bluuebunny/crossref_metadata_2025 huggingface dataset:
Plant 14-3-3s: Omnipotent Metabolic Phosphopartners?
Authors: Sehnke Paul C. , Ferl Robert J.
Published: May 2003

The accurate regulation of metabolism is crucial to the existence all organisms. The inappropriate activation of metabolic enzymes can waste precious energy; likewise, the failure to activate metabolic enzymes can disrupt homeostasis and lead to suboptimal cellular (and organismic) responses. Plants use several means to control their metabolic proteins, including a two-step process of protein phosphorylation and subsequent binding by phosphospecific binding proteins termed 14-3-3 proteins. Sehnke and Ferl discuss how 14-3-3 proteins regulate the activity of nitrate reductase and the H
+
-ATPase pump in plants, and compare the functions of 14-3-3 proteins in plants and animals.
Application of the Chromatin Immunoprecipitation Method to Identify in Vivo Protein-DNA Associations in Fission Yeast
Authors: Takahashi Kohta , Saitoh Shigeaki , Yanagida Mitsuhiro
Published: May 2003

The chromatin immunoprecipitation (ChIP) method provides an ideal tool for detecting direct or indirect interactions between proteins of interest and DNAs with known sequences. Here, we introduce the ChIP protocol used in our laboratory to identify in vivo protein-DNA association in the fission yeast
Schizosaccharomyces pombe.
The cytological and genetic merits of the fission yeast for studying control of the eukaryotic cell cycle and chromosome dynamics are reinforced by application of this ChIP method.
Meeting Report: Targeting Protein Phosphatases-Medicines for the New MilleniumA review of the meeting Protein Phosphatases, FASEB Summer Research Conference, Copper Mountain, CO, 23 to 28 July 2000.
Authors: Shenolikar Shirish , Brautigan David L.
Published: May 2003

Shenolikar and Brautigan summarize the key issues discussed at the conference on protein phosphatases of the Federation of American Societies for Experimental Biology (FASEB). A theme of the meeting was how basic research in the field of protein phosphatases has led to better understanding and treatments for human disease, including type 2 diabetes and obesity. A second important issue presented related to identification and characterization of various phosphatase-binding proteins that regulate phosphatase action.
NF-κB Defects in Humans: The NEMO/Incontinentia Pigmenti Connection
Authors: Courtois Gilles , Israël Alain
Published: May 2003

The components of the nuclear factor-κB (NF-κB) family of transcription factors are critical for regulating the response to immune challenges. Recently, a role for NF-κB in skin biology has been revealed. Within the cascade of proteins whose activities impinge upon the activation of NF-κB, the NEMO (NF-κB essential modulator)/IKKγ protein is required for the activation of the IκB kinases, which in turn, promote the degradation of IκB proteins, leading to the derepression of NF-κB activity. Courtois and Israël discuss the role of NEMO/IKKγ in normal physiological activation of NF-κB and the consequences of defective NF-κB activation, as an effect of NEMO/IKKγ mutations, which can lead to incontinentia pigmenti, a disease marked by alopecia, tooth eruption, skin lesions, and changes in skin pigmentation.
Directed Cell-to-Cell Movement of Functional Proteins: Do Transcription Factors Double as Signal Molecules in Plants?
Authors: Jorgensen Richard A.
Published: May 2003

Cell fate in plants is predominantly determined by position during development. Jorgensen discusses a new mechanism by which cells can regulate gene expression in neighboring cells. Several lines of evidence suggest that transcription factors expressed in one cell can traffic through plasmodesmata to regulate gene expression in adjacent cells that are not expressing the transcription factor themselves. In plants, some transcription factors appear to be intercellular signaling molecules, as well as intracellular regulators of gene expression.
Coregulation of Ion Channels by Neurosteroids and Phosphorylation
Authors: Tasker Jeffrey
Published: May 2003

This Perspective by Tasker is one of two discussing the regulation of ion channels by neurosteroids. Neurosteroids are steroids synthesized in the brain that act as allosteric modulators of ion-channel activity. The dual regulation of γ-aminobutyric acid type A (GABA
A
) receptors and voltage-gated calcium channels by phosphorylation and neurosteroids is the focus of this Perspective.
Molecular Aspects of the Cellular Activities of ADP-Ribosylation Factors
Authors: Randazzo Paul A. , Nie Zhongzhen , Miura Koichi , Hsu Victor W.
Published: May 2003

Adenosine diphosphate-ribosylation factor (Arf) proteins are members of the Arf arm of the Ras superfamily of guanosine triphosphate (GTP)-binding proteins. Arfs are named for their activity as cofactors for cholera toxin-catalyzed adenosine diphosphate-ribosylation of the heterotrimeric G protein Gs. Physiologically, Arfs regulate membrane traffic and the actin cytoskeleton. Arfs function both constitutively within the secretory pathway and as targets of signal transduction in the cell periphery. In each case, the controlled binding and hydrolysis of GTP is critical to Arf function. The activities of some guanine nucleotide exchange factors (GEFs) and guanosine triphosphatase (GTPase)-activating proteins (GAPs) are stimulated by phosphoinositides, including phosphatidylinositol 3,4,5-trisphosphate (PIP3) and phosphatidylinositol 4,5-bisphosphate (PIP2), and phosphatidic acid (PA), likely providing both a means to respond to regulatory signals and a mechanism to coordinate GTP binding and hydrolysis. Arfs affect membrane traffic in part by recruiting coat proteins, including COPI and clathrin adaptor complexes, to membranes. However, Arf function likely involves many additional biochemical activities. Arf activates phospholipase D and phosphatidylinositol 4-phosphate 5-kinase with the consequent production of PA and PIP2, respectively. In addition to mediating Arf's effects on membrane traffic and the actin cytoskeleton, PA and PIP2 are involved in the regulation of Arf. Arf also works with Rho family proteins to affect the actin cytoskeleton. Several Arf-binding proteins suspected to be effectors have been identified in two-hybrid screens. Arf-dependent biochemical activities, actin cytoskeleton changes, and membrane trafficking may be integrally related. Understanding Arf's role in complex cellular functions such as protein secretion or cell movement will involve a description of the temporal and spatial coordination of these multiple Arf-dependent events.
Dueling Enigmas: Neurosteroids and Sigma Receptors in the Limelight
Authors: Gibbs Terrell T. , Farb David H.
Published: May 2003

