Suppressor of cytokine signaling-1 inhibits VAV function through protein degradation

Suppressor of cytokine signaling-1 (SOCS1) is an inducible Src homology 2 (SH2)-containing protein that negatively regulates cytokine and growth factor signaling required during thymic development. Recent evidence indicates that SOCS1 interacts with elongins B and C, which are components of a ubiquitin ligase complex, VCB (VHL/elonginC/B), based on the VHL (von Hippel Lindau) tumor suppressor protein. SOCS1 has previously been shown to operate as an inhibitor of Janus kinases. Here we show that SOCS1 has the distinct function of targeting the hematopoietic specific guanine nucleotide exchange factor, VAV, for ubiquitin-mediated protein degradation. VAV and SOCS1 form a protein complex through interactions between the VAV NH(2)-terminal regulatory region and the SH2 domain of SOCS1 in a phosphotyrosine-independent manner. SOCS1 decreases the steady state levels of cotransfected VAV and onco-VAV and reduces the focus forming activity of onco-VAV. SOCS1 stimulates the polyubiquitination of VAV proteins in vivo, which was stabilized by proteasomal inhibitors. These results suggest that SOCS1 programs VAV degradation by acting as a substrate-specific recognition component of a VCB-like ubiquitin ligase complex.     

The role of pseudokinases in cancer

Kinases play a critical role in regulating many cellular functions including development, differentiation and proliferation. To date, over 518 proteins with kinase activity, comprising ~2-3% of total cellular proteins, have been identified from within the human kinome. Interestingly, approximately 10% of kinases are categorised as pseudokinases since they lack one or more conserved catalytic residues within their kinase domain and were originally thought to have no enzymatic activity. Recently, there has been strong evidence to suggest that some pseudokinsases can not only function as scaffold proteins, but may also possess kinase activity leading to modulation of cell signalling pathways. Altered activity of these pseudokinases can result in impaired cellular function, particularly in malignancies. In this review we are discussing recent evidence that apart from a scaffolding role, pseudokinases also orchestrate cellular processes as active kinases per se in signalling pathways of malignant cells.     

The pebble GTP exchange factor and the control of cytokinesis

Several G proteins of the Rho family have been shown to be required for cytokinesis. The activity of these proteins is regulated by GTP exchange factors (GEFs), which stimulate GDP/GTP exchange, and by GTPase activating proteins (GAPs), which suppress activity by stimulating the intrinsic GTPase activity. The role of Rho family members during cytokinesis is likely to be determined by their spatial and temporal interactions with these factors. Here we focus on the role of the pebble (pbl) gene of Drosophila melanogaster, a RhoGEF that is required for cytokinesis. We summarise the evidence that the primary target of PBL is Rho1 and describe genetic approaches to elucidating the function of PBL and identifying other components of the PBL-activated Rho signalling pathway.     

Lipid rafts in T cell receptor signalling

The molecular events and the protein components that are involved in signalling by the T cell receptor (TCR) for antigen have been extensively studied. Activation of signalling cascades following TCR stimulation depends on the phosphorylation of the receptor by the tyrosine kinase Lck, which localizes to the cytoplasmic face of the plasma membrane by virtue of its post-translational modification. However, the precise order of events during TCR phosphorylation at the plasma membrane, remains to be defined. A current theory that describes early signalling events incorporates the function of lipid rafts, microdomains at the plasma membrane with distinct lipid and protein composition. Lipid rafts have been implicated in diverse biological functions in mammalian cells. In T cells, molecules with a key role in TCR signalling, including Lck, localize to these domains. Importantly, mutant versions of these proteins which fail to localise to raft domains were unable to support signalling by the TCR. Biochemical studies using purified detergent-resistant membranes (DRM) and confocal microscopy have suggested that upon stimulation, the TCR and Lck-containing lipid rafts may come into proximity allowing phosphorylation of the receptor. Further, there are data suggesting that phosphorylation of the TCR could depend on a transient increase in Lck activity that takes place within lipid rafts to initiate signalling. Current results and a model of how lipid rafts may regulate TCR signalling are discussed.     

The complex role of NOTCH receptors and their ligands in the development of hepatoblastoma,cholangiocarcinoma and hepatocellular carcinoma

The NOTCH signalling pathway is one of the key molecular pathways of embryonic development and adult tissues homeostasis in mammals. Mammals have four NOTCH receptors and various ligands that modulate their activity. Many cell disorders, whose genesis involves the NOTCH signalling pathway, have been discovered, including cancer. The mechanisms by which these receptors and their ligands affect liver cell transformation are not yet well understood, and they seem to behave as both oncogenes and tumour‐suppressor proteins. In this review, we discuss the published data regarding the role of these proteins in the development of hepatoblastoma, cholangiocarcinoma and hepatocellular carcinoma malignancies. The alteration of the NOTCH signalling pathway may be one of the main drivers of hepatic neoplastic growth. However, this signalling pathway might also modulate the development of specific liver tumour features. The complexity of the function of NOTCH receptors and their ligands may be due to their interactions with many other cell signalling pathways. Furthermore, the different levels of expression and activation of these receptors could be a reason for their distinct and sometimes contradictory effects.     

