Identification of a novel integrin alphaMbeta2 binding site in CCN1 (CYR61), a matricellular protein expressed in healing wounds and atherosclerotic lesions

CCN1 (cysteine-rich 61) and CCN2 (connective tissue growth factor) are growth factor-inducible immediate-early gene products found in atherosclerotic lesions, restenosed blood vessels, and healing cutaneous wounds. Both CCN proteins have been shown to support cell adhesion and induce cell migration through interaction with integrin receptors. Recently, we have identified integrin alphaMbeta2 as the major adhesion receptor mediating monocyte adhesion to CCN1 and CCN2 and have shown that the alphaMI domain binds specifically to both proteins. In the present study, we demonstrated that activated monocytes adhered to a synthetic peptide (CCN1-H2, SSVKKYRPKYCGS) derived from a conserved region within the CCN1 C-terminal domain, and this process was blocked by the anti-alphaM monoclonal antibody 2LPM19c. Consistently, a glutathione S-transferase (GST) fusion protein containing the alphaMI domain (GST-alphaMI) bound to immobilized CCN1-H2 as well as to the corresponding H2 sequence in CCN2 (CCN2-H2, TSVKTYRAKFCGV). By contrast, a scrambled CCN1-H2 peptide and an 18-residue peptide derived from an adjacent sequence of CCN1-H2 failed to support monocyte adhesion or alphaMI domain binding. To confirm that the CCN1-H2 sequence within the CCN1 protein mediates alphaMbeta2 interaction, we developed an anti-peptide antibody against CCN1-H2 and showed that it specifically blocked GST-alphaMI binding to intact CCN1. Collectively, these results identify the H2 sequence in CCN1 and CCN2 as a novel integrin alphaMbeta2 binding motif that bears no apparent homology to any alphaMbeta2 binding sequence reported to date.     

Critical roles for the COOH-terminal NITY and RGT sequences of the integrin beta3 cytoplasmic domain in inside-out and outside-in signaling

Bidirectional signaling of integrin alphaIIbbeta3 requires the beta3 cytoplasmic domain. To determine the sequence in the beta3 cytoplasmic domain that is critical to integrin signaling, cell lines were established that coexpress the platelet receptor for von Willebrand factor (vWF), glycoprotein Ib-IX, integrin alphaIIb, and mutants of beta3 with truncations at sites COOH terminal to T741, Y747, F754, and Y759. Truncation at Y759 did not affect integrin activation, as indicated by vWF-induced fibrinogen binding, but affected cell spreading and stable adhesion. Thus, the COOH-terminal RGT sequence of beta3 is important for outside-in signaling but not inside-out signaling. In contrast, truncation at F754, Y747, or T741 completely abolished integrin activation. A point mutation replacing Y759 with alanine also abolished integrin activation. Thus, the T755NITY759 sequence of beta3, containing an NXXY motif, is critical to inside-out signaling, whereas the intact COOH terminus is important for outside-in signaling. In addition, we found that the calcium-dependent protease calpain preferentially cleaves at Y759 in a population of beta3 during platelet aggregation and adhesion, suggesting that calpain may selectively regulate integrin outside-in signaling.     

Effects of the eukaryotic pore-forming cytolysin Equinatoxin II on lipid membranes and the role of sphingomyelin

Equinatoxin II (EqtII), a protein toxin from the sea anemone Actinia equina, readily creates pores in sphingomyelin-containing lipid membranes. The perturbation by EqtII of model lipid membranes composed of dimyristoylphosphatidycholine and sphingomyelin (10 mol %) was investigated using wideline phosphorus-31 and deuterium NMR. The preferential interaction between EqtII (0.1 and 0.4 mol %) and the individual bilayer lipids was studied by (31)P magic angle spinning NMR, and toxin-induced changes in bilayer morphology were examined by freeze-fracture electron microscopy. Both NMR and EM showed the formation of an additional lipid phase in sphingomyelin-containing mixed lipid multilamellar suspensions with 0.4 mol % EqtII. The new toxin-induced phase consisted of small unilamellar vesicles 20-40 nm in diameter. Deuterium NMR showed that the new lipid phase contains both dimyristoylphosphatidycholine and sphingomyelin. Solid-state (31)P NMR showed an increase in spin-lattice and a decrease in spin-spin relaxation times in mixed-lipid model membranes in the presence of EqtII, consistent with an increase in the intensity of low frequency motions. The (2)H and (31)P spectral intensity distributions confirmed a change in lipid mobility and showed the creation of an isotropic lipid phase, which was identified as the small vesicle structures visible by electron microscopy in the EqtII-lipid suspensions. The toxin appears to enhance slow motions in the membrane lipids and destabilize the membrane. This effect was greatly enhanced in sphingomyelin-containing mixed lipid membranes compared with pure phosphatidylcholine bilayers, suggesting a preferential interaction between the toxin and bilayer sphingomyelin.     

Two-dimensional display and whole genome comparison of bacterial pathogen genomes of high G+C DNA content

Three highly mutable loci of the wall-less pathogens Mycoplasma bovis, Mycoplasma pulmonis and Mycoplasma agalactiae undergo high-frequency genomic rearrangements and generate extensive antigenic variation of major surface lipoproteins. Adjacent to each locus, an open reading frame exists as a single chromosomal copy and is predicted to encode a site-specific DNA recombinase exhibiting high homology to the recombinases XerD of Escherichia coli and CodV of Bacillus subtilis. Each of the mycoplasmal proteins are members of the lambda integrase family of tyrosine site-specific recombinases and likely mediates site-specific DNA inversions observed within the adjacent, variable loci.     

