Structure and Function of Heterotrimeric G Protein-Regulated Rho Guanine Nucleotide Exchange Factors

Abstract

Activation of certain classes of G protein-coupled receptors (GPCRs) can lead to alterations in the actin cytoskeleton, gene transcription, cell transformation, and other processes that are known to be regulated by Rho family small-molecular-weight GTPases. Although these responses can occur indirectly via cross-talk from canonical heterotrimeric G protein cascades, it has recently been demonstrated that Dbl family Rho guanine nucleotide exchange factors (RhoGEFs) can serve as the direct downstream effectors of heterotrimeric G proteins. Heterotrimeric G␣ 12/13 , G␣ q , and G␤␥ subunits are each now known to directly bind and regulate RhoGEFs. Atomic structures have recently been determined for several of these RhoGEFs and their G protein complexes, providing fresh insight into the molecular mechanisms of signal transduction between GPCRs and small molecular weight G proteins. This review covers what is currently known about the structure, function, and regulation of these recently recognized effectors of heterotrimeric G proteins.Heterotrimeric G proteins are master regulators of cell homeostasis. By coordinating signaling between the ϳ800 G protein-coupled receptors (GPCRs) in the human genome and a relatively small handful of effector enzymes and channels in the cell, they control processes such as muscle contractility, glycogen metabolism, neurotransmission, and the concentration of intracellular ions. Their profound impact on nearly all cellular processes and their therapeutic potential have rendered them one of the most intensely studied signal transduction paradigms at the biochemical and molecular level (Sprang et al., 2007).When heterotrimeric G proteins are in their inactive, GDPbound state, they exist as an inert complex composed of ␣, ␤, and ␥ subunits (G␣␤␥). In this state, they are substrates for activated GPCRs, which catalyze nucleotide exchange on the ␣ subunit (G␣). When bound to GTP, G␣ releases the effector binding surface of the ␤ and ␥ heterodimer (G␤␥) so that both G␣ and G␤␥ can interact with and modulate the activity of specific downstream enzymes and channels. The G␣ subunit has weak guanine nucleotide triphosphatase (GTPase) activity that slowly returns the G protein to its GDP-bound state. G␣⅐GDP then becomes resequestered by G␤␥. Beyond serving as conduits for extracellular signals, heterotrimeric G proteins contribute to the fidelity, duration, and amplitude of GPCR signaling. A given class of heterotrimeric G protein can typically recognize only a subset of GPCRs, and can only interact with one or a few downstream effector targets, ensuring the specificity of signaling from receptor to effector. The rate of GTP hydrolysis on G␣ dictates the length of time that its signal is in play, and this rate -bound conformation. The Ras-like domain is colored cyan, and the ␣-helical domain is gray. The three nucleotide-dependent switch regions (switch I-III) are red. The canonical effector docking site, a shallow canyon formed between switch II and the ␣3 helix, is indicated by the tr...

Mechanistic insight into GPCR-mediated activation of the microtubule-associated RhoA exchange factor GEF-H1

“…Therefore, we asked whether Ga 12/13 competes with Tctex-1 binding, and whether displacement of Tctex-1 by Ga 12/13 may contribute to the activation of GEF-H1. GEF-H1 immune complexes from HEK293T cells expressing GEF-H1, Tctex-1 and Ga 13QL were analysed for the presence of Tctex-1 and Ga 13 . While Tctex-1 was detected in GEF-H1 immunoprecipitates derived from cells expressing GEF-H1 and Tctex-1 alone, expression of Ga 13QL markedly reduced the association of Tctex-1 with GEF-H1 (Fig.…”

“…Ga 12/13 proteins play a well-established role in the activation of RhoGEFs belonging to the regulator of G-protein signalling (RGS) domaincontaining RhoGEF subfamily, which includes p115-RhoGEF, PDZ-RhoGEF, leukemia-associated RhoGEF (LARG) and possibly Lbc-RhoGEF [11][12][13] . Ga 12/13 subunits bind directly to the RGShomology (RH) domain contained within the polypeptide sequences of these GEFs, which is thought to alleviate autoinhibition of their exchange activity.…”

Intramolecular Interactions between the Dbl Homology (DH) Domain and the Carboxyl-terminal region of Myosin II-interacting Guanine Nucleotide Exchange Factor (MyoGEF) Act as an Autoinhibitory Mechanism for the Regulation of MyoGEF Functions

“…Accordingly, it has been demonstrated that RhoA and RhoC are key promoters of breast cancer invasion and metastasis (11,12). Consistent with the fact that numerous RhoGEFs exist in humans (1), multiple RhoGEFs, such as leukemia-associated RhoGEF (LARG), PDZ-RhoGEF, and myosin II-interacting GEF (MyoGEF), are implicated in promoting breast cancer cell invasion through activation of RhoA and/or RhoC (13)(14)(15).…”

Pharmacological Interference With Protein-protein Interactions of Akinase Anchoring Proteins as a Strategy for the Treatment of Disease

“…AKAP--Lbc selectively activates RhoA but not other members of the Rho family such as Rac and Cdc42 [22]. PH domains can control the localization and/or the activity of DH domains [231]. A crystal structure of the complex between RhoA--GDP and the AKAP--Lbc--DHPH domain showed that the PH domain is connected with the DH domain by a helical linker and that the switch I and II regions of RhoA bind to two distinct regions of the DH domain.…”