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

Avian influenza A/H5N1 neuraminidase expressed in yeast with a functional head domain

The study reports heterologous expression in Pichia pastoris of active neuraminidase derived from avian influenza virus A/Viet Nam/DT-036/2005(H5N1). A gene encoding the neuraminidase N1 head domain (residues 63-449) was fused directly in-frame with the Saccharomyces cerevisiae alpha-factor secretion signal in pPICZ(A vector. Recombinant N1 neuraminidase was expressed in P. pastoris as a 72kDa secreted, soluble protein. Glycopeptidase F treatment generated a 45kDa product, indicating that the secreted recombinant N1 neuraminidase is an N-linked glycoprotein. Kinetic studies and inhibition tests with oseltamivir carboxylate demonstrated that the recombinant N1 neuraminidase has similar K(m) and K(i) values to those of the viral N1 neuraminidase. This yeast-based heterologous expression system provided functionally active recombinant N1 neuraminidase that should be useful in anti-influenza drug screening, and also as a potential protein-based vaccine.

Domain architecture and oligomerization properties of the paramyxovirus PIV 5 hemagglutinin-neuraminidase (HN) protein

The mechanism by which the paramyxovirus hemagglutinin-neuraminidase (HN) protein couples receptor binding to activation of virus entry remains to be fully understood, but the HN stalk is thought to play an important role in the process. We have characterized ectodomain constructs of the parainfluenza virus 5 HN to understand better the underlying architecture and oligomerization properties that may influence HN functions. The PIV 5 neuraminidase (NA) domain is monomeric whereas the ectodomain forms a well-defined tetramer. The HN stalk also forms tetramers and higher order oligomers with high alpha-helical content. Together, the data indicate that the globular NA domains form weak intersubunit interactions at the end of the HN stalk tetramer, while stabilizing the stalk and overall oligomeric state of the ectodomain. Electron microscopy of the HN ectodomain reveals flexible arrangements of the NA and stalk domains, which may be important for understanding how these two HN domains impact virus entry.

Structural studies of the parainfluenza virus 5 hemagglutinin-neuraminidase tetramer in complex with its receptor, sialyllactose

The paramyxovirus hemagglutinin-neuraminidase (HN) functions in virus attachment to cells, cleavage of sialic acid from oligosaccharides, and stimulating membrane fusion during virus entry into cells. The structural basis for these diverse functions remains to be fully understood. We report the crystal structures of the parainfluenza virus 5 (SV5) HN and its complexes with sialic acid, the inhibitor DANA, and the receptor sialyllactose. SV5 HN shares common structural features with HN of Newcastle disease virus (NDV) and human parainfluenza 3 (HPIV3), but unlike the previously determined HN structures, the SV5 HN forms a tetramer in solution, which is thought to be the physiological oligomer. The sialyllactose complex reveals intact receptor within the active site, but no major conformational changes in the protein. The SV5 HN structures do not support previously proposed models for HN action in membrane fusion and suggest alternative mechanisms by which HN may promote virus entry into cells.

Purification of the Porcine rubulavirus attachment protein by liquid isoelectric focusing

Porcine rubulavirus (PoRV) is an emerging virus responsible for meningoencephalitis, respiratory distress, and reproductive alterations in pigs. The hemagglutinin-neuraminidase (HN) glycoprotein is the most exposed and antigenic of the virus proteins. HN plays central roles in PoRV infection; i.e., it recognizes sialic acid-containing cell receptors that mediate virus attachment and penetration; in addition, its neuraminidase (sialic acid hydrolysis) activity has been proposed to be a virulence factor. So, HN is an ideal target for therapeutic treatment and prevention of this viral infection. This work describes a simple, fast, and sensitive method to purify the active form of HN protein based on its isoelectric point. HN was purified at a pH of 4.4, at which a single protein band of 66 kDa was observed on SDS-PAGE. Pure HN showed a maximal enzymatic activity at pH 3.5 and 37 degrees C using bovine fetuin as substrate. However, it retains circa 80% of its activity at a wide temperature range from 30 to 55 degrees C. We also describe improvements of neuraminidase determination method, which permits analysis in a microplate spectrophotometer, thereby increasing the sensitivity and reducing the costs of valuable reagents and biological samples.

Regulation of fusion activity by the cytoplasmic domain of a paramyxovirus F protein

SER virus is a member of the family Paramyxoviridae, genus Rubulavirus, which has been isolated from pigs. It is very closely related to SV5 virus serologically, in protein profile, and in nucleotide sequence. However, unlike SV5, SER induces minimal syncytium formation in infected CV-1 or BHK cells. Fluorescence transfer experiments between labeled erythrocytes and infected MDBK cells revealed that SER also induces hemifusion and pore formation with reduced efficiency. The virion polypeptide profiles of SER and SV5 are very similar, except that the SER F1 subunit shows an apparent molecular weight that is about 2 kDa higher than that of SV5. Comparison of the deduced amino acid sequences revealed the SER F (551 aa) to be longer than SV5 F (529 aa) by 22 residues in the cytoplasmic tail (CT) domain. The HN and M gene sequences of the viruses were found to be very similar. The SER F showed minimal fusion activity when coexpressed with either SV5 or SER HN. In contrast, SV5 F was highly fusogenic when coexpressed with either HN protein, indicating that the restricted fusion capacity of SER virus is a property of its F protein. Truncation in the CT of SER F by 22 residues completely rescued its ability to cause syncytium formation, whereas other truncations rescued syncytium formation partially. These results demonstrate that an elongated CT of a paramyxovirus F protein suppresses its membrane fusion activity.