Neurosteroids can be positive or negative regulators of neurotransmitter receptor action, depending on the receptor and the chemical structure of the neurosteroid. This Perspective by Gibbs and Farb is one of two on the subject of neurosteroids. The authors address the possible role of σ receptors in mediating neurosteroid action and describe how the regulation of inhibitory and excitatory ion channels by neurosteroids has implications for the role of these molecules in learning and memory, nociception, and excitotoxicity.
Meeting Report: Plant Developmental Biologists Show Their Colors Toward a Virtual Understanding of Green DevelopmentA review of the meeting "Mechanisms in Plant Development", FASEB Summer Research Conference, Saxtons River, Vermont, 12 to 17 August 2000
Authors: Schrick Kathrin
Published: May 2003

Schrick summarizes the advances made in understanding plant development presented at the FASEB Summer Research Conference, at which lessons learned from
Arabidopsis
were in the limelight. How the underlying mechanisms of plant development are similar to animal developmental pathways are highlighted.
An in Vitro Assay to Study Regulated mRNA Stability
Authors: Fritz David T. , Ford Lance P. , Wilusz Jeffrey
Published: May 2003

The examination of posttranscriptional regulation of mRNA in mammalian cells is critical to discovering the role that mRNA plays in the initiation and maintenance of cellular processes. The complexity of the system defies a holistic approach and, therefore, we have devised an in vitro mRNA turnover assay that enables us to elucidate the factors involved in mRNA deadenylation and degradation. Our system, using an S100 HeLa extract and in vitro transcribed RNAs, accurately mimics the end products of mRNA turnover, which have been previously described using in vivo studies and, in addition, allows for the detailed study of factors that may play a role in regulated deadenylation and degradation. Another important aspect of our system is the ease with which it can be manipulated. We can provide any synthetic RNA molecule to the assay to test for specific sequence activity. Furthermore, the results are clear and accurately interpretable. We have demonstrated that our in vitro system accurately deadenylates and decays a capped and polyadenylated RNA molecule in a processive manner without nonspecific nuclease activity. Finally, we have demonstrated regulated instability in vitro using the AU-rich elements (AREs) from tumor necrosis factor-α (TNF-α) and granulocyte macrophage colony stimulating factor (GM-CSF) embedded within the RNA molecule. The presence of the AREs increased the deadenylation and the decay rates seen in vivo. We feel that this system can be expanded and adapted to examine a variety of mRNA regulatory events in mammalian cells.
Regulation of Enzymatic Activity and Gene Expression by Membrane Fluidity
Authors: Los Dmitry A. , Murata Norio
Published: May 2003

Changes in the cellular environment can lead to alterations in the fluidity of the membranes of prokaryotes and eukaryotes. Changes in temperature and osmotic conditions are two of the best-studied stresses that can affect membrane fluidity. Los and Murata discuss the types of sensors that detect these changes in membrane fluidity and the types of signals that are generated.
Photoactivated Gene Expression for Cell Fate Mapping and Cell Manipulation
Authors: Minden Jonathan , Namba Ruria , Mergliano Jaime , Cambridge Sidney
Published: May 2003

A long-standing goal of developmental biologists is to create developmental fate maps by tracking individual cells through development. Another objective is to perturb the behavior of selected cells and follow the ensuing effects. To this end, we have developed a technique that allows for spatial and temporal control of gene expression in single cells or patches of cells using light to induce gene expression. This technique relies on "caging" the activity of the potent transcriptional activator GAL4VP16 with a photolabile compound, which can be removed with a brief exposure to long-wavelength ultraviolet (UV) light. The caged GAL4VP16 is injected into early-stage embryos, which are aged to the desired point in development, and the cell(s) of interest are irradiated with a brief pulse of long-wavelength UV light. This method has been used extensively in
Drosophila, Xenopus,
and Zebrafish embryos. The methods for purifying, caging, injection, and photoactivation of the GAL4VP16 protein, and methods for the visualization of marked cells are described in detail.
Protein N-myristoylation: Critical Role in Apoptosis and Salt Tolerance
Authors: de Jonge Hugo R. , Hogema Boris , Tilly Ben C.
Published: May 2003