Osmoregulation and fungicide resistance: the Neurospora crassa os-2 gene encodes a HOG1 mitogen-activated protein kinase homologue

Neurospora crassa osmosensitive (os) mutants are sensitive to high osmolarity and therefore are unable to grow on medium containing 4% NaCl. We found that os-2 and os-5 mutants were resistant to the phenylpyrrole fungicides fludioxonil and fenpiclonil. To understand the relationship between osmoregulation and fungicide resistance, we cloned the os-2 gene by using sib selection. os-2 encodes a putative mitogen-activated protein (MAP) kinase homologous to HOG1 and can complement the osmosensitive phenotype of a Saccharomyces cerevisiae hog1 mutant. We sequenced three os-2 alleles and found that all of them were null with either frameshift or nonsense point mutations. An os-2 gene replacement mutant also was generated and was sensitive to high osmolarity and resistant to phenylpyrrole fungicides. Conversely, os-2 mutants transformed with the wild-type os-2 gene could grow on media containing 4% NaCl and were sensitive to phenylpyrrole fungicides. Fludioxonil stimulated intracellular glycerol accumulation in wild-type strains but not in os-2 mutants. Fludioxonil also caused wild-type conidia and hyphal cells to swell and burst. These results suggest that the hyperosmotic stress response pathway of N. crassa is the target of phenylpyrrole fungicides and that fungicidal effects may result from a hyperactive os-2 MAP kinase pathway.     

Determinants in nuclease specificity of Ape1 and Ape2, human homologues of Escherichia coli exonuclease III

Abasic sites and non-conventional 3'-ends, e.g. 3'-oxidized fragments (including 3'-phosphate groups) and 3'-mismatched nucleotides, arise at significant frequency in the genome due to spontaneous decay, oxidation or replication errors. To avert the potentially mutagenic or cytotoxic effects of these chromosome modifications/intermediates, organisms are equipped with apurinic/apyrimidinic (AP) endonucleases and 3'-nucleases that initiate repair. Ape1, which shares homology with Escherichia coli exonuclease III (ExoIII), is the major abasic endonuclease in mammals and an important, yet selective, contributor to 3'-end processing. Mammals also possess a second protein (Ape2) with sequence homology to ExoIII, but this protein exhibits comparatively weak AP site-specific and 3'-nuclease activities. Prompted by homology modeling studies, we found that substitutions in the hydrophobic pocket of Ape1 (comprised of F266, W280 and L282) reduce abasic incision potency about fourfold to 450,000-fold, while introduction of an ExoIII-like pocket into Ape2 enhances its AP endonuclease function. We demonstrate that mutations at F266 and W280 of Ape1 increase 3' to 5' DNA exonuclease activity. These results, coupled with prior comparative sequence analysis, indicate that this active-site hydrophobic pocket influences the substrate specificity of a diverse set of sequence-related proteins possessing the conserved four-layered alpha/beta-fold. Lastly, we report that wild-type Ape1 excises 3'-mismatched nucleotides at a rate up to 374-fold higher than correctly base-paired nucleotides, depending greatly on the structure and sequence of the DNA substrate, suggesting a novel, selective role for the human protein in 3'-mismatch repair.     

Macrophage-colony-stimulating factor-induced activation of extracellular-regulated kinase involves phosphatidylinositol 3-kinase and reactive oxygen species in human monocytes

We previously reported that activation of the phosphatidylinositol (PI) 3-kinase pathway was important in M-CSF-induced monocyte survival. Because M-CSF also induces activation of the mitogen-activated protein (MAP) kinase extracellular-regulated kinase (Erk), we focused on dissecting the mechanism used by M-CSF to induce Erk activation in human monocytes. We found that, in addition to the MAP/Erk kinase inhibitor PD098059, the PI 3-kinase inhibitors LY294002 and wortmannin both suppressed Erk activation in M-CSF-treated monocytes, suggesting that 3-phosphorylated products of PI 3-kinase played a role in Erk activation. Investigating the biochemical pathways regulated by PI 3-kinase to activate Erk, we found that, in response to M-CSF, normal human monocytes induced reactive oxygen species (ROS), which were suppressed by the PI 3-kinase inhibitor wortmannin but not by the solvent control DMSO or the MAP/Erk kinase inhibitor PD098059. We next found that, in the absence of M-CSF, ROS could induce Erk activation in human monocytes. Exogenous H(2)O(2) induced Erk activation in human monocytes, which was suppressed by exogenous catalase. To determine whether ROS induced by M-CSF played a role in Erk activation, we found that N-acetylcysteine and diphenyleneiodonium both suppressed Erk activation in M-CSF-treated monocytes. Erk activation by M-CSF also seemed to play a role in cellular survival in monocytes. These data suggest that, in M-CSF-stimulated human monocytes, PI 3-kinase products and ROS production play a role in Erk activation and monocyte survival.     

The effect of space flight on the production of actinomycin D by Streptomyces plicatus

The effect of space flight on production of the antibiotic actinomycin D by Streptomyces plicatus WC56452 was examined onboard the US Space Shuttle mission STS-80. Paired space flight and ground control samples were similarly prepared using identical hardware, media, and inoculum. The cultures were grown in defined and complex media under dark, anaerobic, thermally controlled (20°C) conditions with samples fixed after 7 and 12 days in orbit, and viable residuals maintained through landing at 17 days, 15 h. Postflight analyses indicated that space flight had reduced the colony-forming unit (CFU) per milliliter count of S. plicatus and increased the specific productivity (pg CFU⁻¹) of actinomycin D. The antibiotic compound itself was not affected, but its production time course was altered in space. Viable flight samples also maintained their sporulation ability when plated on agar medium postflight, while the residual ground controls did not sporulate. Received 21 August 2001/ Accepted in revised form 30 July 2002