Concept: Guanine nucleotide exchange factor
A highly crystallizable T4 lysozyme (T4L) was fused to the N-terminus of the β(2) adrenergic receptor (β(2)AR), a G-protein coupled receptor (GPCR) for catecholamines. We demonstrate that the N-terminal fused T4L is sufficiently rigid relative to the receptor to facilitate crystallogenesis without thermostabilizing mutations or the use of a stabilizing antibody, G protein, or protein fused to the 3rd intracellular loop. This approach adds to the protein engineering strategies that enable crystallographic studies of GPCRs alone or in complex with a signaling partner.
The Rho guanine nucleotide exchange factor (RhoGEF) Trio promotes actin polymerization by directly activating the small GTPase Rac1. Recent studies suggest that autism spectrum disorder (ASD)-related behavioral phenotypes in animal models of ASD can be produced by dysregulation of Rac1’s control of actin polymerization at glutamatergic synapses. Here, in humans, we discover a large cluster of ASD-related de novo mutations in Trio’s Rac1 activating domain, GEF1. Our study reveals that these mutations produce either hypofunctional or hyperfunctional forms of Trio in rodent neurons in vitro. In accordance with pathological increases or decreases in glutamatergic neurotransmission observed in animal models of ASD, we find that these mutations result in either reduced synaptic AMPA receptor expression or enhanced glutamatergic synaptogenesis. Together, our findings implicate both excessive and reduced Trio activity and the resulting synaptic dysfunction in ASD-related pathogenesis, and point to the Trio-Rac1 pathway at glutamatergic synapses as a possible key point of convergence of many ASD-related genes.Trio is a RhoGEF protein that promotes actin polymerization and is implicated in the regulation of glutamatergic synapses in autism spectrum disorder (ASD). Here the authors identify a large cluster of de novo mutations in the GEF1 domain of Trio in whole-exome sequencing data from individuals with ASD, and confirm that some of these mutations lead to glutamatergic dysregulation in vitro.
Skin aging is linked to reduced epidermal proliferation and general extracellular matrix atrophy. This involves factors such as the cell adhesion receptors integrins and amino acid transporters. CD98hc (SLC3A2), a heterodimeric amino acid transporter, modulates integrin signaling in vitro. We unravel CD98hc functions in vivo in skin. We report that CD98hc invalidation has no appreciable effect on cell adhesion, clearly showing that CD98hc disruption phenocopies neither CD98hc knockdown in cultured keratinocytes nor epidermal β1 integrin loss in vivo. Instead, we show that CD98hc deletion in murine epidermis results in improper skin homeostasis and epidermal wound healing. These defects resemble aged skin alterations and correlate with reduction of CD98hc expression observed in elderly mice. We also demonstrate that CD98hc absence in vivo induces defects as early as integrin-dependent Src activation. We decipher the molecular mechanisms involved in vivo by revealing a crucial role of the CD98hc/integrins/Rho guanine nucleotide exchange factor (GEF) leukemia-associated RhoGEF (LARG)/RhoA pathway in skin homeostasis. Finally, we demonstrate that the deregulation of RhoA activation in the absence of CD98hc is also a result of impaired CD98hc-dependent amino acid transports.
Abstract Adhesion G protein-coupled receptors (aGPCR) form the second largest class of GPCR. They are phylogenetically old and have been highly conserved during evolution. Mutations in representatives of this class are associated with severe diseases such as Usher Syndrome, a combined congenital deaf-blindness, or bifrontal parietal polymicrogyria. The main characteristics of aGPCR are their enormous size and the complexity of their N termini. They contain a highly conserved GPCR proteolytic site (GPS) and several functional domains that have been implicated in cell-cell and cell-matrix interactions. Adhesion GPCR have been proposed to serve a dual function as adhesion molecules and as classical receptors. However, until recently there was no proof that aGPCR indeed couple to G proteins or even function as classical receptors. In this review, we have summarized and discussed recent evidence that aGPCR present many functional features of classical GPCR including multiple G protein-coupling abilities, G protein independent signaling and oligomerization but also specific signaling properties only found in aGPCR.