N-myristoylation is a covalent protein modification that can promote the association of proteins with membranes. De Jonge, Hogema, and Tilly discuss how N-myristoylation may be involved in triggering Fas ligand-induced apoptosis in mammals, and in adapting to conditions of high salt in plants. The pro-apoptotic protein BID is unique in that its proteolytic cleavage product, tBID, is posttranslationally myristoylated. In contrast, the plant accessory protein SOS3 undergoes "classical" cotranslational N-myristoylation. N-myristoylation is essential for the proper functioning of these proteins in regulating the signaling pathways (apoptosis and adaptation to salt stress, respectively) in which they are involved.
Positive Versus Negative Signaling of LET-23: Regulation Through the Adaptor Protein, SEM-5
Authors: Worby Carolyn , Margolis Ben
Published: May 2003

Worby and Margolis highlight advances in our understanding of signaling from growth factor receptors using the worm
Caenorhabditis elegans
as a model organism. ARK-1, a cytoplasmic tyrosine kinase, appears to be a negative regulator of multiple pathways in
C. elegans
. The authors discuss several models for how this negative regulation may occur. The adaptor protein (Grb2 in mammals or SEM-5 in
C. elegans
) may serve as a regulated scaffold for the binding of other signaling proteins that include both positive (Ras) and negative (ACK) regulators. Thus, Grb2 may function in a cellular decision point for transducing the incoming signals.
LAT, the Linker for Activation of T Cells: A Bridge Between T Cell-Specific and General Signaling Pathways
Authors: Wange Ronald L.
Published: May 2003

A key event in the regulation of the adaptive immune response is the binding of major histocompatibility complex-bound foreign peptides to T cell antigen receptors (TCRs) that are present on the cell surface of T lymphocytes. Recognition of the presence of cognate antigen in the host animal induces a series of biochemical changes within the T cell; these changes, in the context of additional signals from other surface receptors, ultimately result in massive proliferation of receptor-engaged T cells and the acquisition of effector and memory functions. Early studies established the importance of the activation of the enzymes phospholipase C-γ1 (PLC-γ1) and phosphatidylinositol 3-kinase (PI3K), as well as the small molecular weight heterotrimeric guanine nucleotide binding protein (G protein) Ras, in this process. These biochemical events are dependent on the activity of several protein tyrosine kinases that become activated immediately upon TCR engagement. An unresolved question in the field has been which molecules and what sequence of events tie together the early tyrosine phosphorylation events with the activation of these downstream signaling molecules. A likely candidate for linking the proximal and distal portions of the TCR signaling pathway is the recently described protein, LAT. LAT is a 36-kD transmembrane protein that becomes rapidly tyrosine-phosphorylated after TCR engagement. Phosphorylation of LAT creates binding sites for the Src homology 2 (SH2) domains of other proteins, including PLC-γ1, Grb2, Gads, Grap, 3BP2, and Shb, and indirectly binds SOS, c-Cbl, Vav, SLP-76, and Itk. LAT is localized to the glycolipid-enriched membrane (GEM) subdomains of the plasma membrane by virtue of palmitoylation of two cysteine residues positioned near the endofacial side of the plasma membrane. Notably, in the absence of LAT, TCR engagement does not lead to activation of distal signaling events. This review examines the circumstances surrounding the discovery of LAT and our current understanding of its properties, and discusses current models for how LAT may be functioning to support the transduction of TCR-initiated, T cell-specific signaling events to the distal, general signaling machinery.
Fat Hedgehogs, Slower or Richer?
Authors: van den Heuvel Marcel
Published: May 2003

Hedgehog (Hh) is a protein ligand that is involved in early developmental patterning. The activity of Hh is controlled by cleavage events and various posttranslational modifications, including the addition of cholesterol and palmitic acid. Van den Heuvel describes how acylation of Hh affects the activity of the protein and explores the differences in activity between the vertebrate sonic hedgehog (Shh) and
Drosophila
Hh.
Ypt/Rab GTPases: Regulators of Protein Trafficking
Authors: Segev Nava
Published: May 2003

Ypt/Rab guanosine triphosphatases (GTPases) have emerged in the last decade as key regulators of protein transport in all eukaryotic cells. They seem to be involved in all aspects of vesicle trafficking: vesicle formation, motility, and docking, and membrane remodeling and fusion. The functions of Ypt/Rabs are themselves controlled by upstream regulators that stimulate both their nucleotide cycling and their cycling between membranes. Ypt/Rabs transmit signals to downstream effectors in a guanosine triphosphate (GTP)-dependent manner. The identity of upstream regulators and downstream effectors is known for a number of Ypt/Rabs, and models for their mechanisms of action are emerging. In at least two cases, Ypt/Rab upstream regulators and downstream effectors are found together in a single complex. In agreement with the idea that Ypt/Rabs function in all aspects of vesicular transport, their diverse effectors have recently been shown to function in all identified aspects of vesicle transport. Activators and effectors for individual Ypt/Rabs share no similarity, but are conserved between yeast and mammalian cells. Finally, cross talk demonstrated among the various Ypt/Rabs, and between Ypt/Rabs and other signaling factors, suggests possible coordination among secretory steps, as well as between protein transport and other cellular processes.
Judging a Protein by More Than Its Name: GSK-3
Authors: Woodgett James R.
Published: May 2003

As knowledge of cellular signal transduction has accumulated, general truisms have emerged, including the notion that signaling proteins are usually activated by stimuli and that they, in turn, mediate the actions of specific agonists. Glycogen synthase kinase-3 (GSK-3) is an unusual protein-serine kinase that bucks these conventions. This evolutionarily conserved protein kinase is active in resting cells and is inhibited in response to activation of several distinct pathways, including those acting by elevation of 3′ phosphorylated phosphatidylinositol lipids and adenosine 3′-5′-monophosphate (cAMP). In addition, GSK-3 is distinctly regulated by, and is a core component of, the Wnt pathway. This review describes the unique characteristics of this decidedly oddball protein kinase in terms of its diverse biological functions, plethora of targets, role in several human diseases, and consequential potential as a therapeutic target.
Wnts, Signaling and Sulfates
Authors: Blair Seth S.
Published: May 2003

