Structure of a flavin-binding plant photoreceptor domain: Insights into light-mediated signal transduction

Phototropin, a major blue-light receptor for phototropism in seed plants, exhibits blue-light-dependent autophosphorylation and contains two light, oxygen, or voltage (LOV) domains and a serine/threonine kinase domain. The LOV domains share homology with the PER-ARNT-SIM (PAS) superfamily, a diverse group of sensor proteins. Each LOV domain noncovalently binds a single FMN molecule and exhibits reversible photochemistry in vitro when expressed separately or in tandem. We have determined the crystal structure of the LOV2 domain from the phototropin segment of the chimeric fern photoreceptor phy3 to 2.7-Å resolution. The structure constitutes an FMN-binding fold that reveals how the flavin cofactor is embedded in the protein. The single LOV2 cysteine residue is located 4.2 Å from flavin atom C(4a), consistent with a model in which absorption of blue light induces formation of a covalent cysteinyl-C(4a) adduct. Residues that interact with FMN in the phototropin segment of the chimeric fern photoreceptor (phy3) LOV2 are conserved in LOV domains from phototropin of other plant species and from three proteins involved in the regulation of circadian rhythms in Arabidopsis and Neurospora. This conservation suggests that these domains exhibit the same overall fold and share a common mechanism for flavin binding and light-induced signaling.

Changes in Phosphoinositide Metabolism with Days in Culture Affect Signal Transduction Pathways in Galdieria sulphuraria1

The metabolism of phosphatidylinositol-4,5-bisphosphate (PIP2) changed during the culture period of the thermoacidophilic red alga Galdieria sulphuraria. Seven days after inoculation, the amount of PIP2 in the cells was 910 ± 100 pmol g−1 fresh weight; by 12 d, PIP2 levels increased to 1200 ± 150 pmol g−1 fresh weight. In vitro assays indicated that phosphatidylinositol monophosphate (PIP) kinase specific activity increased from 75 to 230 pmol min−1 mg−1 protein between d 7 and 12. When G. sulphuraria cells were osmostimulated, transient increases of up to 4-fold could be observed in inositol-1,4,5-trisphosphate (IP3) levels within 90 s, regardless of the age of the cells. In d-12 cells, the increase in IP3 was preceded by a transient increase of up to 5-fold in specific PIP kinase activity, whereas no such increase was detected after osmostimulation of d-7 cells. The increase in PIP kinase activity before IP3 signaling in d-12 cells indicates that there is an additional pathway for regulation of phosphoinositide metabolism after stimulation other than an initial activation of phospholipase C. Also, the rapid activation of PIP2 biosynthesis in cells with already-high PIP2 levels suggests that the PIP2 present was not available for signal transduction. By comparing the response of the cells at d 7 and 12, we have identified two potentially distinct pools of PIP2.

A Model for Signal Transduction during Gamete Release in the Fucoid Alga Pelvetia compressa1

Fucoid algae release gametes into seawater following an inductive light period (potentiation), and gamete expulsion from potentiated receptacles of Pelvetia compressa began about 2 min after a light-to-dark transition. Agitation of the medium reversed potentiation, with an exponential time course completed in about 3 h. Light regulated two signaling pathways during potentiation and gamete expulsion: a photosynthetic pathway and a photosynthesis-independent pathway in which red light was active but blue light was not. Uptake of K+ appears to have an important role in potentiation, because a 50% inhibition of potentiation occurred in the presence of the tetraethylammonium ion, a K+-channel blocker. A central role of anion channels in the maintenance of potentiation is suggested by the premature release of gametes in the light when receptacles were incubated with inhibitors of slow-type anion channels. An inhibitor of tyrosine kinases, tyrphostin A63, also inhibited potentiation. A model for gamete release from P. compressa is presented that proposes that illumination results in the accumulation of ions (e.g. K+) throughout the cells of the receptacle during potentiation, which then move into the extracellular matrix during gamete expulsion to generate osmomechanical force, resulting in gamete release.

SPINDLY, a tetratricopeptide repeat protein involved in gibberellin signal transduction in Arabidopsis.

