Assembly and Intracellular Targeting of the βγ Subunits of Heterotrimeric G Proteins

The assembly in living cells of heterotrimeric guanine nucleotide binding proteins from their constituent alpha, beta, and gamma subunits is a complex process, compounded by the multiplicity of the genes that encode them, and the diversity of receptors and effectors with which they interact. Monoclonal anti-beta antibodies (ARC5 and ARC9), raised against immunoaffinity purified beta gamma complexes, recognize beta subunits when not associated with gamma and can thus be used to monitor assembly of beta gamma complexes. Complex formation starts immediately after synthesis and is complete within 30 min. Assembly occurs predominantly in the cytosol, and association of beta gamma complexes with the plasma membrane fraction starts between 15-30 min of chase. Three pools of beta subunits can be distinguished based on their association with gamma subunits, their localization, and their detergent solubility. Association of beta and alpha subunits with detergent-insoluble domains occurs within 1 min of chase, and increases to reach a plateau of near complete detergent resistance within 30 min of chase. Brefeldin A treatment does not interfere with delivery of beta gamma subunits to detergent-insoluble domains, suggesting that assembly of G protein subunits with their receptors occurs distally from the BFA-imposed block of intracellular membrane trafficking and may occur directly at the plasma membrane.