Questions remain about the signaling pathways that control pattern formation during development. Blair describes how sulfated glycosaminoglycans affect several developmentally important signaling pathways, including Wnt-Wingless, Fibroblast growth factor, Hedgehog, and Bone morphogenetic protein-4 signaling. A new secreted sulfatase, Qsulf1, regulates the sensitivity of vertebrate cells to Wnts, possibly by modifying the sulfation of glycosaminoglycans.
Combining Genetics and Cell Biology to Crack the Code of Plant Cell Calcium Signaling
Authors: Allen Gethyn J. , Schroeder Julian I.
Published: May 2003

Plant hormones, light receptors, pathogens, and abiotic signals trigger elevations in the cytosolic calcium concentration, which mediate physiological and developmental responses. Recent studies are reviewed here that reveal how specific genetic mutations impair or modify stimulus-induced calcium elevations in plant cells. These studies provide genetic evidence for the importance of calcium as a second messenger in plant signal transduction. A fundamental question arises: How can different stimuli use the same second messenger, calcium, to mediate different responses? Recent research and models are reviewed that suggest that several important mechanisms contribute to specificity in calcium signaling in plant cells. These mechanisms include (i) activation of different calcium channels in the plasma membrane and organellar membranes, (ii) stimulus-specific calcium oscillation parameters, (iii) cell type-specific responses, and (iv) intracellular localization of calcium gradients and calcium elevations in plant cells.
Cell Signalling - The Proteomics of It All
Authors: Ashman Keith , Moran Michael F. , Sicheri Frank , Pawson Tony , Tyers Mike
Published: May 2003

A challenge for biomedical scientists today is to arrive at an understanding of cellular behavior on a global scale. The advent of DNA microarrays has greatly facilitated discovery of gene expression profiles associated with different cellular states. The problem of understanding cellular signaling at the level of the interacting proteins is in some ways more challenging. Ashman
et al
. discuss the current methods available for studying protein interactions on a global scale, as well as directions for the future. Technical hurdles exist at many stages, from the isolation of protein complexes, to the determination of their composition, to the software and databases needed to analyze the results of large-scale, high-throughput datasets. Ashman
et al
. suggest that, with advances in technology and cooperation among academia and industry, a global protein interaction map that underlies cellular behavior will emerge as an essential resource for basic and applied research.
Identification of Cell Signaling Molecules by Expression Cloning
Authors: Matter Michelle L. , Ginsberg Mark H. , Ramos Joe W.
Published: May 2003

Using expression cloning one can isolate proteins with specific biological functions. This methodology can be adapted for the identification of novel players in the regulation of cell signaling. Here, we describe an expression cloning strategy to identify suppressors of Ras signaling. This screen is based on the observation that the activation of the small guanosine triphosphate (GTP)-binding protein H-Ras initiates a mitogen-activated protein kinase (MAPK)-dependent signaling pathway that inactivates integrin ligand binding. Our strategy depends on flow cytometry and a monoclonal antibody that recognizes integrin activation states. Flow cytometry enhances the screen's sensitivity thereby allowing us to examine function quantitatively at the level of a single cell millions of times in one screen. The following protocol provides a detailed method for the isolation of proteins that regulate cell signaling.
Is Dimerization Of Chemokine Receptors Functionally Relevant?
Authors: Thelen Marcus , Baggiolini Marco
Published: May 2003

Chemokines activate lymphocytes by initiating signaling cascades through cell surface receptors that couple to heterotrimeric GTP-binding proteins. Thelen and Bagglioni describe why the recent proposal that these receptors form heterodimers and activate the Jak kinases for transducing signals should be viewed with caution.
Bridging with GAPs: Receptor Communication Through RGS Proteins
Authors: Druey Kirk M.
Published: May 2003

The regulators of heterotrimeric guanosine triphosphate (GTP)-binding protein (G protein) signaling (RGS proteins) were named for their ability to act as GTP-activating proteins (GAPs) for G proteins and, thus, limit the signal generated by G protein-coupled receptors (GPCRs). In addition to this characteristic biochemical trait, RGS proteins constitute a large family of structurally diverse proteins with variable sequence motifs that permit additional specific interactions. RGS proteins may also serve as a bridge from GPCRs to receptor tyrosine kinases or transmembrane channels, allowing signals from GPCRs to regulate signaling through other types of receptors, and vice versa.
β-Adrenergic Signaling in the Heart: Dual Coupling of the β
2
-Adrenergic Receptor to G
s
and G
i
Proteins
Authors: Xiao Rui-Ping
Published: May 2003