Gibberellins (GAs) are a major class of plant hormones that control many developmental processes, including seed development and germination, flower and fruit development, and flowering time. Genetic studies with Arabidopsis thaliana have identified two genes involved in GA perception or signal transduction. A semidominant mutation at the GIBBERELLIN INSENSITIVE (GAI) locus results in plants resembling GA-deficient mutants but exhibiting reduced sensitivity to GA. Recessive mutations at the SPINDLY (SPY) locus cause a phenotype that is consistent with constitutive activation of GA signal transduction. Here we show that a strong allele of spy is completely epistatic to gai, indicating that SPY acts downstream of GAI. We have cloned the SPY gene and shown that it encodes a new type of signal transduction protein, which contains a tetratricopeptide repeat region, likely serving as a protein interaction domain, and a novel C-terminal region. Mutations in both domains increase GA signal transduction. The presence of a similar gene in Caenorhabditis elegans suggests that SPY represents a class of signal transduction proteins that is present throughout the eukaryotes.

Identification of transferred DNA insertions within Arabidopsis genes involved in signal transduction and ion transport.

The transferred DNA (T-DNA) of Agrobacterium tumefaciens serves as an insertional mutagen once integrated into a host plant's genome. As a means of facilitating reverse genetic analysis in Arabidopsis thaliana, we have developed a method that allows one to search for plants carrying F-DNA insertions within any sequenced Arabidopsis gene. Using PCR, we screened a collection of 9100 independent T-DNA-transformed Arabidopsis lines and found 17 T-DNA insertions within the 63 genes analyzed. The genes surveyed include members of various gene families involved in signal transduction and ion transport. As an example, data are shown for a T-DNA insertion that was found within CPK-9, a member of the gene family encoding calmodulin-domain protein kinases.

Signal transduction in systemic acquired resistance.

Systemic acquired resistance (SAR) is an important component of plant defense against pathogen infection. Accumulation of salicylic acid (SA) is required for the induction of SAR. However, SA is apparently not the translocated signal but is involved in transducing the signal in target tissues. Interestingly, SA accumulation is not required for production and release of the systemic signal. In addition to playing a pivotal role in SAR signal transduction, SA is important in modulating plant susceptibility to pathogen infection and genetic resistance to disease. It has been proposed that SA inhibition of catalase results in H2O2 accumulation and that therefore H2O2 serves as a second messenger in SAR signaling. We find no accumulation of H2O2 in tissues expressing SAR; thus the role of H2O2 in SAR signaling is questionable.

Cytoplasmic RNA modulators of an inside-out signal-transduction cascade

A vaccinia virus-based RNA expression system enabled high-level cytoplasmic expression of RNA aptamers directed against the intracellular domain of the β2 integrin LFA-1, a transmembrane protein that mediates cell adhesion to intercellular adhesion molecule-1 (ICAM-1). In two different cell types, cytoplasmic expression of integrin-binding aptamers reduced inducible cell adhesion to ICAM-1. The aptamers specifically target, and thereby define, a functional cytoplasmic subdomain important for the regulation of cell adhesion in leukocytes. Our approach of aptamer-controlled blocking of signaling pathways in vivo could potentially be applied wherever targeted modulation of a signal-transduction cascade is desired.

The TOR (target of rapamycin) signal transduction pathway regulates the stability of translation initiation factor eIF4G in the yeast Saccharomyces cerevisiae

Initiation factor eIF4G is an essential protein required for initiation of mRNA translation via the 5′ cap-dependent pathway. It interacts with eIF4E (the mRNA 5′ cap-binding protein) and serves as an anchor for the assembly of further initiation factors. With treatment of Saccharomyces cerevisiae with rapamycin or with entry of cells into the diauxic phase, eIF4G is rapidly degraded, whereas initiation factors eIF4E and eIF4A remain stable. We propose that nutritional deprivation or interruption of the TOR signal transduction pathway induces eIF4G degradation.

ErbB2 expression increases the spectrum and potency of ligand-mediated signal transduction through ErbB4

