβ1γ1 had a higher binding affinity than Gβ1γ1. RACK1 interaction with Gαtβ1γ1 was surprising, because few proteins other than GPCRs interact with the heterotrimer. Indeed, all known Gβγ effectors bind to the Gα-interacting region, suggesting a novel mechanism for RACK1-G protein interaction.
C. Ribas, A. Takesono, M. Sato, J. D. Hildebrandt, S. M. Lanier, Pertussis toxin-insensitive activation of the heterotrimeric G-proteins Gi/Go by the NG108-15 G-protein activator. J. Biol. Chem. 277, 50223-50225 (2002). [Abstract] [Full Text]
E. J. Dell, J. Connor, S. Chen, E. G. Stebbins, N. P. Skiba, D. Mochly-Rosen, H. E. Hamm, The βγ subunit of heterotrimeric G proteins interacts with RACK1 and two other WD repeat proteins. J. Biol. Chem. 277, 49888-49895 (2002). [Abstract] [Full Text]