MAPKK-independent activation of p38alpha mediated by TAB1-dependent autophosphorylation of p38alpha

Phosphorylation of mitogen-activated protein kinases (MAPKs) on specific tyrosine and threonine sites by MAP kinase kinases (MAPKKs) is thought to be the sole activation mechanism. Here, we report an unexpected activation mechanism for p38alpha MAPK that does not involve the prototypic kinase cascade. Rather it depends on interaction of p38alpha with TAB1 [transforming growth factor-beta-activated protein kinase 1 (TAK1)-binding protein 1] leading to autophosphorylation and activation of p38alpha. We detected formation of a TRAF6-TAB1-p38alpha complex and showed stimulus-specific TAB1-dependent and TAB1-independent p38alpha activation. These findings suggest that alternative activation pathways contribute to the biological responses of p38alpha to various stimuli.     

Structural mutations of the T cell receptor zeta chain and its role in T cell activation

T cell hybridomas that express zeta zeta, but not zeta eta, dimers in their T cell receptors (TCRs) produce interleukin-2 (IL-2) and undergo an inhibition of spontaneous growth when activated by antigen, antibodies to the receptor, or antibodies to Thy-1. Hybridomas without zeta and eta were reconstituted with mutated zeta chains. Cytoplasmic truncations of up to 40% of the zeta molecule reconstituted normal surface assembly of TCRs, but antigen-induced IL-2 secretion and growth inhibition were lost. In contrast, cross-linking antibodies to the TCR activated these cells. A point mutation conferred the same signaling phenotype as did the truncations and caused defective antigen-induced tyrosine kinase activation. Thus zeta allows the binding of antigen/major histocompatibility complex (MHC) to alpha beta to effect TCR signaling.     

Beta-arrestin 2: a receptor-regulated MAPK scaffold for the activation of JNK3

beta-Arrestins, originally discovered in the context of heterotrimeric guanine nucleotide binding protein-coupled receptor (GPCR) desensitization, also function in internalization and signaling of these receptors. We identified c-Jun amino-terminal kinase 3 (JNK3) as a binding partner of beta-arrestin 2 using a yeast two-hybrid screen and by coimmunoprecipitation from mouse brain extracts or cotransfected COS-7 cells. The upstream JNK activators apoptosis signal-regulating kinase 1 (ASK1) and mitogen-activated protein kinase (MAPK) kinase 4 were also found in complex with beta-arrestin 2. Cellular transfection of beta-arrestin 2 caused cytosolic retention of JNK3 and enhanced JNK3 phosphorylation stimulated by ASK1. Moreover, stimulation of the angiotensin II type 1A receptor activated JNK3 and triggered the colocalization of beta-arrestin 2 and active JNK3 to intracellular vesicles. Thus, beta-arrestin 2 acts as a scaffold protein, which brings the spatial distribution and activity of this MAPK module under the control of a GPCR.     

Coupling of stress in the ER to activation of JNK protein kinases by transmembrane protein kinase IRE1

Malfolded proteins in the endoplasmic reticulum (ER) induce cellular stress and activate c-Jun amino-terminal kinases (JNKs or SAPKs). Mammalian homologs of yeast IRE1, which activate chaperone genes in response to ER stress, also activated JNK, and IRE1alpha-/- fibroblasts were impaired in JNK activation by ER stress. The cytoplasmic part of IRE1 bound TRAF2, an adaptor protein that couples plasma membrane receptors to JNK activation. Dominant-negative TRAF2 inhibited activation of JNK by IRE1. Activation of JNK by endogenous signals initiated in the ER proceeds by a pathway similar to that initiated by cell surface receptors in response to extracellular signals.     