Interleukin 3-dependent murine 32D cells do not detectably express members of the ErbB receptor family and do not proliferate in response to known ligands for these receptors. 32D transfectants were generated expressing human ErbB4 alone (32D.E4) or with ErbB2 (32D.E2/E4). Epidermal growth factor (EGF), neuregulin 1-β (NRG1-β), betacellulin (BTC), transforming growth factor-α (TGF-α), heparin binding-EGF (HB-EGF), and amphiregulin were analyzed for their ability to mediate mitogenesis in these transfectants. 32D.E4 responded mitogenically to NRG1-β and BTC. Surprisingly, EGF also induced significant DNA synthesis and TGF-α was negligibly mitogenic on 32D.E4 cells, whereas HB-EGF and amphiregulin were inactive. Although coexpression of ErbB2 with ErbB4 in 32D.E2/E4 cells did not significantly alter DNA synthesis in response to NRG1-β or BTC, it greatly enhanced mitogenesis elicited by EGF and TGF-α and unmasked the ability of HB-EGF to induce proliferation. EGF-related ligands that exhibited potent mitogenic activity on 32D.E2/E4 cells at low concentrations induced adherence, morphological alterations, and up-regulation of the Mac-1 integrin and FcγRII/III at higher concentrations. While 125I-EGF could be specifically crosslinked to both 32D.E4 and 32D.E2/E4 cells, its crosslinking capacity was greatly enhanced in the cotransfected cells. The ability of the various ligands to mediate proliferation and/or adhesion in the two transfectants correlated with their capacity to induce substrate tyrosine phosphorylation and to initiate and sustain activation of mitogen-activated protein kinase. We conclude that the ability of ErbB4 to mediate signal transduction through EGF-like ligands is broader than previously assumed and can be profoundly altered by the concomitant expression of ErbB2.

Interleukin-2 induces β2-integrin-dependent signal transduction involving the focal adhesion kinase-related protein B (fakB)

β2 integrin molecules are involved in a multitude of cellular events, including adhesion, migration, and cellular activation. Here, we studied the influence of β2 integrins on interleukin-2 (IL-2)-mediated signal transduction in human CD4+ T cell lines obtained from healthy donors and a leukocyte adhesion deficiency (LAD) patient. We show that IL-2 induces tyrosine phosphorylation of a 125-kDa protein and homotypic adhesion in β2 integrin (CD18)-positive but not in β2-integrin-negative T cells. EDTA, an inhibitor of integrin adhesion, blocks IL-2-induced tyrosine phosphorylation of the 125-kDa protein but not other proteins in β2-integrin-positive T cells. Likewise, a β2 integrin (CD18) antibody selectively inhibits induction of the 125-kDa phosphotyrosine protein, whereas cytokine-mediated tyrosine phosphorylation of other proteins is largely unaffected. Immunoprecipitation experiments indicate that the IL-2-induced 125-kDa phosphotyrosine protein is the focal adhesion kinase-related protein B (fakB). Thus, IL-2 induces strong tyrosine phosphorylation of fakB in β2-integrin-positive but not in β2-integrin-negative T cells, and CD18 mAb selectively blocks IL-2-induced fakB-tyrosine phosphorylation in β2-integrin-positive T cells. In parallel experiments, IL-2 does not induce or augment tyrosine phosphorylation of p125FAK. In conclusion, our data indicate that IL-2 induces β2-integrin-dependent signal transduction events involving the tyrosine kinase substrate fakB.

Platelet signal transduction defect with Gα subunit dysfunction and diminished Gαq in a patient with abnormal platelet responses

G proteins play a major role in signal transduction upon platelet activation. We have previously reported a patient with impaired agonist-induced aggregation, secretion, arachidonate release, and Ca2+ mobilization. Present studies demonstrated that platelet phospholipase A2 (cytosolic and membrane) activity in the patient was normal. Receptor-mediated activation of glycoprotein (GP) IIb-IIIa complex measured by flow cytometry using antibody PAC-1 was diminished despite normal amounts of GPIIb-IIIa on platelets. Ca2+ release induced by guanosine 5′-[γ-thio]triphosphate (GTP[γS]) was diminished in the patient’s platelets, suggesting a defect distal to agonist receptors. GTPase activity (a function of α-subunit) in platelet membranes was normal in resting state but was diminished compared with normal subjects on stimulation with thrombin, platelet-activating factor, or the thromboxane A2 analog U46619. Binding of 35S-labeled GTP[γS] to platelet membranes was decreased under both basal and thrombin-stimulated states. Iloprost (a stable prostaglandin I2 analog) -induced rise in cAMP (mediated by Gαs) and its inhibition (mediated by Gαi) by thrombin in the patient’s platelet membranes were normal. Immunoblot analysis of Gα subunits in the patient’s platelet membranes showed a decrease in Gαq (<50%) but not Gαi, Gαz, Gα12, and Gα13. These studies provide evidence for a hitherto undescribed defect in human platelet G-protein α-subunit function leading to impaired platelet responses, and they provide further evidence for a major role of Gαq in thrombin-induced responses.