β-adrenergic receptor (AR) subtypes are archetypical members of the G protein-coupled receptor (GPCR) superfamily. Whereas both β
1
AR and β
2
AR stimulate the classic G
s
-adenylyl cyclase-3′,5′-adenosine monophosphate (cAMP)-protein kinase A (PKA) signaling cascade, β
2
AR couples to both G
s
and G
i
proteins, activating bifurcated signaling pathways. In the heart, dual coupling of the β
2
AR to G
s
and G
i
results in compartmentalization of the G
s
-stimulated cAMP signal, thus selectively affecting plasma membrane effectors (such as L-type Ca
2+
channels) and bypassing cytoplasmic target proteins (such as phospholamban and myofilament contractile proteins). More important, the β
2
AR-to-G
i
branch delivers a powerful cell survival signal that counters apoptosis induced by the concurrent G
s
-mediated signal or by a wide range of assaulting factors. This survival pathway sequentially involves G
i
, Gβγ, phosphoinositide 3-kinase, and Akt. Furthermore, cardiac-specific transgenic overexpression of βAR subtypes in mice results in distinctly different phenotypes in terms of the likelihood of cardiac hypertrophy and heart failure. These findings indicate that stimulation of the two βAR subtypes activates overlapping, but different, sets of signal transduction mechanisms, and fulfills distinct or even opposing physiological and pathophysiological roles. Because of these differences, selective activation of cardiac β
2
AR may provide catecholamine-dependent inotropic support without cardiotoxic consequences, which might have beneficial effects in the failing heart.
Highlights of Alternative Splicing Regulation Session: Yes, No, Maybe--A History of Paradigm ShiftsHighlights from the Sixth Annual Meeting of the RNA Society, Banff, Alberta, Canada, 29 May to 3 June 2001.
Authors: Cooper Thomas A.
Published: May 2003

Cooper summarizes the discussions and presentations from the session entitled "Control of Splice Site Selection" held at the Sixth Annual Meeting of the RNA Society. Paradigms are shifting as experiments show that some of the proteins involved in regulating splicing can act as splicing enhancers or repressors, depending on the cellular context. The complex interactions among the ribonucleoproteins (RNPs) and proteins, and the role of cis elements, in controlling cell-specific splicing are highlighted. The importance of properly regulated splicing is emphasized by examples of disease pathologies in which alternative splicing is aberrant.
Insulin Signaling: Lessons from the
Drosophila
Tuberous Sclerosis Complex, a Tumor Suppressor
Authors: Montagne Jacques , Radimerski Thomas , Thomas George
Published: May 2003

The genes that encode the proteins composing the tuberous sclerosis complex (TSC) are tumor suppressors. Experiments in the model organism
Drosophila melanogaster
have provided insight into the identity of these genes and their functions in regulating cell size and proliferation. Montagne
et al
. describe the various genetic interactions that show TSC to be a regulator of the insulin signaling pathway and a regulator of progression through the cell cycle, which explains its effects on cell size and tissue and tumor growth.
Signaling By Fibroblast Growth Factors: The Inside Story
Authors: Goldfarb Mitchell
Published: May 2003

Polypeptide growth factors bind to the extracellular domains of cell surface receptors, triggering activation of receptor-intrinsic or receptor-associated protein kinases. Although this central thesis is widely accepted, one family of proteins, the fibroblast growth factors (FGFs), have for more than a decade attracted a research "counterculture" looking for direct FGF actions inside cells. Goldfarb discusses how the search for alternative signaling pathways is moving mainstream with the help of two recent publications reporting specific intracellular targets for FGF and FGF-like proteins.
Signaling Pathways Regulating Golgi Structure and Function
Authors: Preisinger Christian , Barr Francis A.
Published: May 2003

At the end of the cell division cycle, organelles must be equally distributed to two daughter cells. This includes the many stacks of the Golgi apparatus. Several kinases have been implicated in regulating Golgi disassembly during mitosis, but much about this process remains obscure.
Melatonin Receptor Signaling: Finding the Path Through the Dark
Authors: Masana Monica I. , Dubocovich Margarita L.
Published: May 2003

Melatonin, dubbed "the hormone of darkness," is involved in relaying photoperiodic information to the organism. Not only is melatonin involved in the regulation of circadian rhythms and sleep, but it also has roles in visual, cerebrovascular, reproductive, neuroendocrine, and neuroimmunological functions. Melatonin mediates its effects through G protein-coupled receptors: MT
1
, MT
2
, and, possibly,

MT
3

. Pharmacological agents have been instrumental in identifying these receptor types. Masana and Dubocovich discuss how the level of receptor expression may alter their efficacy, so that caution is necessary when extrapolating the pharmacological properties of ligands defined on recombinant systems to the receptors in the organism. With these cautions in mind, they describe the various signaling pathways and physiological roles ascribed to the three melatonin receptor types.
Signaling to the Mammalian Circadian Clocks: In Pursuit of the Primary Mammalian Circadian Photoreceptor
Authors: Pando Matthew P. , Sassone-Corsi Paolo
Published: May 2003

The mammalian circadian system is critical for the proper regulation of behavioral and physiological rhythms. The central oscillator, or master clock, is located in the hypothalamic suprachiasmatic nucleus (SCN). Additional circadian clocks are dispersed throughout most organs and tissues of an animal. The most prominent stimuli capable of synchronizing circadian oscillations to the environment is light. This occurs through daily photic signaling to the SCN, which ultimately results in the appropriate phasing of the various biological rhythms. Two critical aspects of circadian biology that will be discussed here are photic signaling and the communication between central and peripheral clocks. After 10 years of investigation, the primary mammalian circadian photoreceptor remains elusive. Recent findings suggest that multiple photoreceptive molecules may contribute to the perception of environmental light cycles. In addition, the relatively recent identification of cell-autonomous peripheral clocks has opened up an entirely new area of investigation. Deciphering the communication networks responsible for harmonious central and peripheral clock function is a critical step toward the development of effective therapies for circadian-related disorders.
Regulation and Physiological Roles of Serum- and Glucocorticoid-Induced Protein Kinase Isoforms
Authors: Lang Florian , Cohen Philip
Published: May 2003