Plant mitogen-activated protein kinases and their roles in mediation of signal transduction in abiotic stresses

Mitogen-activated protein kinase (MAPK) cascade plays a central role in transfer information from diverse receptors/sensors to a wide range of cellular responses in plants. MAP kinases are organized into a complex network for efficient transmission of specific stimuli, including the abiotic stress signaling. In recent years, the mutants of loss-offunction and gain-of-function, and other additional tools are used to investigate the plant MAPK cascades. This review has summarized the recent progress on the MAPK cascade involved in mediation of the transduction of several pronounced abiotic stress signalings, such as salt, drought, low and high temperature, wound, hormone, and deficient nutrients. Currently, although part of the components of the MAPK cascade responding to the abiotic stresses have been identified, the integral molecular mechanisms of the abiotic stresses signaling transduction mediated viaMAPK cascade are largely unknown and need to be elucidated further in the future.     

植物MAPK级联途径及其功能研究进展

促分裂原活化蛋白激酶(Mitogen Actived Protein Kinase,MAPK)是一种蛋白激酶,蛋白激酶又称蛋白磷酸化酶,其可通过将ATP上的磷酸基团转移到底物蛋白质氨基酸残基上,来催化底物蛋白质磷酸化。MAPK级联途径广泛存在于真核生物中。植物中的MAPK级联途径与动物和酵母类似,都包括MAPKKK、MAPKK和MAPK 3种蛋白激酶。大量的研究表明,植物中的MAPK级联途径不仅能被多种生物与非生物胁迫所激活,同时也参与激素信号转导以及植物的生长发育进程。文章就植物中MAPK级联途径及其功能的研究情况进行了综述,并展望了相关研究的发展趋势。     

DUBA: a deubiquitinase that regulates type I interferon production

Production of type I interferon (IFN-I) is a critical host defense triggered by pattern-recognition receptors (PRRs) of the innate immune system. Deubiquitinating enzyme A (DUBA), an ovarian tumor domain-containing deubiquitinating enzyme, was discovered in a small interfering RNA-based screen as a regulator of IFN-I production. Reduction of DUBA augmented the PRR-induced IFN-I response, whereas ectopic expression of DUBA had the converse effect. DUBA bound tumor necrosis factor receptor-associated factor 3 (TRAF3), an adaptor protein essential for the IFN-I response. TRAF3 is an E3 ubiquitin ligase that preferentially assembled lysine-63-linked polyubiquitin chains. DUBA selectively cleaved the lysine-63-linked polyubiquitin chains on TRAF3, resulting in its dissociation from the downstream signaling complex containing TANK-binding kinase 1. A discrete ubiquitin interaction motif within DUBA was required for efficient deubiquitination of TRAF3 and optimal suppression of IFN-I. Our data identify DUBA as a negative regulator of innate immune responses.     

Keeping G proteins at bay: a complex between G protein-coupled receptor kinase 2 and Gbetagamma

The phosphorylation of heptahelical receptors by heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor kinases (GRKs) is a universal regulatory mechanism that leads to desensitization of G protein signaling and to the activation of alternative signaling pathways. We determined the crystallographic structure of bovine GRK2 in complex with G protein beta1gamma2 subunits. Our results show how the three domains of GRK2-the RGS (regulator of G protein signaling) homology, protein kinase, and pleckstrin homology domains-integrate their respective activities and recruit the enzyme to the cell membrane in an orientation that not only facilitates receptor phosphorylation, but also allows for the simultaneous inhibition of signaling by Galpha and Gbetagamma subunits.     

Common and distinct elements in cellular signaling via EGF and FGF receptors

Signaling pathways that are activated by epidermal growth factor (EGF) or fibroblast growth factor (FGF) receptors have been identified and compared (detailed Connections Maps are available at Science's Signal Transduction Knowledge Environment). Both receptors stimulate a similar complement of intracellular signaling pathways. However, whereas activated EGF receptors (EGFRs) function as the main platform for recruitment of signaling proteins, signaling through the FGF receptors (FGFRs) is mediated primarily by assembly of a multidocking protein complex. Moreover, FGFR signaling is subject to additional intracellular and extracellular control mechanisms that do not affect EGFR signaling. The differential circuitry of the intracellular networks that are activated by EGFR and FGFR may affect signal specificity and physiological responses.     

