The human polycomb group EED protein interacts with the integrase of human immunodeficiency virus type 1

Human EED, a member of the superfamily of WD-40 repeat proteins and of the Polycomb group proteins, has been identified as a cellular partner of the human immunodeficiency virus type 1 (HIV-1) matrix (MA) protein (R. Peytavi et al., J. Biol. Chem. 274:1635-1645, 1999). In the present study, EED was found to interact with HIV-1 integrase (IN) both in vitro and in vivo in yeast. In vitro, data from mutagenesis studies, pull-down assays, and phage biopanning suggested that EED-binding site(s) are located in the C-terminal domain of IN, between residues 212 and 264. In EED, two putative discrete IN-binding sites were mapped to its N-terminal moiety, at a distance from the MA-binding site, but EED-IN interaction also required the integrity of the EED last two WD repeats. EED showed an apparent positive effect on IN-mediated DNA integration reaction in vitro, in a dose-dependent manner. In situ analysis by immunoelectron microscopy (IEM) of cellular distribution of IN and EED in HIV-1-infected cells (HeLa CD4(+) cells or MT4 lymphoid cells) showed that IN and EED colocalized in the nucleus and near nuclear pores, with maximum colocalization events occurring at 6 h postinfection (p.i.). Triple colocalizations of IN, EED, and MA were also observed in the nucleoplasm of infected cells at 6 h p.i., suggesting the ocurrence of multiprotein complexes involving these three proteins at early steps of the HIV-1 virus life cycle. Such IEM patterns were not observed with a noninfectious, envelope deletion mutant of HIV-1.     

Structural analysis of human rhinovirus complexed with ICAM-1 reveals the dynamics of receptor-mediated virus uncoating

Intercellular adhesion molecule 1 (ICAM-1) functions as the cellular receptor for the major group of human rhinoviruses, being not only the target of viral attachment but also the mediator of viral uncoating. The configurations of HRV3-ICAM-1 complexes prepared both at 4 degrees C and physiological temperature (37 degrees C) were analyzed by cryoelectron microscopy and image reconstruction. The particle diameters of two complexes (with and without RNA) representing uncoating intermediates generated at 37 degrees C were each 4% larger than that of those prepared at 4 degrees C. The larger virus particle arose by an expansive movement of the capsid pentamers along the fivefold axis, which loosens interprotomer contacts, particularly at the canyon region where the ICAM-1 receptor bound. Particle expansion required receptor binding and preceded the egress of the viral RNA. These observations suggest that receptor-mediated uncoating could be a consequence of restrained capsid motion, where the bound receptors maintain the viral capsid in an expanded open state for subsequent genome release.     

High mutant frequency in populations of a DNA virus allows evasion from antibody therapy in an immunodeficient host

The degree of genetic heterogeneity of DNA virus populations in nature and its consequences for disease control are virtually unknown. The parvovirus minute virus of mice (MVMi) was used here to investigate (i) the frequency of antibody-escape mutants in populations of a DNA virus and (ii) the ability of a DNA virus to evade in the long-term a passive monoclonal antibody (MAb) therapy in an immunodeficient natural host. Independent clonal populations of MVMi harbored a high proportion of mutants resistant to neutralizing MAb (mutant frequency = [2.8 +/- 0.5] x 10(-5)) that rapidly evolved under antibody pressure in culture to become mixtures dominated by genotypically diverse escape mutants. Immunodeficient mice naturally infected with clonal populations of MVMi and subsequently treated by intravenous injections of MAb were initially protected from the characteristic viral induced lethal leukopenia. However, some treated animals developed a delayed severe leukopenic syndrome associated with the emergence of genetically heterogeneous populations of MAb-resistant mutants in the MVMi main target organs. The 11 plaque-purified viruses analyzed from an antibody-resistant population obtained from one animal corresponded to four different mutant genotypes, although their consensus sequence remained wild type. All cloned escape mutants harbored single radical amino acid changes within a stretch of seven residues in a surface-exposed loop at the threefold axes of the capsid. This antigenic site, which can tolerate radical changes preserving MVMi pathogenic potential, may thereby allow the virus to evade the immune control. These findings indicate a high genetic heterogeneity and rapid adaptation of populations of a mammal DNA virus in vivo and provide a genetic basis for the failure of passive immunotherapy in the natural host.     

