Novel Mechanisms Regulating the Heterotrimeric G Protein Complex

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Hormones and neurotransmitters modulate a variety of physiological processes in cell growth and behavior. Their cognate cell surface receptors, which have seven transmembrane (7TM) domains, act by coupling to G proteins, promoting the dissociation of GDP and the subsequent loading of GTP. Signaling abates when GTP is hydrolyzed and GTPase activity is accelerated by Regulators of G Signaling (RGS) proteins having GTPase accelerating protein (GAP) activity. Recently, we discovered a naturally-occurring 7TM-RGS protein in Arabidopsis (AtRGS1) that we hypothesize to be a D-glucose receptor that has a D-glucose-dependent GAP activity. It is the first example of a receptor-GAP and is the prototype for a new class of D-glucose receptors. We also showed that the Arabidopsis G¿ subunit has rapid nucleotide exchange making nucleotide hydrolysis the rate limiting step. This property is in marked contrast to the slow nucleotide exchange property of all tested G¿ subunits where GDP release is the rate limiting step of the G protein cycle. Thus, we hypothesize that regulation of the G protein cycle is at the GTP hydrolysis step and is mediated by AtRGS1. Finally, we showed that a D-glucose metabolite dramatically increases the nucleotide hydrolysis rate of the G¿ subunit. Clearly, the Arabidopsis G protein cycle contains several interesting properties, namely activation of a G¿ subunit that does not require a GEF, regulation of the cycle at the GTP hydrolysis step, and a 7TM protein that may be the ligand-regulated GAP controlling the cycling rate. Because the Arabidopsis G¿ has the basic core structure and function of human G¿, an understanding of how the Arabidopsis G¿ activation is regulated will provide insight into novel mechanisms to control human G¿ activation. The goal here is to understand how the G¿ protein is activated in the context of sugar signaling. Both hypothesis- and discovery-driven approaches will be taken to determine precisely what structure imparts regulatory control. Our initial study of the Arabidopsis G protein cycle illustrated how the G-protein cycle can be regulated by mechanisms apart by the classical GEF. Consequently, a greater degree of plasticity of the cycle is now appreciated and new entry points for regulation are revealed. Understanding the structure underlying these new mechanisms will provide a new means to regulate other G protein cycles. In humans, 7TM receptors and RGS proteins interact either directly or indirectly via adaptor proteins; these are two of several possible mechanisms providing selectivity between receptors and RGS proteins. AtRGS1 is the most extreme example of a mechanism providing receptor-RGS protein selectivity in that both GEF and GAP are two domains on one molecule. Understanding how AtRGS1 regulates the G protein cycle in a ligand dependent manner opens up new possibilities to regulate G protein cycles through drug therapies. Finally, use of Arabidopsis as a model will enable us to solve how cells respond to D-glucose within the context of a multicellular organism.