Rho GTPase-based signaling networks control cellular dynamics by coordinating protrusions and retractions in space and time. Here, we reveal a signaling network that generates pulses and propagating waves of cell contractions. These dynamic patterns emerge via self-organization from an activator-inhibitor network, in which the small GTPase Rho amplifies its activity by recruiting its activator, the guanine nucleotide exchange factor GEF-H1. Rho also inhibits itself by local recruitment of actomyosin and the associated RhoGAP Myo9b. This network structure enables spontaneous, self-limiting patterns of subcellular contractility that can explore mechanical cues in the extracellular environment. Indeed, actomyosin pulse frequency in cells is altered by matrix elasticity, showing that coupling of contractility pulses to environmental deformations modulates network dynamics. Thus, our study reveals a mechanism that integrates intracellular biochemical and extracellular mechanical signals into subcellular activity patterns to control cellular contractility dynamics.
The consequence of a loss of balance between G-protein activation and deactivation in cancers has been interrogated by studying infrequently occurring mutants of trimeric G-protein α-subunits and GPCRs. Prior studies on members of a newly identified family of non-receptor guanine nucleotide exchange factors (GEFs), GIV/Girdin, Daple, NUCB1 and NUCB2 have revealed that GPCR-independent hyperactivation of trimeric G proteins can fuel metastatic progression in a variety of cancers. Here we report that elevated expression of each GEF in circulating tumor cells (CTCs) isolated from the peripheral circulation of patients with metastatic colorectal cancer is associated with a shorter progression-free survival (PFS). The GEFs were stronger prognostic markers than two other markers of cancer progression, S100A4 and MACC1, and clustering of all GEFs together improved the prognostic accuracy of the individual family members; PFS was significantly lower in the high-GEFs versus the low-GEFs groups [H.R = 5, 20 (95% CI; 2,15-12,57)]. Because nucleotide exchange is the rate-limiting step in cyclical activation of G-proteins, the poor prognosis conferred by these GEFs in CTCs implies that hyperactivation of G-protein signaling by these GEFs is an important event during metastatic progression, and may be more frequently encountered than mutations in G-proteins and/or GPCRs.
- Proceedings of the National Academy of Sciences of the United States of America
- Published over 1 year ago
Cells migrate by directing Ras-related C3 botulinum toxin substrate 1 (Rac1) and cell division control protein 42 (Cdc42) activities and by polymerizing actin toward the leading edge of the cell. Previous studies have proposed that this polarization process requires a local positive feedback in the leading edge involving Rac small GTPase and actin polymerization with PI3K likely playing a coordinating role. Here, we show that the pleckstrin homology and RhoGEF domain containing G3 (PLEKHG3) is a PI3K-regulated Rho guanine nucleotide exchange factor (RhoGEF) for Rac1 and Cdc42 that selectively binds to newly polymerized actin at the leading edge of migrating fibroblasts. Optogenetic inactivation of PLEKHG3 showed that PLEKHG3 is indispensable both for inducing and for maintaining cell polarity. By selectively binding to newly polymerized actin, PLEKHG3 promotes local Rac1/Cdc42 activation to induce more local actin polymerization, which in turn promotes the recruitment of more PLEKHG3 to induce and maintain cell front. Thus, autocatalytic reinforcement of PLEKHG3 localization to the leading edge of the cell provides a molecular basis for the proposed positive feedback loop that is required for cell polarization and directed migration.