Novel natural peptide substrates for endopeptidase 24.15, neurolysin,and angiotensin-converting enzyme

Endopeptidase 24.15 (EC; ep24.15), neurolysin (EC; ep24.16), and angiotensin-converting enzyme (EC; ACE) are metallopeptidases involved in neuropeptide metabolism in vertebrates. Using catalytically inactive forms of ep24.15 and ep24.16, we have identified new peptide substrates for these enzymes. The enzymatic activity of ep24.15 and ep24.16 was inactivated by site-directed mutagenesis of amino acid residues within their conserved HEXXH motifs, without disturbing their secondary structure or peptide binding ability, as shown by circular dichroism and binding assays. Fifteen of the peptides isolated were sequenced by electrospray ionization tandem mass spectrometry and shared homology with fragments of intracellular proteins such as hemoglobin. Three of these peptides (PVNFKFLSH, VVYPWTQRY, and LVVYPWTQRY) were synthesized and shown to interact with ep24.15, ep24.16, and ACE, with K(i) values ranging from 1.86 to 27.76 microm. The hemoglobin alpha-chain fragment PVNFKFLSH, which we have named hemopressin, produced dose-dependent hypotension in anesthetized rats, starting at 0.001 microg/kg. The hypotensive effect of the peptide was potentiated by enalapril only at the lowest peptide dose. These results suggest a role for hemopressin as a vasoactive substance in vivo. The identification of these putative intracellular substrates for ep24.15 and ep24.16 is an important step toward the elucidation of the role of these enzymes within cells.     

Archaeoglobus fulgidus CopB is a thermophilic Cu2+-ATPase: functional role of its histidine-rich-N-terminal metal binding domain

P1B-type ATPases transport heavy metal ions across cellular membranes. Archaeoglobus fulgidus CopB is a member of this subfamily. We have cloned, expressed in Escherichia coli, and functionally characterized this enzyme. CopB and its homologs are distinguished by a metal binding sequence Cys-Pro-His in their sixth transmembrane segment (H6) and a His-rich N-terminal metal binding domain (His-N-MBD). CopB is a thermophilic protein active at 75 degrees C and high ionic strength. It is activated by Cu2+ with high apparent affinity (K1/2 = 0.28 microm) and partially by Cu+ and Ag+ (22 and 55%, respectively). The higher turnover was associated with a faster phosphorylation rate in the presence of Cu2+. A truncated CopB lacking the first 54 amino acids was constructed to characterize the His-N-MBD. This enzyme showed reduced ATPase activity (50% of wild type) but no changes in metal selectivity, ATP dependence, or phosphorylation levels. However, a slower rate of dephosphorylation of the E2P(Cu2+) form was observed for truncated CopB. The data suggest that the presence of the His residue in the putative transmembrane metal binding site of CopB determines a selectivity for this enzyme that is different for that observed in Cu+/Ag+-ATPases carrying a Cys-Pro-Cys sequence. The His-NMBD appears to have a regulatory role affecting the metal transport rate by controlling the metal release/dephosphorylation rates.     

In vitro synthesis of lactose permease to probe the mechanism of membrane insertion and folding

Insertion and folding of polytopic membrane proteins is an important unsolved biological problem. To study this issue, lactose permease, a membrane transport protein from Escherichia coli, is transcribed, translated, and inserted into inside-out membrane vesicles in vitro. The protein is in a native conformation as judged by sensitivity to protease, binding of a monoclonal antibody directed against a conformational epitope, and importantly, by functional assays. By exploiting this system it is possible to express the N-terminal six helices of the permease (N(6)) and probe changes in conformation during insertion into the membrane. Specifically, when N(6) remains attached to the ribosome it is readily extracted from the membrane with urea, whereas after release from the ribosome or translation of additional helices, those polypeptides are not urea extractable. Furthermore, the accessibility of an engineered Factor Xa site to Xa protease is reduced significantly when N(6) is released from the ribosome or more helices are translated. Finally, spontaneous disulfide formation between Cys residues at positions 126 (Helix IV) and 144 (Helix V) is observed when N(6) is released from the ribosome and inserted into the membrane. Moreover, in contrast to full-length permease, N(6) is degraded by FtsH protease in vivo, and N(6) with a single Cys residue at position 148 does not react with N-ethylmaleimide. Taken together, the findings indicate that N(6) remains in a hydrophilic environment until it is released from the ribosome or additional helices are translated and continues to fold into a quasi-native conformation after insertion into the bilayer. Furthermore, there is synergism between N(6) and the C-terminal half of permease during assembly, as opposed to assembly of the two halves as independent domains.     

