The guanine-nucleotide binding regulatory proteins (G-proteins) are heterotrimers which function as transmembrane signal transducers by coupling receptors for extracellular stimuli to intracellular effectors (enzymes, ion channels). G-proteins constitute a diverse family distinguished by specific receptor and effector interactions which in turn are determined by the structure of the three constituent subunits. The alpha subunit binds guanine nucleotides and has a well established role in effector modulation. The beta and gamma subunits are tightly associated as a beta-gamma complex, comprising a single functional entity which, like the alpha subunit, is absolutely required for G protein interaction with receptor. An effector modulatory role for the beta-gamma complex is becoming increasingly apparent in several systems. The present research emphasizes the role of the beta-gamma complex in G-protein-mediated signal transduction. A structurally divergent neurally expressed G beta subunit, beta-5, was cloned from brain by Mel Simon and coworkers, and later found in an alternatively spliced """"""""long"""""""" form in retina (beta-5 L). G beta-5 was recently found to exhibit functional specialization, as it was able to activate PLC but not the MAPK or JNK cascades. Furthermore G beta-5/gamma-2 inhibited AC type II and interacted selectively with the G alpha-q isoform, properties novel among G beta-gamma complexes studied to date. The effector selectivity of G beta-5 was further explored by in vitro kinase assays, and its pattern of expression in mouse tissues and cultured cell lines analyzed by protein immunoblotting (WB), in situ hybridization (ISH) and ribonuclease protection assays (RPA). In HEK-293 cells, cotransfected G beta-1/gamma-2 but not G beta-5/gamma-2 activated epitope-tagged Akt/ PKB in a wortmannin-sensitive fashion. The failure of G beta-5 to activate both the MAPK cascade and Akt/PKB suggests that the inability to interact with a common intermediate such as PI3K gamma may account for G beta-5 selectivity. Analysis of cultured cell lines by Northern blotting, RPA and WB demonstrated G beta-5 expression in alpha-T3, PC-12, SH-SY5Y and GT1-7 neuronal cell lines, but not in non-neuronal C6 glioma, COS-7 or HEK-293 cells. Mouse brain sections were analyzed by ISH with G beta-5-specific riboprobe and revealed specific signal throughout the brain including cortical layers of the forebrain, the superior and inferior colliculi in the midbrain, and cerebellar Purkinje cells. These results suggest the specialized function of G beta-5 is of widespread importance throughout the central nervous system. Immunoaffinity purification of Gbeta5 from detergent-extracted membranes of mouse brain identified regulators of G protein signaling RGS6 and RGS7 as tightly bound partners.