- Proceedings of the National Academy of Sciences of the United States of America
- Published 3 months ago
Activation of heterotrimeric G proteins by cytoplasmic nonreceptor proteins is an alternative to the classical mechanism via G protein-coupled receptors (GPCRs). A subset of nonreceptor G protein activators is characterized by a conserved sequence named the Gα-binding and activating (GBA) motif, which confers guanine nucleotide exchange factor (GEF) activity in vitro and promotes G protein-dependent signaling in cells. GBA proteins have important roles in physiology and disease but remain greatly understudied. This is due, in part, to the lack of efficient tools that specifically disrupt GBA motif function in the context of the large multifunctional proteins in which they are embedded. This hindrance to the study of alternative mechanisms of G protein activation contrasts with the wealth of convenient chemical and genetic tools to manipulate GPCR-dependent activation. Here, we describe the rational design and implementation of a genetically encoded protein that specifically inhibits GBA motifs: GBA inhibitor (GBAi). GBAi was engineered by introducing modifications in Gαi that preclude coupling to every known major binding partner [GPCRs, Gβγ, effectors, guanine nucleotide dissociation inhibitors (GDIs), GTPase-activating proteins (GAPs), or the chaperone/GEF Ric-8A], while favoring high-affinity binding to all known GBA motifs. We demonstrate that GBAi does not interfere with canonical GPCR-G protein signaling but blocks GBA-dependent signaling in cancer cells. Furthermore, by implementing GBAi in vivo, we show that GBA-dependent signaling modulates phenotypes during Xenopus laevis embryonic development. In summary, GBAi is a selective, efficient, and convenient tool to dissect the biological processes controlled by a GPCR-independent mechanism of G protein activation mediated by cytoplasmic factors.
Like tissues, single cells are subjected to continual stresses and damage. As such, cells have a robust wound repair mechanism comprised of dynamic membrane resealing and cortical cytoskeletal remodeling. One group of proteins, the Rho family of small guanosine triphosphatases (GTPases), is critical for this actin and myosin cytoskeletal response in which they form distinct dynamic spatial and temporal patterns/arrays surrounding the wound. A key mechanistic question, then, is how these GTPase arrays are formed. Here, we show that in the Drosophila melanogaster cell wound repair model Rho GTPase arrays form in response to prepatterning by Rho guanine nucleotide exchange factors (RhoGEFs), a family of proteins involved in the activation of small GTPases. Furthermore, we show that Annexin B9, a member of a class of proteins associated with the membrane resealing, is involved in an early, Rho family-independent, actin stabilization that is integral to the formation of one RhoGEF array. Thus, Annexin proteins may link membrane resealing to cytoskeletal remodeling processes in single cell wound repair.
Eukaryotic cells restrict protein synthesis under various stress conditions, by inhibiting the eukaryotic translation initiation factor 2B (eIF2B). eIF2B is the guanine nucleotide exchange factor for eIF2, a heterotrimeric G protein consisting of α-, β- and γ-subunits. eIF2B exchanges GDP for GTP on the γ-subunit of eIF2 (eIF2γ), and is inhibited by stress-induced phosphorylation of eIF2α. eIF2B is a heterodecameric complex of two copies each of the α-, β-, γ-, δ- and ε-subunits; its α-, β- and δ-subunits constitute the regulatory subcomplex, while the γ- and ε-subunits form the catalytic subcomplex. The three-dimensional structure of the entire eIF2B complex has not been determined. Here we present the crystal structure of Schizosaccharomyces pombe eIF2B with an unprecedented subunit arrangement, in which the α2β2δ2 hexameric regulatory subcomplex binds two γε dimeric catalytic subcomplexes on its opposite sides. A structure-based in vitro analysis by a surface-scanning site-directed photo-cross-linking method identified the eIF2α-binding and eIF2γ-binding interfaces, located far apart on the regulatory and catalytic subcomplexes, respectively. The eIF2γ-binding interface is located close to the conserved ‘NF motif’, which is important for nucleotide exchange. A structural model was constructed for the complex of eIF2B with phosphorylated eIF2α, which binds to eIF2B more strongly than the unphosphorylated form. These results indicate that the eIF2α phosphorylation generates the ‘nonproductive’ eIF2-eIF2B complex, which prevents nucleotide exchange on eIF2γ, and thus provide a structural framework for the eIF2B-mediated mechanism of stress-induced translational control.