生长素结合蛋白研究进展

植物细胞感受生长素的信号是由生长素结合蛋白和生长素分子相结合。从而引起生长素信号传递的一系列级联反应而完成的．生长素与其结合蛋白的结合是生长素引发生理反应的第一步，也是必需的过程．     

Raf functions downstream of Ras1 in the Sevenless signal transduction pathway.

Specification of the R7 cell fate in the developing Drosophila eye requires activation of the Sevenless (Sev) receptor tyrosine kinase, located on the surface of the R7 precursor cell, by its interaction with the Boss protein, expressed on the surface of the neighbouring R8 cell. Four genes that participate in the intracellular transmission of this signal have so far been identified and molecularly characterized: Ras1, Sos, Gap1 and sina (refs 4-8). The Drosophila homologue of the mammalian Raf-1 serine/threonine kinase, which has been implicated in signal transduction pathways activated by many receptor tyrosine kinases (reviewed in refs 9 and 10), is encoded by the raf locus (also known as l(1)polehole, Draf-1 or Draf). Here we show that the Drosophila Raf serine/threonine kinase also plays a crucial role in the R7 pathway: the response to Sev activity is dependent on raf function, and a constitutively activated Raf protein can induce R7 cell development in the absence of sev function. We also present genetic evidence suggesting that Raf acts downstream of Ras1 and upstream of Sina in this signal transduction cascade.     

A novel putative auxin carrier family regulates intracellular auxin homeostasis in plants

The phytohormone auxin acts as a prominent signal, providing, by its local accumulation or depletion in selected cells, a spatial and temporal reference for changes in the developmental program. The distribution of auxin depends on both auxin metabolism (biosynthesis, conjugation and degradation) and cellular auxin transport. We identified in silico a novel putative auxin transport facilitator family, called PIN-LIKES (PILS). Here we illustrate that PILS proteins are required for auxin-dependent regulation of plant growth by determining the cellular sensitivity to auxin. PILS proteins regulate intracellular auxin accumulation at the endoplasmic reticulum and thus auxin availability for nuclear auxin signalling. PILS activity affects the level of endogenous auxin indole-3-acetic acid (IAA), presumably via intracellular accumulation and metabolism. Our findings reveal that the transport machinery to compartmentalize auxin within the cell is of an unexpected molecular complexity and demonstrate this compartmentalization to be functionally important for a number of developmental processes.     

Regulatory properties of LFA-1 alpha and beta chains in human T-lymphocyte activation

Lymphocyte function-associated antigen-1 (LFA-1) is a heterodimer composed of an alpha and beta chain that is expressed on the surface of most leukocytes and is an essential molecule for adhesion reactions between cells participating in the immune response. A putative ligand for LFA-1 is the intercellular adhesion molecule ICAM-1 (refs 3-5). Leukocyte adhesion abnormality is found in patients with LFA-1 deficiency. It is not clear whether binding of ligand to the LFA-1 molecule merely spatially orientates cells towards each other or can also induce signals that regulate cell activation and differentiation. We have recently developed a T-cell proliferation assay which uses immobilized anti-CD3 monoclonal antibodies as stimulant and is independent of LFA-1-mediated cellular adhesion. As there is no interference by anti-LFA-1 monoclonal antibodies with the adhesion-dependent activation steps, this T-cell activation system allows us to investigate whether transmembrane signals are induced by binding of ligand to LFA-1 on T cells. Our data indicate that binding of ligand to LFA-1 results in the transduction of regulatory signal across the plasma membrane, rather like other molecules (CD2, CD4, CD8) (refs 8-11) with signal-modifying properties involved in the adhesion of T cells to target/stimulator cells. Indeed, adhesion molecules might generally be important in signal transduction, even in cells not belonging to the immune system.     

Protein kinase C switches the Raf kinase inhibitor from Raf-1 to GRK-2

Feedback inhibition is a fundamental principle in signal transduction allowing rapid adaptation to different stimuli. In mammalian cells, the major feedback inhibitor for G-protein-coupled receptors (GPCR) is G-protein-coupled receptor kinase 2 (GRK-2), which phosphorylates activated receptors, uncouples them from G proteins and initiates their internalization. The functions of GRK-2 are indispensable and need to be tightly controlled. Dysregulation promotes disorders such as hypertension or heart failure. In our search for a control mechanism for this vital kinase, here we show that the Raf kinase inhibitor protein (RKIP) is a physiological inhibitor of GRK-2. After stimulation of GPCR, RKIP dissociates from its known target, Raf-1 (refs 6-8), to associate with GRK-2 and block its activity. This switch is triggered by protein kinase C (PKC)-dependent phosphorylation of the RKIP on serine 153. The data delineate a new principle in signal transduction: by activating PKC, the incoming receptor signal is enhanced both by removing an inhibitor from Raf-1 and by blocking receptor internalization. A physiological role for this mechanism is shown in cardiomyocytes in which the downregulation of RKIP restrains beta-adrenergic signalling and contractile activity.     

