Streptococcus pneumoniae G5 domains bind different ligands

Many bacterial pathogens express small G5 domains that exist in the context of various membrane‐anchored proteins and these G5 domains have been associated with colonization, cellular adhesion, and biofilm formation. However, despite over a decade since the computational prediction of these G5 domains, many remain uncharacterized, particularly those from Streptococcus pneumoniae. Of five previously predicted G5 domains we found that four of these, all derived from S. pneumoniae, are independently folded modules. As one of these exhibits extreme line broadening due to self‐association, we were able to use NMR solution studies to probe the potential ligand interactions of the remaining three G5 domains. None of these G5 domains engage N‐acetylglucosamine (NAG) as previously predicted but do interact with other small molecules that may modulate adherence to both bacteria and host cells. Specifically, while all G5 domains tested engage Zn, only one of these G5 domains engage heparin. NMR solution structural studies of the IgA1 Protease G5 (IgA1P‐G5) and endo‐beta‐N‐acetylglucosaminidase‐D G5 (ENDD‐G5) also facilitated identification of the ligand binding sites and confirm the typical G5 fold that comprises two connected β‐sheets with no canonical core. NMR relaxation experiments indicate flexibility on both ends and within the connecting regions between the β‐sheets. Our studies thus establish a basis for future biological experiments to test whether the ligands presented here are involved in bacterial adherence, either to bacteria or to host cells.     

Characterizing and controlling the inherent dynamics of cyclophilin-A

With the recent advances in NMR relaxation techniques, protein motions on functionally important timescales can be studied at atomic resolution. Here, we have used NMR-based relaxation experiments at several temperatures and both 600 and 900 MHz to characterize the inherent dynamics of the enzyme cyclophilin-A (CypA). We have discovered multiple chemical exchange processes within the enzyme that form a “dynamic continuum” that spans 20–30 Å comprising active site residues and residues proximal to the active site. By combining mutagenesis with these NMR relaxation techniques, a simple method of counting the dynamically sampled conformations has been developed. Surprisingly, a combination of point mutations has allowed for the specific regulation of many of the exchange processes that occur within CypA, suggesting that the dynamics of an enzyme may be engineered.     

Migration patterns of Hoopoe Upupa epops and Wryneck Jynx torquilla: an analysis of European ring recoveries

For many bird species, recovery of ringed individuals remains the best source of information about their migrations. In this study, we analyzed the recoveries of ringed European Hoopoe (Upupa epops) and the Eurasian Wryneck (Jynx torquilla) from 1914 to 2005 from all European ringing schemes. The aim was to define general migration directions and to make inferences about the winter quarters, knowing that hardly any recoveries are available from sub-Saharan Africa. For the autumn migration, there is evidence of a migratory divide for the Hoopoe in Central Europe, at approximately 10–12°E. Autumn migration directions of Wrynecks gradually change from SW to SE depending on the longitude (west to east) of the ringing place. In both species, only a few recoveries were available indicating spring migration directions, but they showed similar migration axes as for autumn migration, and hence no evidence for loop-migration. Due to a paucity of recoveries on the African continent, we can make only limited inferences about wintering grounds: extrapolating migration directions are only indicative of the longitude of the wintering area. The directions of autumn migration indicate a typical pattern observed in European long-distance migrants: west-European Hoopoes and Wrynecks are likely to winter in western Africa, while central- and east-European birds probably winter more in the east. Due to the migratory divide, for the Hoopoe, this phenomenon is more pronounced.     

Residues in the Heptad Repeat A Region of the Fusion Protein Modulate the Virulence of Sendai Virus in Mice

