Our ability to treat disorders such as depression, anxiety, and schizophrenia has benefited greatly from our understanding of G protein signaling networks. Heterotrimeric G protein signaling is known to regulate a wide variety of physiological process, including mood. Thus, it is not surprising that defects in G protein signaling lead to disease. Accordingly, advances in our understanding of G protein signaling have led directly to the development of drugs such as the Tri-Cyclic Anti-depressant/Anti-psychotics, and the Benzodiazepine family of anxiolytics. The development of improved therapeutics, however, will require both the discovery of new drug targets, and a refined understanding of how these new targets regulate signaling. Recent research has uncovered a new class of activators for heterotrimeric G protein coupled systems, however, these proteins are of limited therapeutic usefulness until we obtain a better understanding of their mechanism of action and the cellular context in which they function. In this proposal, I aim to characterize Arr4, a novel non-receptor activator of heterotrimeric G protein signaling. I recently identified that Arr4 interacts with Gpa1, a yeast homolog of G alpha-i. Furthermore, I found that Arr4 functions as a bona fide guanine-nucleotide exchange factor, or GEF, for Gpa1. Here, I aim to determine (i) the role of typical and atypical G beta-gamma subunits in regulating Arr4 activity, (ii) map the binding surface between Arr4 and G alpha subunits (iii) identify where in cells Arr4 functions as a GEF. To address these aims, I will purify the various proteins involved for biochemical analysis. Furthermore, standard genetic manipulation of yeast, as well as a number of established in vivo and in vitro assays for G protein activation will be used. Throughout this proposal, I will be characterizing the activity of yeast Arr4 on the yeast G alpha, Gpa1. Because of the high degree of conservation between the yeast and human homologues of these proteins, I will also conduct parallel experiments using the human versions of these proteins. The yeast model system has a long history of providing information that inspires analogous discoveries in mammalian systems. The goal of this proposal is to determine how non-receptor GEFs, such as Arr4, activate their cognate G proteins, and also to, by comparison to GPCRs and GEFs for small GTPases, identify themes in GEF activity. This information will aid in our ability to more completely utilize GEFs as drug targets. Treatment of mood disorders has benefited from drugs that target G protein signaling. A new class of proteins was recently found to regulate G protein signaling, and here I aim to characterize this class of proteins, in order to create better drugs to treat mood disorders. [unreadable] [unreadable] [unreadable]