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 (Gbeta5-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. Immunoaffinity purification of Gbeta5 from detergent-extracted membranes of mouse brain identified regulators of G protein signaling RGS6 and RGS7 as tightly bound partners. We characterized the expression of Gbeta5 in brain and in several neuroendocrine cell lines to learn more about the function and regulation of Gbeta5 in native systems. One Gbeta5-positive cell line was rat pheochromocytoma PC12 cells. We discovered that Gbeta5 and RGS7 proteins were expressed in the cell nuclei of these neuron-like cells. We analyzed NGF-differentiated PC12 cells and mouse brain by immunocytochemistry and by subcellular fractionation and found that both Gbeta5 and RGS7 are expressed in the plasma membrane, cytosol and cell nucleus. Using GFP fusion proteins in transfected PC12 cells, we found that Gbeta5 mutants defective in their ability to interact with RGS7 failed to undergo nuclear localization, suggesting an RGS protein-mediated nuclear localization mechanism. The findings of Gbeta5 and RGS7 protein expression in the neuronal nucleus raise the possibility of information transfer by Gbeta5-RGS protein complexes between the plasma membrane and the nuclear compartments.