While the molecular basis of fusion (F) protein refolding during membrane fusion has been studied extensively in vitro, little is known about the biological significance of membrane fusion activity in parainfluenza virus replication and pathogenesis in vivo. Two recombinant Sendai viruses, F-L179V and F-K180Q, were generated that contain F protein mutations in the heptad repeat A region of the ectodomain, a region of the protein known to regulate F protein activation. In vitro, the F-L179V virus caused increased syncytium formation (cell-cell membrane fusion) yet had a rate of replication and levels of F protein expression and cleavage similar to wild-type virus. The F-K180Q virus had a reduced replication rate along with reduced levels of F protein expression, cleavage, and fusogenicity. In DBA/2 mice, the hyperfusogenic F-L179V virus induced greater morbidity and mortality than wild-type virus, while the attenuated F-K180Q virus was much less pathogenic. During the first week of infection, virus replication and inflammation in the lungs were similar for wild-type and F-L179V viruses. After approximately 1 week of infection, the clearance of F-L179V virus was delayed, and more extensive interstitial inflammation and necrosis were observed in the lungs, affecting entire lobes of the lungs and having significantly greater numbers of syncytial cell masses in alveolar spaces on day 10. On the other hand, the slower-growing F-K180Q virus caused much less extensive inflammation than wild-type virus, presumably due to its reduced replication rate, and did not cause observable syncytium formation in the lungs. Overall, the results show that residues in the heptad repeat A region of the F protein modulate the virulence of Sendai virus in mice by influencing both the spread and clearance of the virus and the extent and severity of inflammation. An understanding of how the F protein contributes to infection and inflammation in vivo may assist in the development of antiviral therapies against respiratory paramyxoviruses.Sendai virus (SeV), a murine parainfluenza virus (PIV), belongs to the genus Respirovirus within the family Paramyxoviridae (33). Sendai virus is the murine counterpart of human parainfluenza virus 1 (HPIV1), and these two viruses share high sequence homology and antigenic cross-reactivity (23, 38, 58). Both Sendai virus and HPIV1 cause respiratory diseases in their hosts that range from mild to severe, with the greatest morbidity and mortality occurring in immunocompromised hosts (3, 17). In pediatric medicine, HPIV1 is an important cause of bronchiolitis, pneumonia, and laryngotracheobronchitis, or croup (11). Other members of the genus Respirovirus include human and bovine forms of PIV3 (30).Like other paramyxoviruses, Sendai virus is an enveloped, nonsegmented, negative-strand RNA virus that invades host cells by fusion (F) protein-mediated membrane fusion at the plasma membrane (33). The receptor binding protein for Sendai virus, as well as the other parainfluenza viruses, is the hemagglutinin-neuraminidase (HN) protein. During viral entry, the HN protein binds sialic acid-containing receptors on the surfaces of host cells and then triggers the F protein to refold and cause membrane fusion (34, 40). Paramyxovirus replication occurs in the cytoplasm of infected cells, where the viral nucleocapsid is formed by the encapsidation of the viral genome with the viral nucleoprotein (N), phosphoprotein (P), and the large RNA-dependent RNA-polymerase (L) protein (33). The assembly and budding of infectious parainfluenza virions from the plasma membrane are mediated largely by the matrix (M) protein, which interacts with the viral nucleocapsid and the cytoplasmic tails of the HN and F proteins (56, 63).The paramyxovirus F protein mediates both virus-cell fusion and cell-cell fusion. Similar to other class I viral fusion proteins, paramyxovirus F proteins are expressed on the surfaces of infected cells and virions as trimers that are trapped in metastable (high energy) conformations (29, 54, 71, 73). In order to become activated for membrane fusion, uncleaved F₀ precursor protein trimers must be cleaved into a fusion-capable complex formed by F₁ and F₂ subunits (55). Field isolates of Sendai virus that have a monobasic cleavage site are cleavage activated by tryptase Clara secreted from respiratory epithelial cells (32, 69) while the pantropic F1-R laboratory isolate of Sendai