A novel sensor to map auxin response and distribution at high spatio-temporal resolution

Auxin is a key plant morphogenetic signal but tools to analyse dynamically its distribution and signalling during development are still limited. Auxin perception directly triggers the degradation of Aux/IAA repressor proteins. Here we describe a novel Aux/IAA-based auxin signalling sensor termed DII-VENUS that was engineered in the model plant Arabidopsis thaliana. The VENUS fast maturing form of yellow fluorescent protein was fused in-frame to the Aux/IAA auxin-interaction domain (termed domain II; DII) and expressed under a constitutive promoter. We initially show that DII-VENUS abundance is dependent on auxin, its TIR1/AFBs co-receptors and proteasome activities. Next, we demonstrate that DII-VENUS provides a map of relative auxin distribution at cellular resolution in different tissues. DII-VENUS is also rapidly degraded in response to auxin and we used it to visualize dynamic changes in cellular auxin distribution successfully during two developmental responses, the root gravitropic response and lateral organ production at the shoot apex. Our results illustrate the value of developing response input sensors such as DII-VENUS to provide high-resolution spatio-temporal information about hormone distribution and response during plant growth and development.     

Activation of the beta 1 isozyme of phospholipase C by alpha subunits of the Gq class of G proteins.

Many hormones, neurotransmitters and growth factors, on binding to G protein-coupled receptors or receptors possessing tyrosine kinase activity, increase intracellular levels of the second messengers inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. This is due to activation of phosphoinositide-specific phospholipase(s) C (PLC), the isozymes of which are classified into groups, alpha, beta, gamma and delta. The beta, gamma and delta groups themselves contain PLC isozymes which have both common and unique structural domains. Only the gamma 1 isozyme has been implicated in a signal transduction mechanism. This involves association with, and tyrosine phosphorylation by, the ligand-bound epidermal growth factor and platelet-derived growth factor receptors, probably by means of the PLC-gamma 1-specific src homology (SH2) domain. Because EGF receptor-mediated tyrosine phosphorylation of PLC-gamma 1 stimulates catalytic activity in vitro and G proteins have been implicated in the activation of PLC, we investigated which PLC isozymes are subject to G protein regulation. We have purified an activated G protein alpha subunit that stimulates partially purified phospholipase C and now report that this G protein specifically activates the beta 1 isozyme, but not the gamma 1 and delta 1 isozymes of phospholipase C. We also show that this protein is related to the Gq class of G protein alpha subunits.     

Identification of CRE1 as a cytokinin receptor from Arabidopsis

Cytokinins are a class of plant hormones that are central to the regulation of cell division and differentiation in plants. It has been proposed that they are detected by a two-component system, because overexpression of the histidine kinase gene CKI1 induces typical cytokinin responses and genes for a set of response regulators of two-component systems can be induced by cytokinins. Two-component systems use a histidine kinase as an environmental sensor and rely on a phosphorelay for signal transduction. They are common in microorganisms, and are also emerging as important signal detection routes in plants. Here we report the identification of a cytokinin receptor. We identified Arabidopsis cre1 (cytokinin response 1) mutants, which exhibited reduced responses to cytokinins. The mutated gene CRE1 encodes a histidine kinase. CRE1 expression conferred a cytokinin-dependent growth phenotype on a yeast mutant that lacked the endogenous histidine kinase SLN1 (ref. 10), providing direct evidence that CRE1 is a cytokinin receptor. We also provide evidence that cytokinins can activate CRE1 to initiate phosphorelay signalling.     

Stimulation of specific GTP binding and hydrolysis activities in lymphocyte membrane by interleukin-2

Interleukin-2 (IL-2) is a polypeptide growth factor which stimulates the proliferation and differentiation of T lymphocytes. The receptor for IL-2 is expressed on activated T lymphocytes, cloned IL-2 dependent cells and several other cell types. Analysis of the primary structure and of immune-precipitated receptor suggests that this molecule has no intrinsic signal transduction function, unlike other growth factors. IL-2 interaction with a high affinity receptor has been shown, however, to activate the calcium/phospholipid-dependent protein kinase C (PK-C) presumably via phosphoinositide hydrolysis. Members of a family of closely related guanine nucleotide binding proteins (G proteins) regulate a diverse group of metabolic events. Two of them, Gs and Gi, stimulate and inhibit adenylate cyclase activity respectively, and other G proteins are involved in diverse signal transduction system. Another member, Go, has no known function and activation of phospholipase C has been attributed to the action of an unidentified G protein, Gp. Since it has been observed that IL-2 inhibits the catalytic activity of adenylate cyclase and that agents such as PGE2 which stimulate adenylate cyclase activity inhibit the lymphoproliferative response to IL-2, association of GTP binding proteins with IL-2 signal transduction was investigated. In this report we describe for the first time the participation of a GTP binding protein in the action of a polypeptide growth factor, interleukin-2.     

