# The RHO Family GTPases: Mechanisms of Regulation and Signaling
## 1 Introduction  
The RHO (RAS homolog) family is an integral part of the RAS superfamily of guanine nucleotide-binding proteins. RHO family proteins are crucial for several reasons: (i) approximately 1% of the human genome encodes proteins that either regulate or are regulated by direct interaction with RHO proteins; (ii) they control almost all fundamental cellular processes in eukaryotes including morphogenesis, polarity, movement, cell division, gene expression, and cytoskeleton reorganization [1]; and (iii) they are associated with a series of human diseases (Figure 1) [2]. The RHO family of proteins functions as molecular switches in the cell and cycle between being in a GDP-bound, inactive state and a GTP-bound, active state [3]. Invaluable insights have been gained by structural and biochemical studies of RHO GTPases and their complexes with interacting partners thus far, increasing our understanding of both how the switch mechanism of the RHO GTPases is regulated and how a RHO GTPase can interact with four classes of structurally and functionally unrelated protein families (Figure 1) [3]. The cellular regulation of this cycle involves guanine nucleotide exchange factors (GEFs), which accelerate intrinsic GDP/GTP exchange, and GTPase-activating proteins (GAPs), which stimulate intrinsic GTP hydrolysis activity [4]. The formation of the active GTP-bound state of the GTPase is accompanied by a conformational change in two regions (known as switch I and II), which provides a platform for the selective interaction with structurally and functionally diverse proteins (the so-called downstream effectors; Table 1) that initiate a network of cytoplasmic and nuclear signaling cascades [5, 6]. A prerequisite of RHO protein function is membrane association, which is achieved by isoprenylation, a posttranslational modification. In this respect, RHO proteins are regulated by a third control mechanism that directs their membrane targeting to specific subcellular sites. Specifically, guanine nucleotide dissociation inhibitors (GDIs) bind selectively to prenylated RHO proteins and control their cycle between the cytosol and membrane. Activation of RHO proteins results in their association with effector molecules that subsequently activatea wide variety of downstream signaling cascades, thereby regulating many important physiological and pathophysiological processes in eukaryotic cells [7]. The molecular mechanisms of RHO GTPase regulation have been well characterized, but our understanding of the signal transduction to downstream targets and, most notably, the autoinhibitory mechanisms of GEFs, GAPs, and effectors remain unclear. Very important and challenging, the elucidation of these critical control mechanisms will open new directions for the design of additional therapeutic interventions. Signaling by these GTPases is controlled by other mechanisms including post-translational modifications such as phosphorylation, ubiquitylation, sumoylation, and acetylation (see for more details [8; 9]). Figure 1: Molecular principles of RHO GTPase regulation and signaling. Most RHO GTPases (20 canonical human members) act as molecular switches by cycling between a GDP-bound, inactive state and a GTP-bound, active state.