virus has a mutated cleavage site and is cleaved by more ubiquitously expressed proteases (41, 67). Paramyxovirus F proteins have several regions involved in F protein conformational changes during membrane fusion: a hydrophobic fusion peptide, two 4-3 heptad repeat regions (designated heptad repeat A [HRA] and HRB), a transmembrane domain, and a cytoplasmic tail. The prefusion form of the PIV5 F₀ protein has a mushroom-like shape formed by a large globular head attached to a rod-like stalk formed by the HRB region (76). Upon triggering by the HN protein, the HRB region dissociates, the HRA region springs into a coiled coil, and the fusion peptide is inserted into the target membrane (52). Membrane fusion is catalyzed by the formation of a coiled-coil hairpin structure (2, 7, 75, 78), formed by the HRA and HRB regions, that juxtaposes the membrane-interacting fusion peptide and transmembrane domains (52). We recently performed a mutational analysis on a 10-residue sequence in the HRA region of the Sendai virus F protein (37) that forms a β-strand-turn-α-helix structure in the prefusion conformation and part of a triple-stranded coiled coil in the hairpin conformation (75, 76). The mutated residues were found to play important roles in regulating the activation and membrane fusion activity of the Sendai virus F protein, showing that F protein refolding is regulated by residues that undergo dramatic changes in secondary and tertiary structure between the prefusion and hairpin conformations.Upon triggering by the HN protein, cell surface-expressed F protein trimers mediate cell-cell fusion (syncytium formation) and extend infection in a local area (55). In nonpolarized epithelial cells, virus-induced syncytium formation has long been considered a hallmark of in vitro cytopathogenesis by respiratory paramyxoviruses. However, many questions remain regarding the extent of envelope glycoprotein expression, parainfluenza virus budding, and syncytium formation at the basolateral surfaces of polarized cells (4, 77). In an in vitro model of human airway epithelium (HAE), HPIV3 has been shown to infect ciliated epithelial cells exclusively, predominantly at the apical surface, causing little virus-mediated cytopathology, no spread of the virus beyond ciliated cells, and no syncytium formation (77). As the HAE model lacks innate and adaptive immune cells, this model would not reveal the formation of syncytia involving all cell types in the respiratory tract that are present during infection, including those that play a role in the host response to infection. In immunocompetent mice, the replication of field isolates of Sendai virus is restricted to the respiratory tract, and progeny virions bud from the apical surfaces of polarized epithelial cells (68). While syncytial cell formation in the bronchiolar epithelia of mice infected with Sendai virus has been reported previously (28), the timing of giant cell formation and its contribution to the spread of the virus and the disease it induces in the respiratory tract remain unknown.To test the hypothesis that the fusogenicity of the F protein contributes to the pathogenicity of Sendai virus in mice, the natural host of this virus, we generated two recombinant Sendai viruses containing F protein mutations in the heptad repeat A region that were found previously to either increase or decrease its fusogenic activity when the F protein was expressed from plasmid DNA constructs (37). In the present study, the effects of the F protein mutations on virus replication, F protein expression, F protein cleavage, and syncytium formation were characterized in vitro. The hyperfusogenic F-L179V virus was found to induce greater morbidity and mortality in DBA/2 mice than wild-type virus, whereas the hypofusogenic and attenuated F-K180Q virus was found to be much less pathogenic. After 1 week of infection, the F-L179V virus induced more extensive interstitial inflammation and necrosis in the lungs than the wild-type virus, including a greater number of syncytial cell masses. On the other hand, the attenuated F-K180Q virus caused much less extensive inflammation than wild-type virus and did not cause observable syncytium formation in the lungs. A comparison of 50% minimal lethal dose (MLD₅₀) values, lung titers, and histopathologic changes reveals a correlation between the membrane fusion activity of the F protein and the virulence of Sendai virus in mice.     