Beta-arrestin 2 mediates endocytosis of type III TGF-beta receptor and down-regulation of its signaling

beta-Arrestins bind to activated seven transmembrane-spanning (7TMS) receptors (G protein-coupled receptors) after the receptors are phosphorylated by G protein-coupled receptor kinases (GRKs), thereby regulating their signaling and internalization. Here, we demonstrate an unexpected and analogous role of beta-arrestin 2 (betaarr2) for the single transmembrane-spanning type III transforming growth factor-beta (TGF-beta) receptor (TbetaRIII, also referred to as betaglycan). Binding of betaarr2 to TbetaRIII was also triggered by phosphorylation of the receptor on its cytoplasmic domain (likely at threonine 841). However, such phosphorylation was mediated by the type II TGF-beta receptor (TbetaRII), which is itself a kinase, rather than by a GRK. Association with betaarr2 led to internalization of both receptors and down-regulation of TGF-beta signaling. Thus, the regulatory actions of beta-arrestins are broader than previously appreciated, extending to the TGF-beta receptor family as well.     

A domain in TNF receptors that mediates ligand-independent receptor assembly and signaling

A conserved domain in the extracellular region of the 60- and 80-kilodalton tumor necrosis factor receptors (TNFRs) was identified that mediates specific ligand-independent assembly of receptor trimers. This pre-ligand-binding assembly domain (PLAD) is physically distinct from the domain that forms the major contacts with ligand, but is necessary and sufficient for the assembly of TNFR complexes that bind TNF-alpha and mediate signaling. Other members of the TNFR superfamily, including TRAIL receptor 1 and CD40, show similar homotypic association. Thus, TNFRs and related receptors appear to function as preformed complexes rather than as individual receptor subunits that oligomerize after ligand binding.     

植物盐胁迫相关信号转导机制的研究

植物的抗盐反应是一个复杂的过程,受许多基因的调控,外界的高Na＋通过IP3诱导胞内Ca2＋的升高,SOS3接收Ca^2＋信号,激活SOS2的激酶活性,SOS2通过调节位于质膜和液泡膜上的Na＋/H＋反向转运体把Na＋运到体外或液泡中,ABA、ROS、AtHK1、MAPK级联反应和LEA也参与盐胁迫造成的渗透胁迫和损伤的反应过程,但是要通过生物学手段利用盐信号传递过程中一些成分提高作物的抗盐能力,还需要对植物盐胁迫相关信号转导机制进行更加深入的研究.     

Perception of brassinosteroids by the extracellular domain of the receptor kinase BRI1

An assay was developed to study plant receptor kinase activation and signaling mechanisms. The extracellular leucine-rich repeat (LRR) and transmembrane domains of the Arabidopsis receptor kinase BRI1, which is implicated in brassinosteroid signaling, were fused to the serine/threonine kinase domain of XA21, the rice disease resistance receptor. The chimeric receptor initiates plant defense responses in rice cells upon treatment with brassinosteroids. These results, which indicate that the extracellular domain of BRI1 perceives brassinosteroids, suggest a general signaling mechanism for the LRR receptor kinases of plants. This system should allow the discovery of ligands for the LRR kinases, the largest group of plant receptor kinases.     

Spinophilin blocks arrestin actions in vitro and in vivo at G protein-coupled receptors

Arrestin regulates almost all G protein-coupled receptor (GPCR)-mediated signaling and trafficking. We report that the multidomain protein, spinophilin, antagonizes these multiple arrestin functions. Through blocking G protein receptor kinase 2 (GRK2) association with receptor-Gbetagamma complexes, spinophilin reduces arrestin-stabilized receptor phosphorylation, receptor endocytosis, and the acceleration of mitogen-activated protein kinase (MAPK) activity following endocytosis. Spinophilin knockout mice were more sensitive than wild-type mice to sedation elicited by stimulation of alpha2 adrenergic receptors, whereas arrestin 3 knockout mice were more resistant, indicating that the signal-promoting, rather than the signal-terminating, roles of arrestin are more important for certain response pathways. The reciprocal interactions of GPCRs with spinophilin and arrestin represent a regulatory mechanism for fine-tuning complex receptor-orchestrated cell signaling and responses.     