Noncoenzyme function of vitamin PP and its biologically active derivatives: the current state of the problem

This review is devoted to the current state of investigations of vitamin PP and nicotinamide dinucleotides noncoenzyme functions. Particular attention has been focused on the role of these compounds in post-translation modification of proteins (mono- and poly-ADP-ribosylation), in regulation of gene activity, calcium homeostasis and Ca2+ signalling as well as in modulation of synaptic transmission. Biological significance of these processes in cell function was elicited. The role of deregulation of vitamin PP mediated signalling mechanisms involved in control over the cell function under conditions of different diseases was emphasized.     

Protein phosphatases in MAPK signalling: we keep learning from yeast

Because of their key role in cell signalling, a rigorous regulation of mitogen-activated protein kinases (MAPKs) is essential in eukaryotic physiology. Whereas the use of binding motifs and scaffold proteins guarantees the selective activation of a specific MAPK pathway, activating kinases and downregulating phosphatases control the appropriate intensity and timing of MAPK activation. Tyrosine, serine/threonine and dual-specificity phosphatases co-ordinately dephosphorylate and thereby inactivate MAPKs. In budding yeast, enzymes that belong to these three types of phosphatases have been shown to counteract the MAPKs that govern the cellular response to varied extracellular stimuli. Studies carried out with these yeast phosphatases have expanded our knowledge of essential key aspects of the biology of these negative regulators, such as their function, the mechanisms that operate in their modulation by MAPK pathways and their binding to MAPK substrates. Furthermore, yeast MAPK phosphatases have been shown to play additional and essential roles in MAPK-mediated signalling, controlling MAPK localization or cross-talk among pathways. This review stresses the importance of these negative regulators in eukaryotic signalling by discussing the recent developments and perspectives in the study of yeast MAPK phosphatases.     

IQGAP1 and its binding proteins control diverse biological functions

IQGAP proteins have been identified in a wide spectrum of organisms, ranging from yeast to humans. The most extensively studied family member is the ubiquitously expressed scaffold protein IQGAP1, which participates in multiple essential aspects of mammalian biology. IQGAP1 mediates these effects by binding to and regulating the function of numerous interacting proteins. Over ninety proteins have been reported to associate with IQGAP1, either directly or as part of a larger complex. In this review, we summarise those IQGAP1 binding partners that have been identified in the last five years. The molecular mechanisms by which these interactions contribute to the functions of receptors and their signalling cascades, small GTPase function, cytoskeletal dynamics, neuronal regulation and intracellular trafficking are evaluated. The evidence that has accumulated recently validates the role of IQGAP1 as a scaffold protein and expands the repertoire of cellular activities in which it participates.     

SOCS proteins, cytokine signalling and immune regulation

Suppressor of cytokine signalling (SOCS) proteins are inhibitors of cytokine signalling pathways. Studies have shown that SOCS proteins are key physiological regulators of both innate and adaptive immunity. These molecules positively and negatively regulate macrophage and dendritic-cell activation and are essential for T-cell development and differentiation. Evidence is also emerging of the involvement of SOCS proteins in diseases of the immune system. In this Review we bring together data from recent studies on SOCS proteins and their role in immunity, and propose a cohesive model of how cytokine signalling regulates immune-cell function.     

The RGS protein Crg2 regulates both pheromone and cAMP signalling in Cryptococcus neoformans

G proteins orchestrate critical cellular functions by transducing extracellular signals into internal signals and controlling cellular responses to environmental cues. G proteins typically function as switches that are activated by G protein-coupled receptors (GPCRs) and negatively controlled by regulator of G protein signalling (RGS) proteins. In the human fungal pathogen Cryptococcus neoformans, three G protein alpha subunits (Gpa1, Gpa2 and Gpa3) have been identified. In a previous study, we identified the RGS protein Crg2 involved in regulating the pheromone response pathway through Gpa2 and Gpa3. In this study, a role for Crg2 was established in the Gpa1-cAMP signalling pathway that governs mating and virulence. We show that Crg2 physically interacts with Gpa1 and crg2 mutations increase cAMP production. crg2 mutations also enhance mating filament hyphae production, but reduce cell-cell fusion and sporulation efficiency during mating. Although crg2 mutations and the Gpa1 dominant active allele GPA1(Q284L) enhanced melanin production under normally repressive conditions, virulence was attenuated in a murine model. We conclude that Crg2 participates in controlling both Gpa1-cAMP-virulence and pheromone-mating signalling cascades and hypothesize it may serve as a molecular interface between these two central signalling conduits.     

