Guanine nucleotide binding proteins (G proteins) are membrane associated, heterotrimeric proteins composed of three subunits: alpha, beta and gamma. The G protein beta subunit assumes a barrel shaped beta propeller structure containing WD40 repeats preceded by an N terminal alpha helix. The beta subunit forms a stable dimer with the gamma subunit. The alpha subunit only contacts the beta subunit in the dimer, lying on the opposite face from the gamma subunit. G proteins and their receptors (GPCRs) regulate systems as diverse as sensory perception, cell growth and hormonal regulation.