植物MAPK级联途径应答的非生物胁迫研究进展

促分裂原活化蛋白激酶（mitogen-activated protein kinase, MAPK）是一种高度保守的蛋白激酶，广泛存在于真核生物中。MAPK级联途径是一种重要的高度保守的细胞信号转导途径，主要包括MAPKKK、MAPKK和MAPK三种蛋白激酶，通过磷酸化顺序被激活。MAPK级联途径参与了植物生长发育、激素调节、生物胁迫以及非生物胁迫的应答响应。从植物MAPK级联途径成员的分类、结构特征及对非生物胁迫的响应等方面进行了综述，将为系统的理解植物MAPK级联途径及其在非生物逆境胁迫下的表达调控等提供参考。     

水稻BWMK1互作基因的分离

为探明BWMK1介导的信号传导途径,采用酵母双杂交系统,构建水稻稻瘟病菌诱导后的cDNA文库,以BWMK1为诱饵,对文库进行筛选,通过验证、测序和基因编码分析,获得了7个BWMK1互作基因BWIP1～BWIP7,分别位于水稻基因组第12、2、3、10、6、4、5染色体上；7个互作基因中,除BWlP2和BWIP7未找到同...     

Structure of nerve growth factor complexed with the shared neurotrophin receptor p75

Neurotrophins are secreted growth factors critical for the development and maintenance of the vertebrate nervous system. Neurotrophins activate two types of cell surface receptors, the Trk receptor tyrosine kinases and the shared p75 neurotrophin receptor. We have determined the 2.4 A crystal structure of the prototypic neurotrophin, nerve growth factor (NGF), complexed with the extracellular domain of p75. Surprisingly, the complex is composed of an NGF homodimer asymmetrically bound to a single p75. p75 binds along the homodimeric interface of NGF, which disables NGF's symmetry-related second p75 binding site through an allosteric conformational change. Thus, neurotrophin signaling through p75 may occur by disassembly of p75 dimers and assembly of asymmetric 2:1 neurotrophin/p75 complexes, which could potentially engage a Trk receptor to form a trimolecular signaling complex.     

Pyogenic bacterial infections in humans with IRAK-4 deficiency

Members of the Toll-like receptor (TLR) and interleukin-1 receptor (IL-1R) superfamily share an intracytoplasmic Toll-IL-1 receptor (TIR) domain, which mediates recruitment of the interleukin-1 receptor-associated kinase (IRAK) complex via TIR-containing adapter molecules. We describe three unrelated children with inherited IRAK-4 deficiency. Their blood and fibroblast cells did not activate nuclear factor kappaB and mitogen-activated protein kinase (MAPK) and failed to induce downstream cytokines in response to any of the known ligands of TIR-bearing receptors. The otherwise healthy children developed infections caused by pyogenic bacteria. These findings suggest that, in humans, the TIR-IRAK signaling pathway is crucial for protective immunity against specific bacteria but is redundant against most other microorganisms.     

Snapshot of activated G proteins at the membrane: the Galphaq-GRK2-Gbetagamma complex

G protein-coupled receptor kinase 2 (GRK2) plays a key role in the desensitization of G protein-coupled receptor signaling by phosphorylating activated heptahelical receptors and by sequestering heterotrimeric G proteins. We report the atomic structure of GRK2 in complex with Galphaq and Gbetagamma, in which the activated Galpha subunit of Gq is fully dissociated from Gbetagamma and dramatically reoriented from its position in the inactive Galphabetagamma heterotrimer. Galphaq forms an effector-like interaction with the GRK2 regulator of G protein signaling (RGS) homology domain that is distinct from and does not overlap with that used to bind RGS proteins such as RGS4.