Cooperative interactions at the SLP‐76 complex are critical for actin polymerization

T‐cell antigen receptor (TCR) engagement induces formation of multi‐protein signalling complexes essential for regulating T‐cell functions. Generation of a complex of SLP‐76, Nck and VAV1 is crucial for regulation of the actin machinery. We define the composition, stoichiometry and specificity of interactions in the SLP‐76, Nck and VAV1 complex. Our data reveal that this complex can contain one SLP‐76 molecule, two Nck and two VAV1 molecules. A direct interaction between Nck and VAV1 is mediated by binding between the C‐terminal SH3 domain of Nck and the VAV1 N‐terminal SH3 domain. Disruption of the VAV1:Nck interaction deleteriously affected actin polymerization. These novel findings shed new light on the mechanism of actin polymerization after T‐cell activation.     

Tobacco Rar1, EDS1 and NPR1/NIM1 like genes are required for N-mediated resistance to tobacco mosaic virus

The tobacco N gene confers resistance to tobacco mosaic virus (TMV) and encodes a Toll-interleukin-1 receptor/nucleotide binding site/leucine-rich repeat (TIR-NBS-LRR) class protein. We have developed and used a tobacco rattle virus (TRV) based virus induced gene silencing (VIGS) system to investigate the role of tobacco candidate genes in the N-mediated signalling pathway. To accomplish this we generated transgenic Nicotiana benthamiana containing the tobacco N gene. The transgenic lines exhibit hypersensitive response (HR) to TMV and restrict virus spread to the inoculated site. This demonstrates that the tobacco N gene can confer resistance to TMV in heterologous N. benthamiana. We have used this line to study the role of tobacco Rar1-, EDS1-, and NPR1/NIM1- like genes in N-mediated resistance to TMV using a TRV based VIGS approach. Our VIGS analysis suggests that these genes are required for N function. EDS1-like gene requirement for the N function suggests that EDS1 could be a common component of bacterial, fungal and viral resistance signalling mediated by the TIR-NBS-LRR class of resistance proteins. Requirement of Rar1- like gene for N-mediated resistance to TMV and some powdery mildew resistance genes in barley provide the first example of converging points in the disease resistance signalling pathways mediated by TIR-NBS-LRR and CC-NBS-LRR proteins. The TRV based VIGS approach as described here to study N-mediated resistance signalling will be useful for the analysis of not only disease resistance signalling pathways but also of other signalling pathways in genetically intractable plant systems.     

Endosomal sorting and signalling: an emerging role for sorting nexins

The endocytic network comprises a series of interconnected tubulo-vesicular membranous compartments that together regulate various sorting and signalling events. Although it is clear that defects in endocytic function underlie a variety of human diseases, our understanding of the molecular entities that regulate these sorting and signalling events remains limited. Here we discuss the sorting nexins family of proteins and propose that they have a fundamental role in orchestrating the formation of protein complexes that are involved in endosomal sorting and signalling.     

Investigating RGS proteins in yeast

Regulator of G protein signalling (RGS) proteins are vital in the adaptation of cells to stimulation via G protein-coupled receptors. Yeast have been integral in elucidating the important role that RGS proteins play within cellular processes. In addition to extensive characterisation of the endogenous RGS proteins, these organisms have enabled the identification and analysis of numerous mammalian homologues. The simplicity and plasticity of the yeast pheromone-response pathway has facilitated studies which would have been impossible in mammalian systems and it is certain that yeast will continue to have a great impact on this field of research in the future.     

A perspective on non-catalytic Src homology (SH) adaptor signalling proteins

Intracellular adaptor signalling proteins are members of a large family of mediators crucial for signal transduction pathways. Structurally, these molecules contain one Src Homology 2 (SH2) domain and one or more Src Homology 3 (SH3) domain(s); with either a catalytic subunit, or with other non-catalytic modular subunits. Cells depend on these regulatory signalling molecules to transmit information to the nucleus from both external and internal cues including growth factors, cytokines and steroids. Although there is a vast library of adaptor signalling proteins expressed ubiquitously in cells, the vital role these SH containing proteins play in regulating cellular signalling lacks the recognition they deserve. Their target selection method via the SH domains is simple yet highly effective. The SH3 domain(s) interact with proteins that contain proline-rich motifs, whereas the SH2 domain only binds to proteins containing phosphotyrosine residues. This unique characteristic physically enables proteins from a diverse range of networks to assemble for amplification of a signalling event. The biological consequence generated from these adaptor signalling proteins in a constantly changing microenvironment have profound regulatory effect on cell fate decision particularly when this is involved in the progression of a diseased state.