| G proteins are biological signaling switches, which typically exist in a multi-protein complex at the cell periphery. Included in these systems is a seven-transmembrane spanning G protein coupled receptor (GPCR), a guanine nucleotide binding protein G alpha subunit (Galpha) and the obligate dimer Gbetagamma. GPCRs bind extracellular ligands, and transmit signals to intracellular G proteins. Ligand binding causes the Galpha subunit to exchange GDP for GTP. GTP-bound Galpha no longer interacts with Gbetagamma, and both entities are free to interact with downstream effector proteins. Galpha regulates the duration of the signal because Galpha is a GTPase, an activity that can be accelerated by GTPase Accelerating Proteins (GAPs) such as RGS proteins.;The standard G protein signaling model, in its basic tenets, has remained largely unchanged for decades. This paradigm, however, fails to take into account recent findings that G proteins are not restricted to the cell periphery. A number of reports suggest that Galpha subunits in particular exist at, and even signal at, intracellular locations. These findings open up the possibility that Galpha subunits regulate novel signaling pathways, away from the cell periphery, and potentially independent of the traditional "Galpha-Gbetagamma-GPCR" paradigm. One particular example comes from the yeast model system, where the G protein Gpa1 regulates a cell fusion process called mating. It was recently identified that Gpa1 regulates a Gbetagamma-mediated signal at the cell periphery, and PI3K-mediated signal from the cell interior. Studies presented in this thesis focus on: (1) the regulation of Gpa1 signaling by post-translational modifications such as palmitoylation and ubiquitination, and (2) the activation of Gpa1 by non-receptor activators. Specifically, we detail the discovery that Arr4 functions as an exchange factor for Gpa1, and may contribute to the activation of Gpa1 at the endosome. Also, we find that Gpa1 localization to the plasma membrane and endosome is regulated by multiple forms of ubiquitination and also by dynamic palmitoylation. Finally, we create a novel method to be used for the identification of proteins involved in palmitoylation. Collectively, these discoveries further the growing understanding that G proteins function at intracellular locations. |