Serum- and glucocorticoid-induced protein kinase 1 (
SGK1
) was identified in 1993 as an immediate early gene whose mRNA levels increase dramatically within 30 minutes when cells are exposed to serum or glucocorticoids, or both. Subsequently, many other agonists, acting through a variety of signal transduction pathways, have been shown to induce
SGK1
gene transcription in cells and tissues. SGK1 is a member of the "AGC" subfamily, which includes protein kinases A, G, and C, and its catalytic domain is most similar to protein kinase B (PKB). Like PKB, SGK1 is activated by phosphorylation in response to signals that stimulate phosphatidylinositol 3-kinase, and this is mediated by 3-phosphoinositide-dependent protein kinase 1 (PDK1) and another protein kinase that has yet to be identified. Thus, SGK1 is remarkable in being activated at both the transcriptional and posttranslational levels by a huge number of extracellular signals. In contrast, little is known about the transcriptional regulation of the two closely related isoforms SGK2 and SGK3, although they can be activated by phosphorylation. The substrate specificity of SGK isoforms superficially resembles that of PKB in that serine and threonine residues lying in Arg-Xaa-Arg-Xaa-Xaa-Ser/Thr sequences (where Xaa is a variable amino acid) are phosphorylated. However, although they may have some substrates in common, evidence is emerging that SGK1 and PKB phosphorylate distinct proteins and have different functions in vivo. In particular, SGK1 plays an important role in activating certain potassium, sodium, and chloride channels, suggesting an involvement in the regulation of processes such as cell survival, neuronal excitability, and renal sodium excretion. Moreover, sustained high levels of SGK1 protein and activity may contribute to conditions such as hypertension and diabetic nephropathy. This raises the possibility that specific inhibitors of SGK1 may have therapeutic potential for the treatment of several diseases.
Plant Histidine Kinases: An Emerging Picture of Two-Component Signal Transduction in Hormone and Environmental Responses
Authors: Urao Takeshi , Yamaguchi-Shinozaki Kazuko , Shinozaki Kazuo
Published: May 2003

In the
Arabidopsis thaliana
genome, 11 genes encode bacterial-type two-component histidine kinases. Genetic and biochemical analyses indicate that five two-component histidine kinase-like proteins (ETR1, ETR2, EIN4, ERS1, and ERS2) function as ethylene receptors. A hybrid histidine kinase, CRE1 (also known as AHK4), acts as a cytokinin receptor, and a set of response regulators may be involved in cytokinin signal transduction. In addition to CRE1, histidine kinases CKI1 and CKI2 are likely to play important roles in cytokinin signaling. A database search of the entire
Arabidopsis
genome sequence has identified two additional homologs of CRE1.
Arabidopsis
seems to employ a hybrid histidine kinase, ATHK1, as an osmosensor. Plants widely use two-component systems in the detection of, and signal transduction by, the growth regulators ethylene and cytokinin, as well as in their responses to environmental stimuli.
The Cbl Family: Ubiquitin Ligases Regulating Signaling by Tyrosine Kinases
Authors: Sanjay Archana , Horne William C. , Baron Roland
Published: May 2003

The Cbl proteins compose a family of ubiquitin ligases that play a central role in the down-regulation of signaling cascades involving receptor and nonreceptor tyrosine kinases. Analysis of the activity of these proteins suggests that they can regulate the signaling process through ubiquitination of the plasma membrane receptors and various downstream signaling components, including the Cbl proteins themselves. Structural analysis of the Cbl proteins shows that, in many instances, they interact with phosphorylated tyrosine residues on their targets. Furthermore, phosphorylation of specific tyrosine residues on the Cbl proteins may provide an additional level of control on the ubiquitinating activity of these proteins.
β-Arrestin and Mdm2, Unsuspected Partners in Signaling from the Cell Surface
Authors: Strous Ger J. , Schantl Julia A.
Published: May 2003

Mdm2 is a ubiquitin-protein ligase known to ubiquitinate p53, promoting its degradation by the ubiquitin-proteasome system. Shenoy and co-workers showed that Mdm2 can act as a key factor in the sequestration of the cell surface β
2
-adrenergic receptor (β-AR) through interactions with β-arrestin. Strous and Schantl discuss how Mdm2 may be a switch connecting extracellular signals mediated through G protein-coupled receptors (GPCRs) to p53 and its functions in apoptosis and cell cycle progression.
Phospholipid Signaling in Plants: Holding On to Phospholipase D
Authors: Munnik Teun , Musgrave Alan
Published: May 2003

Plant cells contain various phospholipase-based signaling pathways. In fact, their repertoire of phospholipase D (PLD) molecules far outnumbers those of mammalian and yeast cells. Munnik and Musgrave take a broad look at PLD function in animal, yeast, and plant cells, and suggest that a PLD-based connection between membranes and microtubules is a biological property worth considering across species.
The Complex and Intriguing Lives of PIP
2
with Ion Channels and Transporters
Authors: Hilgemann Donald W. , Feng Siyi , Nasuhoglu Cem
Published: May 2003

