G PROTEINS Independent of GPCRs and Nucleotide Exchange

Sato et al. used a strain of yeast lacking the pheromone receptor to functionally screen mammalian cDNA libraries and identify nonreceptor proteins that activate heterotrimeric guanine nucleotide-binding protein (G protein) signaling. They identified a previously uncharacterized protein that they called activator of G protein signaling (AGS) 8, which was derived from cardiac tissue in a rat model of transient myocardial ischemia. Expression of AGS8 mRNA was increased in ischemic ventricles but not in other models of cardiac dysfunction. The effects of hypoxia were specific: Whereas expression of AGS8 mRNA increased in response to hypoxia in cultured ventricular myocytes, it was unaffected in aortic smooth muscle cells, aortic endothelial cells, and cardiac fibroblasts. AGS8 was functionally active in yeast strains that expressed various G subunits, including a mutant G_i2 believed to remain bound to GDP (rather than exchanging GTP for GDP), but required the presence of Gß and downstream components of the mitogen-activated protein kinase signaling pathway (the kinase Ste20 and the scaffolding protein Ste5). Moreover, an in vitro binding assay indicated that AGS8 interacted with Gß but not with G_i1/2, G_i3, G_o, or G_s. In COS-7 cells, coexpression of AGS8 with Gß and the Gß effector PLC-ß2 did not inhibit Gß-stimulated inositol phosphate production; rather, it reversed inhibition of Gß-stimulated inositol phosphate production by G₁₃. A similar reversal by AGS8 of inhibitory effects of G on Gß function was apparent in yeast. Thus, AGS8 appears to be a regulator of G protein signaling that is induced in ischemic heart to act through a mechanism involving interaction with Gß.

M. Sato, M. J. Cismowski, E. Toyota, A. V. Smrcka, P. A. Lucchesi, W. M. Chilian, S. M. Lanier, Identification of a receptor-independent activator of G protein signaling (AGS8) in ischemic heart and its interaction with Gß. Proc. Natl. Acad. Sci. U.S.A. 103, 797-802 (2006). [Abstract] [Full Text]