Regulation of large and small G proteins by ubiquitination

Abstract

Edited by Mike Shipston Many sensory and chemical signal inputs are transmitted by intracellular GTP-binding (G) proteins. G proteins make up two major subfamilies: "large" G proteins comprising three subunits and "small" G proteins, such as the proto-oncogene product RAS, which contains a single subunit. Members of both subfamilies are regulated by post-translational modifications, including lipidation, proteolysis, and carboxyl methylation. Emerging studies have shown that these proteins are also modified by ubiquitination. Much of our current understanding of this posttranslational modification comes from investigations of the large G-protein ␣ subunit from yeast (Gpa1) and the three RAS isotypes in humans, NRAS, KRAS, and HRAS. G␣ undergoes both mono-and polyubiquitination, and these modifications have distinct consequences for determining the sites and mechanisms of its degradation. Genetic and biochemical reconstitution studies have revealed the enzymes and binding partners required for addition and removal of ubiquitin, as well as the delivery and destruction of both the mono-and polyubiquitinated forms of the G protein. Complementary studies of RAS have identified multiple ubiquitination sites, each having distinct consequences for binding to regulatory proteins, shuttling to and from the plasma membrane, and degradation. Here, we review what is currently known about these two well-studied examples, Gpa1 and the human RAS proteins, that have revealed additional mechanisms of signal regulation and dysregulation relevant to human physiology. We also compare and contrast the effects of G-protein ubiquitination with other post-translational modifications of these proteins. A variety of sensory and chemical signals are detected by receptors at the cell surface. In many cases, these inputs are transmitted by intracellular GTP-binding proteins (Fig. 1). Generally speaking, large G proteins are activated by seven transmembrane segment receptors, also known as G-proteincoupled receptors (GPCRs). 3 When these receptors bind to an agonist ligand, the G-protein ␣ subunit releases GDP, binds to GTP, and dissociates from the G␤␥ subunit dimer. GTP-bound G␣, G␤␥, or both go on to activate intracellular effectors that transduce the signal. Similarly, many small G proteins are activated by growth factor receptors and their associated guanine nucleotide-exchange factors (GEFs). As with GPCRs, GEFs promote the exchange of GTP for GDP and subsequent activation of downstream effectors. For both large and small G proteins, signaling persists until GTP is converted to GDP, and this inactivation step is accelerated by binding to GTPase-activating proteins (GAPs). Thus, GEFs and GAPs work in opposition to one another to regulate the activity of their cognate G proteins, both large and small. Here, we review newer mechanisms of G-protein regulation by ubiquitination, with a focus on a large G protein from yeast and three small G proteins from humans: NRAS, KRAS, and HRAS. We compare and contrast the effects of ubiquitination with ...

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