Molecular switch in signal transduction: Reaction paths of the conformational changes in ras p21

Conformational changes in ras p21 triggered by the hydrolysis of GTP play an essential role in the signal transduction pathway. The path for the conformational change is determined by molecular dynamics simulation with a holonomic constraint directing the system from the known GTP-bound structure (with the γ-phosphate removed) to the GDP-bound structure. The simulation is done with a shell of water molecules surrounding the protein. In the switch I region, the side chain of Tyr-32, which undergoes a large displacement, moves through the space between loop 2 and the rest of the protein, rather than on the outside of the protein. As a result, the charged residues Glu-31 and Asp-33, which interact with Raf in the homologous RafRBD–Raps complex, remain exposed during the transition. In the switch II region, the conformational changes of α2 and loop 4 are strongly coupled. A transient hydrogen bonding complex between Arg-68 and Tyr-71 in the switch II region and Glu-37 in switch I region stabilizes the intermediate conformation of α2 and facilitates the unwinding of a helical turn of α2 (residues 66–69), which in turn permits the larger scale motion of loop 4. Hydrogen bond exchange between the protein and solvent molecules is found to be important in the transition. Possible functional implications of the results are discussed.

Mechanical stretch induces vascular permeability factor in human mesangial cells: Mechanisms of signal transduction

Hemodynamic abnormalities have been implicated in the pathogenesis of the increased glomerular permeability to protein of diabetic and other glomerulopathies. Vascular permeability factor (VPF) is one of the most powerful promoters of vascular permeability. We studied the effect of stretch on VPF production by human mesangial cells and the intracellular signaling pathways involved. The application of mechanical stretch (elongation 10%) for 6 h induced a 2.4-fold increase over control in the VPF mRNA level (P < 0.05). There was a corresponding 3-fold increase in VPF protein level by 12 h (P < 0.001), returning to the baseline by 24 h. Stretch-induced VPF secretion was partially prevented both by the protein kinase C (PKC) inhibitor H7 (50 μM: 72% inhibition, P < 0.05) and by pretreatment with phorbol ester (phorbol-12-myristate-13 acetate 10−7 M: 77% inhibition, P < 0.05). A variety of protein tyrosine kinase (PTK) inhibitors, genistein (20 μg/ml), herbimycin A (3.4 μM), and a specific pp60src peptide inhibitor (21 μM) also significantly reduced, but did not entirely prevent, stretch-induced VPF protein secretion (respectively 63%, 80%, and 75% inhibition; P < 0.05 for all). The combination of both PKC and PTK inhibition completely abolished the VPF response to mechanical stretch (100% inhibition, P < 0.05). Stretch induces VPF gene expression and protein secretion in human mesangial cells via PKC- and PTK-dependent mechanisms.

Induction of host signal transduction pathways by Helicobacter pylori

Adherence of Helicobacter pylori to cultured gastric epithelial cells is associated with several cellular events, including the tyrosine phosphorylation of a 145-kDa host protein; the reorganization of the host cell actin and associated cellular proteins, like vasodilator-stimulated phosphoprotein, adjacent to the attached bacterial cell; and the subsequent release of the cytokine, interleukin 8 (IL-8). H. pylori isolated from patients with ulcer disease and gastric cancer contain a DNA insertion, the cag pathogenicity island (PAI), that is not present in bacteria isolated from individuals with asymptomatic infection. Mutations in a number of PAI genes abolish tyrosine phosphorylation and IL-8 synthesis but not the cytoskeletal rearrangements. Kinase inhibition studies suggest there are two distinct pathways operative in stimulating IL-8 release from host cells and one of these H. pylori pathways is independent of the tyrosine phosphorylation step.

Signal transduction through homologs of the Ste20p and Ste7p protein kinases can trigger hyphal formation in the pathogenic fungus Candida albicans

The CST20 gene of Candida albicans was cloned by functional complementation of a deletion of the STE20 gene in Saccharomyces cerevisiae. CST20 encodes a homolog of the Ste20p/p65PAK family of protein kinases. Colonies of C. albicans cells deleted for CST20 revealed defects in the lateral formation of mycelia on synthetic solid “Spider” media. However, hyphal development was not impaired in some other media. A similar phenotype was caused by deletion of HST7, encoding a functional homolog of the S. cerevisiae Ste7p protein kinase. Overexpression of HST7 partially complemented the deletion of CST20. Cells deleted for CST20 were less virulent in a mouse model for systemic candidiasis. Our results suggest that more than one signaling pathway can trigger hyphal development in C. albicans, one of which has a protein kinase cascade that is analogous to the mating response pathway in S. cerevisiae and might have become adapted to the control of mycelial formation in asexual C. albicans.