UCSF

G protein-coupled receptors (GPCRs) are master regulators of human physiology and comprise the largest class of transmembrane receptors in the human genome. These cell-surface receptors respond to a wide variety of extracellular agonists––hormones, peptides, small molecules, ions, neurotransmitters––and convey this information to the cell by interacting with intracellular proteins. Heterotrimeric G proteins are the primary intracellular signaling partners of GPCRs and bind to agonist-activated GPCRs. Coupling to GPCRs causes G protein guanine nucleotide exchange. Release of inhibitory GDP and binding of activating GTP dissociates the heterotrimeric G protein, liberating the Gα and Gβγ subunits to activate downstream signaling pathways. The molecular mechanism governing GPCR-mediated activation of G proteins was first described more than 40 years ago. Nevertheless, there are fundamental aspects of GPCR–G protein biology that remain uncharacterized. This dissertation uses biochemical reconstitution, pharmacological manipulation, and structural biology to improve existing models of GPCR–G protein interactions. In chapter 1, we investigate how endogenous ligand–receptor interactions drive differential patterns of G protein engagement (Gαq vs. Gαs) at a single GPCR. In chapter 2, we characterize the mechanism by which oncogenic mutations in Gαq drive constitutive G protein signaling in GPCR-independent pathways. We envision this work will advance our fundamental understanding of G protein biology and further translational efforts to manipulate G protein biology in human physiology.