Phosphatidylinositol-4,5-bisphosphate (PIP
2
), the precursor of several signaling molecules in eukayotic cells, is itself also used by cells to signal to membrane-associated proteins. PIP
2
anchors numerous signaling molecules and cytoskeleton at the cell membrane, and the metabolism of PIP
2
is closely connected to membrane trafficking. Recently, ion transporters and channels have been discovered to be regulated by PIP
2
. Systems reported to be activated by PIP
2
include (i) plasmalemmal calcium pumps (PMCA), (ii) cardiac sodium-calcium exchangers (NCX1), (iii) sodium-proton exchangers (NHE1-4), (iv) a sodium-magnesium exchanger of unknown identity, (v) all inward rectifier potassium channels (KATP, IRK, GIRK, and ROMK channels), (vi) epithelial sodium channels (ENaC), and (vii) ryanodine-sensitive calcium release channels (RyR). Systems reported to be inhibited by PIP
2
include (i) cyclic nucleotide-gated channels of the rod (CNG), (ii) transient receptor potential-like (TRPL)
Drosophila
phototransduction channels, (iii) capsaicin-activated transient receptor potential (TRP) channels (VR1), and (iv) IP
3
-gated calcium release channels (IP3R). Systems that appear to be completely insensitive to PIP
2
include (i) voltage-gated sodium channels, (ii) most voltage-gated potassium channels, (iii) sodium-potassium pumps, (iv) several neurotransmitter transporters, and (v) cystic fibrosis transmembrane receptor (CFTR)-type chloride channels. Presumably, local changes of the concentration of PIP
2
in the plasma membrane represent cell signals to those mechanisms sensitive to PIP
2
changes. Unfortunately, our understanding of how local PIP
2
concentrations are regulated remains very limited. One important complexity is the probable existence of phospholipid microdomains, or lipid rafts. Such domains may serve to localize PIP
2
and thereby PIP
2
signaling, as well as to organize PIP
2
binding partners into signaling complexes. A related biological role of PIP
2
may be to control the activity of ion transporters and channels during biosynthesis or vesicle trafficking. Low PIP
2
concentrations in the secretory pathway would inactivate all of the systems that are stimulated by PIP
2
. How, in detail, is PIP
2
used by cells to control ion channel and transporter activities? Further progress requires an improved understanding of lipid kinases and phosphatases, how they are regulated, where they are localized in cells, and with which ion channels and transporters they might localize.
Signals from Eph and Ephrin Proteins: A Developmental Tool Kit
Authors: Boyd Andrew W. , Lackmann Martin
Published: May 2003

Interactions between Eph receptors and their ligands the ephrin proteins are critically important in many key developmental processes. Emerging evidence also supports a role for these molecules in postembryonic tissues, particularly in pathological processes, including tissue injury and tumor metastasis. We review the signaling mechanisms that allow the 14 Eph and nine ephrin proteins to deliver intracellular signals that regulate cell shape and movement. What emerges is that the initiation of these signals is critically dependent on which Eph and ephrin proteins are expressed, the level of their expression, and, in some cases, which splice variants are expressed. Diversity at the level of initial interaction and in the downstream signaling processes regulated by Eph-ephrin signaling provides a subtle, versatile system of regulation of intercellular adhesion, cell shape, and cell motility.
VEGF Receptor Signal Transduction
Authors: Matsumoto Taro , Claesson-Welsh Lena
Published: May 2003

The family of vascular endothelial growth factors (VEGFs) currently includes VEGF-A, -B, -C, -D, -E, and placenta growth factor (PlGF). Several of these factors, notably VEGF-A, exist as different isoforms, which appear to have unique biological functions. The VEGF family proteins bind in a distinct pattern to three structurally related receptor tyrosine kinases, denoted VEGF receptor-1, -2, and -3. Neuropilins, heparan-sulfated proteoglycans, cadherins, and integrin αvβ3 serve as coreceptors for certain but not all VEGF proteins. Moreover, the angiogenic response to VEGF varies between different organs and is dependent on the genetic background of the animal. Inactivation of the genes for VEGF-A and VEGF receptor-2 leads to embryonal death due to the lack of endothelial cells. Inactivation of the gene encoding VEGF receptor-1 leads to an increased number of endothelial cells, which obstruct the vessel lumen. Inactivation of VEGF receptor-3 leads to abnormally organized vessels and cardiac failure. Although VEGF receptor-3 normally is expressed only on lymphatic endothelial cells, it is up-regulated on vascular as well as nonvascular tumors and appears to be involved in the regulation of angiogenesis. A large body of data, such as those on gene inactivation, indicate that VEGF receptor-1 exerts a negative regulatory effect on VEGF receptor-2, at least during embryogenesis. Recent data imply a positive regulatory role for VEGF receptor-1 in pathological angiogenesis. The VEGF proteins are in general poor mitogens, but binding of VEGF-A to VEGF receptor-2 leads to survival, migration, and differentiation of endothelial cells and mediation of vascular permeability. This review outlines the current knowledge about the signal transduction properties of VEGF receptors, with focus on VEGF receptor-2.
Meeting Report: Signaling Schemes for TGF-βA review of the meeting "The TGF-β Superfamily: Signaling and Development," FASEB Summer Research Conference, Tucson, Arizona, 7 to 12 July 2001.
Authors: Roberts Anita B. , Derynck Rik
Published: May 2003