Expression, purification, and biochemical characterization of a recombinant Iectin of Sarcocystis muris (Apicomplexa) cyst merozoites

The mature major microneme protein of Sarcocystis muris cyst merozoites, which is known as a dimeric lectin with high affinity to galactose and some of its derivatives, was expressed in Escherichia coli as a histidine-tagged fusion protein. The recombinant polypeptide, which was recognized by a monoclonal antibody directed against the native lectin, was purified from inclusion bodies after solubilization and refolding, using a combination of metal chelate and lactose affinity chromatography. The apparent molecular mass of the refolded polypeptide as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoreses was 16 kDa, whereas gel filtration chromatography clearly demonstrated that the recombinant protein, like its native counterpart, exists as a homodimer of two non-covalently associated subunits. Inhibition of haemagglutination suggests that the combining site of the recombinant lectin recognizes N-acetyl-galactosamine as the dominant sugar, thus confirming the correct folding of the monosaccharide combining site in the renatured lectin. To the best of our knowledge, this work represents the first reported detailed characterization of a recombinant lectin from apicomplexan parasites, and may contribute to a better understanding of the process of host cell recognition and invasion by these obligate intracellular protozoa.     

Cytoplasmic Domain of Sendai Virus HN Protein Contains a Specific Sequence Required for Its Incorporation into Virions

In the assembly of paramyxoviruses, interactions between viral proteins are presumed to be specific. The focus of this study is to elucidate the protein-protein interactions during the final stage of viral assembly that result in the incorporation of the viral envelope proteins into virions. To this end, we examined the specificity of HN incorporation into progeny virions by transiently transfecting HN cDNA genes into Sendai virus (SV)-infected cells. SV HN expressed from cDNA was efficiently incorporated into progeny Sendai virions, whereas Newcastle disease virus (NDV) HN was not. This observation supports the theory of a selective mechanism for HN incorporation. To identify the region on HN responsible for the selective incorporation, we constructed chimeric SV and NDV HN cDNAs and evaluated the incorporation of expressed proteins into progeny virions. Chimera HN that contained the SV cytoplasmic domain fused to the transmembrane and external domains of the NDV HN was incorporated to SV particles, indicating that amino acids in the cytoplasmic domain are responsible for the observed specificity. Additional experiments using the chimeric HNs showed that 14 N-terminal amino acids are sufficient for the specificity. Further analysis identified five consecutive amino acids (residues 10 to 14) that were required for the specific incorporation of HN into SV. These residues are conserved among all strains of SV as well as those of its counterpart, human parainfluenza virus type 1. These results suggest that this region near the N terminus of HN interacts with another viral protein(s) to lead to the specific incorporation of HN into progeny virions.     

Topography of a flexible ribonucleoprotein helix: protein-protein contacts in Sendai virus nucleocapsids.

Contacts among the three polypeptide species in the flexible helical nucleocapsids of a paramyxovirus were examined with bifunctional protein cross-linking reagents. Polypeptides L and P, minor components of Sendai virus nucleocapsids implicated in viral RNA polymerase activity, were efficiently cross-linked into large complexes, indicating that they enjoy abundant contacts with neighboring protein molecules in the helix. Less reactivity was found in the case of the major structural polypeptide, NP; about half of all molecules of NP formed large cross-linked complexes, most of the rest remaining as monomers along with a small proportion of homodimers and low-order oligomers. Marked heterogeneity in the cross-linking reactivity of NP molecules, which may reflect the conformational quasi-equivalence inherent in a flexible helix, was indicated by the production of several conformers of homodimers and other low-order oligomers of NP, and by failure of the kinetics of NP cross-linking to conform to a simple statistical model of random polmerization. The validity of the statistical model was shown by cross-linking experiments with the rigid helical virus, tobacco mosaic virus.     

Crystallization of biologically active hemagglutinin-neuraminidase glycoprotein dimers proteolytically cleaved from human parainfluenza virus type 1.

We isolated, purified, and characterized the hemagglutinin-neuraminidase (HN) of human parainfluenza virus type 1, with the ultimate goal of producing crystals suitable for three-dimensional X-ray structure analysis. Pronase was used to cleave the globular head of the HN molecule directly from virus particles, forming HN monomers and dimers. The purified dimers retained neuraminidase and hemadsorption activity and were recognized by 14 anti-HN monoclonal antibodies, demonstrating intact HN antigenic structure and function. N-terminal sequence analysis of the dimers showed that cleavage had occurred at amino acid 136 or 137, freeing the C-terminal 438 or 439 amino acids. On electron micrography, the dimer appeared as two box-shaped structures, each approximately 5 by 5 nm. When the purified HN dimers were crystallized in hanging drops by vapor diffusion against 20% polyethylene glycol 3350, they formed both rectangular plates and needlelike crystals. The rectangular crystals diffracted X-rays, indicating an ordered atomic structure. However, the resolution was approximately 10 A (1 nm), insufficient for three-dimensional structural analysis. Experiments to improve the resolution by increasing the size and quality of the crystals are in progress.