Angiogenesis in collagen I requires alpha2beta1 ligation of a GFP*GER sequence and possibly p38 MAPK activation and focal adhesion disassembly

Angiogenesis depends on proper collagen biosynthesis and cross-linking, and type I collagen is an ideal angiogenic scaffold, although its mechanism is unknown. We examined angiogenesis using an assay wherein confluent monolayers of human umbilical vein endothelial cells were overlain with collagen in a serum-free defined medium. Small spaces formed in the cell layer by 2 h, and cells formed net-like arrays by 6-8 h and capillary-like lumens by 24 h. Blocking of alpha2beta1, but not alpha1 or alpha(v)beta3 integrin function halted morphogenesis. We found that a triple-helical, homotrimeric peptide mimetic of a putative alpha2beta1 binding site: alpha1(I)496-507 GARGERGFP*GER (where single-letter amino acid nomenclature is used, P* = hydroxyproline) inhibited tube formation, whereas a peptide carrying another putative site: alpha1(I)127-138 GLP*GERGRP*GAP* or control peptides did not. A chemical inhibitor of p38 mitogen-activated protein kinase (p38 MAPK), SB202190, blocked tube formation, and p38 MAPK activity was increased in collagen-treated cultures, whereas targeting MAPK kinase (MEK), focal adhesion kinase (FAK), or phosphatidylinositol 3-kinase (PI3K) had little effect. Collagen-treated cells had fewer focal adhesions and 3- to 5-fold less activated FAK. Thus capillary morphogenesis requires endothelial alpha2beta1 integrin engagement of a single type I collagen integrin-binding site, possibly signaling via p38 MAPK and focal adhesion disassembly/FAK inactivation.     

Down-regulation of glutathione and Bcl-2 synthesis in mouse B16 melanoma cells avoids their survival during interaction with the vascular endothelium

B16 melanoma (B16M) cells with high GSH content show high metastatic activity. However, the molecular mechanisms linking GSH to metastatic cell survival are unclear. The possible relationship between GSH and the ability of Bcl-2 to prevent cell death was studied in B16M cells with high (F10) and low (F1) metastatic potential. Analysis of a Bcl-2 family of genes revealed that B16M-F10 cells, as compared with B16M-F1 cells, overexpressed preferentially Bcl-2 (approximately 5.7-fold). Hepatic sinusoidal endothelium-induced B16M-F10 cytotoxicity in vitro increased from approximately 19% (controls) to approximately 97% in GSH-depleted B16M-F10 cells treated with an antisense Bcl-2 oligodeoxynucleotide (Bcl-2-AS). l-Buthionine (S,R)-sulfoximine-induced GSH depletion or Bcl-2-AS decreased the metastatic growth of B16M-F10 cells in the liver. However, the combination of l-buthionine (S,R)-sulfoximine and Bcl-2-AS abolished metastatic invasion. Bcl-2-overexpressing B16M-F1/Tet-Bcl-2 and B16M-F10/Tet-Bcl-2 cells, as compared with controls, showed an increase in GSH content, no change in the rate of GSH synthesis, and a decrease in GSH efflux. Thus, Bcl-2 overexpression may increase metastatic cell resistance against oxidative/nitrosative stress by inhibiting release of GSH. In addition, Bcl-2 availability regulates the mitochondrial GSH (mtGSH)-dependent opening of the permeability transition pore complex. Death in B16M-F10 cells was sharply activated at mtGSH levels below 30% of controls values. However, this critical threshold increased to approximately 60% of control values in Bcl-2-AS-treated B16M-F10 cells. GSH ester-induced replenishment of mtGSH levels (even under conditions of cytosolic GSH depletion) prevented cell death. Our results indicate that survival of B16M cells with high metastatic potential can be challenged by inhibiting their GSH and Bcl-2 synthesis.     

Heparan sulfate 6-o-sulfotransferase is essential for muscle development in zebrafish

Heparan sulfate proteoglycans function in development and disease. They consist of a core protein with attached heparan sulfate chains that are altered by a series of carbohydrate-modifying enzymes and sulfotransferases. Here, we report on the identification and characterization of a gene encoding zebrafish heparan sulfate 6-O-sulfotransferase (hs6st) that shows high homology to other heparan sulfate 6-O-sulfotransferases. When expressed as a fusion protein in cultured cells, the protein shows specific 6-O-sulfotransferase activity and preferentially acts on the iduronosyl N-sulfoglycosamine. In the developing embryo, hs6st is expressed in the brain, the somites, and the fins; the same structures that were affected upon morpholino-mediated functional knockdown. Morpholino injections significantly inhibited 6-O- but not 2-O-sulfation as assessed by HPLC. Morphants display disturbed somite specification independent of the somite oscillator mechanism and have impaired muscle differentiation. In conclusion, our results show that transfer of sulfate to specific positions on glycosaminoglycans is essential for muscle development.