Ion channels activated by inositol 1,4,5-trisphosphate in plasma membrane of human T-lymphocytes

Hydrolysis of membrane-associated phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)-P2) to water soluble inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a common response by many different kinds of cells to a wide variety of external stimuli (see refs 1 and 2 for review). Ins (1,4,5)P3 is a putative second messenger which increases intracellular Ca2+ by mobilizing internal Ca2+ stores, a hypothesis which has been substantiated by studies with chemically permeabilized cells and with isolated microsomal membrane fractions. But the possibility that Ins(1,4,5)P3 could induce in intact cells an influx of external Ca2+ through transmembrane channels, originally hypothesized by Michell in 1975, has never been directly tested. We report here single-channel recordings of an Ins(1,4,5)P3-activated conductance in excised patches of T-lymphocyte plasma membrane. The Ins(1,4,5)P3-activated transmembrane channel appears to be identical to the recently described mitogen-regulated, voltage-insensitive Ca2+ permeable channel involved in T-cell activation. We suggest that Ins(1,4,5)P3 acts as the second messenger mediating transmembrane Ca2+ influx through specific Ca2+-permeable channels in mitogen-stimulated T-cell activation.     

Guard cell abscisic acid signalling and engineering drought hardiness in plants

Guard cells are located in the epidermis of plant leaves, and in pairs surround stomatal pores. These control both the influx of CO2 as a raw material for photosynthesis and water loss from plants through transpiration to the atmosphere. Guard cells have become a highly developed system for dissecting early signal transduction mechanisms in plants. In response to drought, plants synthesize the hormone abscisic acid, which triggers closing of stomata, thus reducing water loss. Recently, central regulators of guard cell abscisic acid signalling have been discovered. The molecular understanding of the guard cell signal transduction network opens possibilities for engineering stomatal responses to control CO2 intake and plant water loss.     

A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence

Plants sense potential microbial invaders by using pattern-recognition receptors to recognize pathogen-associated molecular patterns (PAMPs). In Arabidopsis thaliana, the leucine-rich repeat receptor kinases flagellin-sensitive 2 (FLS2) (ref. 2) and elongation factor Tu receptor (EFR) (ref. 3) act as pattern-recognition receptors for the bacterial PAMPs flagellin and elongation factor Tu (EF-Tu) (ref. 5) and contribute to resistance against bacterial pathogens. Little is known about the molecular mechanisms that link receptor activation to intracellular signal transduction. Here we show that BAK1 (BRI1-associated receptor kinase 1), a leucine-rich repeat receptor-like kinase that has been reported to regulate the brassinosteroid receptor BRI1 (refs 6,7), is involved in signalling by FLS2 and EFR. Plants carrying bak1 mutations show normal flagellin binding but abnormal early and late flagellin-triggered responses, indicating that BAK1 acts as a positive regulator in signalling. The bak1-mutant plants also show a reduction in early, but not late, EF-Tu-triggered responses. The decrease in responses to PAMPs is not due to reduced sensitivity to brassinosteroids. We provide evidence that FLS2 and BAK1 form a complex in vivo, in a specific ligand-dependent manner, within the first minutes of stimulation with flagellin. Thus, BAK1 is not only associated with developmental regulation through the plant hormone receptor BRI1 (refs 6,7), but also has a functional role in PRR-dependent signalling, which initiates innate immunity.     

Regulation of focal adhesion-associated protein tyrosine kinase by both cellular adhesion and oncogenic transformation.

Increasing evidence indicates that the integrin family of cell adhesion receptors can transduce biochemical signals from the extracellular matrix to the cell interior to modulate cell growth and differentiation. We have shown that integrin/ligand interactions can trigger tyrosine phosphorylation of a protein of M(r) 120,000 (pp120), so it is possible that signal transduction by integrins might involve activation of intracellular protein tyrosine kinases as an early event in cell binding to the extracellular matrix. Here we report that pp120 is identical to the focal adhesion-associated protein tyrosine kinase pp125FAK (refs 3, 4). We show that tyrosine phosphorylation of this protein is modulated both by cell adhesion and transformation by pp60v-src, and that these changes in phosphorylation are correlated with increased pp125FAK tyrosine kinase activity. A model is proposed to relate these findings to the molecular basis of anchorage-independent growth of transformed cells.     

Toward understanding the molecular mechanisms governing plant hormone actions: A brief introduction to the major research program “Molecular mechanisms of plant hormone actions” funded by the national natural science foundation of china (NSFC)

Decades of physiological and molecular studies have demonstrated that plants rely on a diverse set of small molecule hormones to regulate every aspect of their biological processes including development,growth and adaptation.The     

Auxin transport is sufficient to generate a maximum and gradient guiding root growth

The plant growth regulator auxin controls cell identity, cell division and cell expansion. Auxin efflux facilitators (PINs) are associated with auxin maxima in distal regions of both shoots and roots. Here we model diffusion and PIN-facilitated auxin transport in and across cells within a structured root layout. In our model, the stable accumulation of auxin in a distal maximum emerges from the auxin flux pattern. We have experimentally tested model predictions of robustness and self-organization. Our model explains pattern formation and morphogenesis at timescales from seconds to weeks, and can be understood by conceptualizing the root as an 'auxin capacitor'. A robust auxin gradient associated with the maximum, in combination with separable roles of auxin in cell division and cell expansion, is able to explain the formation, maintenance and growth of sharply bounded meristematic and elongation zones. Directional permeability and diffusion can fully account for stable auxin maxima and gradients that can instruct morphogenesis.