The transforming growth factor-β (TGF-β) superfamily of signaling molecules regulates many developmental processes in a range of organisms from worms to humans. Understanding the mechanisms by which they exert their repertoire of effects has required identification of the components of signaling pathways that they control. Roberts and Derynck focus on this aspect of TGF-β biology in their review of a recent Federation of American Societies for Experimental Biology (FASEB) meeting on TGF-β signaling and development and summarize current signaling paradigms and future prospects in TGF-β signaling from the cell surface to the nucleus.
Plant Receptor-Like Kinase Gene Family: Diversity, Function, and Signaling
Authors: Shiu Shin-Han , Bleecker Anthony B.
Published: May 2003

Plant receptor-like kinases (RLKs) are transmembrane proteins with putative amino-terminal extracellular domains and carboxyl-terminal intracellular kinase domains, with striking resemblance in domain organization to the animal receptor tyrosine kinases such as epidermal growth factor receptor. The recently sequenced
Arabidopsis
genome contains more than 600 RLK homologs, representing nearly 2.5% of the annotated protein-coding genes in
Arabidopsis
. Although only a handful of these genes have known functions and fewer still have identified ligands or downstream targets, the studies of several RLKs such as
CLAVATA1
, Brassinosteroid Insensitive 1, Flagellin Insensitive 2, and
S
-locus receptor kinase provide much-needed information on the functions mediated by members of this large gene family. RLKs control a wide range of processes, including development, disease resistance, hormone perception, and self-incompatibility. Combined with the expression studies and biochemical analysis of other RLKs, more details of RLK function and signaling are emerging.
TGF-β Superfamily Signaling and Left-Right Asymmetry
Authors: Whitman Malcolm , Mercola Mark
Published: May 2003

Despite an outwardly bilaterally symmetrical appearance, most internal organs of vertebrates display considerable left-right (LR) asymmetry in their anatomy and physiology. The orientation of LR asymmetry with respect to the dorsoventral and anteroposterior body axes is invariant such that fewer than 1 in 10,000 individuals exhibit organ reversals. The stereotypic orientation of LR asymmetry is ensured by distinct left- and right-side signal transduction pathways that are initiated by divergent members of the transforming growth factor-β (TGF-β) superfamily of secreted proteins. During early embryogenesis, the TGF-β-like protein Nodal (or a Nodal-related ortholog) is expressed by the left lateral plate mesoderm and provides essential LR cues to the developing organs. In chick embryos at least, bone morphogenetic protein (BMP) signaling is active on the right side of the embryo and must be inhibited on the left in order for Nodal to be expressed. Thus, at a key point in the determination of LR asymmetry, left-sided signaling is mediated by the transcription factors Smad2 and Smad3 (regulated by Nodal), whereas signaling on the right depends on Smad1 and Smad5 (which are regulated by BMP). This review summarizes the considerable progress that has been made in recent years in understanding the complex network of feedback and feedforward circuitry that regulates both the left- and right-sided pathways. Also discussed is the problem of how signal transduction mediated by the Smad proteins can pattern LR asymmetry without interfering with coincident dorsoventral patterning, which relies on the same Smad proteins.
New Responsibilities for the PI3K Regulatory Subunit p85α
Authors: Okkenhaug Klaus , Vanhaesebroeck Bart
Published: May 2003

Class IA phosphoinositide-3 kinases (PI3Ks) are heterodimeric enzymes that regulate many signal transduction pathways. The p85 regulatory subunit recruits the p110 catalytic subunit to the membrane, where p110 phosphorylates inositol lipids. Recent studies present evidence for an additional role for p85α in the regulation of actin cytoskeleton. Okkenhaug and Vanhaesebroeck discuss these results and ask whether experiments describing p85α knockout mice need to be reinterpreted.
PDK2: A Complex Tail in One Akt
Authors: Chan Tung O. , Tsichlis Philip N.
Published: May 2003

The kinase Akt contains two phosphatidylinositol-3 kinase (PI3K)-dependent phosphorylation sites, one in the activation loop (Thr
308
) and one in the carboxyl-terminal tail (Ser
473
), both of which are conserved among the members of the AGC kinase family. Under physiological conditions, the phosphorylation of Thr
308
appears to be coordinately regulated with the phosphorylation of Ser
473
. Under experimental conditions, however, the two sites can be uncoupled, suggesting that their phosphorylation is controlled by different kinases and phosphatases. Phosphoinositide-dependent kinase 1 (PDK1), the kinase that phosphorylates the activation loop site, has been unambiguously identified. However, PDK2, a kinase that is hypothesized to phosphorylate the hydrophobic carboxyl-terminal site, remains elusive. This Perspective examines the regulation and biological significance of Akt phosphorylation at Ser
473
. The authors propose that Ser
473
undergoes both autophosphorylation and phosphorylation by other kinases. Both events may be promoted by interactions between PDK1 and phosphorylated or phosphomimetically altered hydrophobic phosphorylation motifs in kinases associated with Akt. These interactions may induce conformational changes in Akt that make Ser
473
accessible to phosphorylation.
Membrane Cholesterol, Protein Phosphorylation, and Lipid Rafts
Authors: Edidin Michael
Published: May 2003

The functions of cholesterol and membrane microdomains in transmembrane signaling remain controversial. Edidin discusses the questions surrounding lipid rafts, membrane microdomains that have been biochemically defined but are difficult to visualize in vivo. He also discusses whether experiments showing correlation of changes in plasma membrane cholesterol with differentiation and the formation of adherens junctions in endothelial cells are consistent with a model in which lipid rafts influence the regulation of these processes.