The 'invisible hand': regulation of RHO GTPases by RHOGDIs

Abstract

The 'invisible hand' is a term originally coined by Adam Smith in The Theory of Moral Sentiments to describe the forces of self-interest, competition and supply and demand that regulate the resources in society. This metaphor continues to be used by economists to describe the self-regulating nature of a market economy. The same metaphor can be used to describe the RHO-specific guanine nucleotide dissociation inhibitor (RHOGDI) family, which operates in the background, as an invisible hand, using similar forces to regulate the RHO GTPase cycle.

Figure 1. The RHOGDI cycle

(a) Newly synthesized RHO family GTPases are geranylgeranylated and then post-translationally modified by the protease Ras-converting enzyme 1 (RCE1) and by isoprenylcysteine carboxyl methyltransferase (ICMT) at the cytoplasmic face of the endoplasmic reticulum (ER). (b) After geranylgeranylation, RHO proteins associate with RHO specific guanine nucleotide dissociation inhibitors (RHOGDIs), which sequester them in the cytosol and protect them from degradation. (c) Free prenylated cytosolic RHO GTPases are unstable and are rapidly degraded by the proteasome. (d) Several RHO GTPases can associate with RHOGDI and compete for its binding. Overexpression of a GTPase can displace the endogenous RHO proteins from RHOGDI targeting them for degradation. (e) The rate of cycling of the RHOGDI -RHO GTPase complex between the cytosol and the membrane can be regulated by post-translational modifications on both the RHO GTPases and the RHOGDI, which modulate the affinity of the interaction. A slower pathway for recycling RHO proteins through vesicle trafficking has also been postulated. (f) Once at the membrane, the RHO GTPases can be activated by guanine nucleotide exchange factors (GEFs) and bind to downstream effectors. Following inactivation by GTPase-activating proteins (GAPs), RHO GTPases are extracted from the membrane by RHOGDI. (g) Active RHOA can also be targeted for degradation by the ubiquitin ligase SMAD ubiquitylation regulatory factor 1 (Smurf1). GGTase, geranylgeranyl transferase.

Figure 2. Structure of the RHOGDI–RHO GTPase complex

(A) Space filling model showing CDC42 in complex with RHO-specific guanine nucleotide dissociation inhibitor 1 (RHOGDI1). The domains that participate in the interaction are highlighted and labelled on the struucture. (B) Cartoon representation of the crystal structure of prenylated CDC42 in complex with RHOGDI . The phosphorylated residues are shown in magenta, CDC42 is shown in green, and RHOGDI1 is shown in gold. These structures are reproduced from the NCBI Molecular Modeling Database(ID: 1275). C, carboxyl terminus; GG, geranylgeranyl; N, amino terminus; PBR, Poly Basic Region.

Figure 3. Mechanisms of regulation of the RHOGDI-RHOGTPase interaction

a | Release by lipids: The presence of acidic phospholipids can promote the release of RHOGTPases from RHO-specific guanine nucleotide dissociation inhibitors (RHOGDIs). Phospholipids mediate a partial opening of the complex that exposes the GTPases to RHO-specific guanine nucleotide exchange factors (RHOGEFs) or other dissociation factors such as the ones described in b. b | Release by protein–protein interactions: p75 neurotrophin receptor (p75^NTR) and ezrin, radixin and moesin (ERM) proteins interact with RHOGDIs and facilitate the release of RHOA, which can then be activated by specific RHOGEFs. The interaction of p75^NTR with RHOGDIs is enhanced by myelin derived proteins such as myelin-associated glycoprotein (MAG) and neurite outgrowth inhibitor (NOGO). ERM proteins also need to be activated to interact with RHOGDI. c | Release by phosphorylation. Phosphorylation of RHOGDI Ser, Thr and Tyr residues promote the release of RHO GTPases. Depending on the residues phosphorylated, this release can be specific for a single RHO GTPase, or affect multiple RHO GTPases simultaneously (Table 1). For example p21-activated kinase (PAK)- or FER-mediated phosphorylation promote the specific release of RAC1, but not RHOA or CDC42. Some kinases act in concert to target the RAC1–RHODI complex to the membrane and regulate the local release of RAC1 at specific site on the membrane, where it is subsequently activated by RHOGEFs. Diacylglycerol kinase-ζ (DGKζ) forms a complex with PAK, RAC1 and RHOGDI. In response to platelet derived growth factor (PDGF), DGKζ stimulates the production of phosphatidic acid (PA), which induces PAK activity. Active PAK then phosphorylates RHOGDI and releases RAC1 for activation. Positively charged phospholipids are indicated in blue, negatively charged phopholipids in red. DAG, diacylglycerol; PDGFR, PDGF receptor.