Expression and characterization of soluble human parainfluenza virus type 1 hemagglutinin-neuraminidase glycoprotein

Human parainfluenza virus types 1 (hPIV-1), 2, and 3 represent significant respiratory pathogens for which no antiviral treatment is currently available. To characterize the biochemical functions of the hPIV-1 hemagglutinin-neuraminidase (HN) glycoprotein, a potential target for antiviral therapy, we cloned and expressed a soluble portion of hPIV-1 HN (amino acid residues 137-575), lacking the N-terminal hydrophobic membrane anchorage region, in insect cells using the baculovirus secretion expression system. The expressed HN protein was purified through cation-exchange chromatography followed by metal affinity chromatography, using the 6xHis epitope introduced at the carboxyl terminus of the recombinant protein. N-terminal amino acid sequence analysis of purified HN indicated that the honeybee melittin secretion signal peptide was correctly removed during post-translational processing. Further characterization revealed that the purified HN protein was N-glycosylated and exhibited neuraminidase activity whose characteristics resembled those of the native HN protein of hPIV-1 virions. The establishment of this expression and purification system has allowed us to further explore the biochemical characteristics of paramyxovirus HN and to obtain material that could be suitable for X-ray crystallography studies.

Oligomerization, secretion, and biological function of an anchor-free parainfluenza virus type 2 (PI2) fusion protein

A number of studies indicate that the transmembrane domain, the cytoplasmic domain, or both regions of viral surface glycoproteins are involved in quaternary structure formation. In this report, the transmembrane domain and cytoplasmic tail coding sequence of the fusion (F) glycoprotein gene from parainfluenza type 2 virus was truncated by PCR and the resulting gene (PI2F') was expressed in HeLa-T4 cells by using the vaccinia virus-T7 transient expression system. Pulse-chase experiments indicated that the anchor-free PI2F' was expressed and processed into F(1) and F(2) subunits. Both the processed and the unprocessed anchor-free PI2F' proteins were found to be efficiently secreted into the culture medium. Examination of the oligomeric form of the anchor-free PI2F' by chemical cross-linking demonstrated that it assembles posttranslationally into dimers and trimers with a pattern similar to that of the wild-type PI2F protein. In an effort to better understand the biological properties of the truncated form of PI2F', we anchored PI2F' by a glycosyl-phosphatidylinositol (GPI) linkage. The GPI-anchored PI2F' protein, when coexpressed with PI2HN, did not induce cell fusion seen as syncytium formation, but was found to initiate lipid mixing (hemifusion) as observed by transfer of R-18 rhodamine from red blood cells to the GPI-PI2F'/PI2HN cotransfected cells. The results therefore indicate that the extracellular domain of the PI2 fusion protein contains not only the structural information sufficient to direct assembly into higher oligomers, but also is competent to initiate membrane fusion, suggesting that the anchor-free PI2F' may be useful for further structural studies.

The use of a quantitative fusion assay to evaluate HN-receptor interaction for human parainfluenza virus type 3

Sialic acid is the receptor determinant for the human parainfluenza virus type 3 (HPF3) hemagglutinin-neuraminidase (HN) glycoprotein, the molecule responsible for binding of the virus to cell surfaces. In order for the fusion protein (F) of HPF3 to promote membrane fusion, HN must interact with its receptor. In addition to its role in receptor binding and fusion promotion, the HPF3 HN molecule contains receptor-destroying (sialidase) activity. The putative active sites are in the extracellular domain of this type II integral membrane protein. However, HN is not available in crystalline form; the exact locations of these sites, and the structural requirements for binding to the cellular receptor, which has not yet been isolated, are unknown. Nor have small molecular synthetic inhibitors of attachment or fusion that would provide insight into these processes been identified. The strategy in the present study was to develop an assay system that would provide a measure of a specific step in the viral cycle-functional interaction between viral glycoproteins and the cell during attachment and fusion-and serve to screen a variety of substances for inhibitory potential. The assay is based on our previous finding that CV-1 cells persistently infected (p.i.) with HPF3 do not fuse with one another but that the addition of uninfected CV-1 cells, supplying the critical sialic acid containing receptor molecules that bind HN, results in rapid fusion. In the present assay two HeLa cell types were used: we persistently infected HeLa-LTR-betagal cells, assessed their fusion with uninfected HeLa-tat cells, and then quantitated the beta-galactosidase (betagal) produced as a result of this fusion. The analog alpha-2-S-methyl-5-N-thioacetylneuraminic acid (alpha-Neu5thioAc2SMe) interfered with fusion, decreasing betagal production by 84% at 50 mM and by 24% at 25 mM. In beginning to extend our studies to different types of molecules, we tested an unsaturated derivative of sialic acid, 2,3-dehydro-2-deoxy-n-acetyl neuraminic acid (DANA), which is known to inhibit influenza neuraminidase by virtue of being a transition-state analog. We found that 10 mM DANA inhibited neuraminidase activity in HPF3 viral preparations. More significantly, this compound was active in our assay of HN-receptor interaction; 10 mM DANA completely blocked fusion and betagal production, and hemadsorption inhibition by DANA suggested that DANA blocks attachment. In plaque reduction assays performed with the compounds, the active analog alpha-Neu5thioAc2SMe reduced plaque formation by 50% at a 50 mM concentration; DANA caused a 90% inhibition in the plaque reduction assay at a concentration of 25 mM. Our results indicate that specific sialic acid analogs that mimic the cellular receptor determinant of HPF3 can block virus cell interaction and that an unsaturated n-acetyl-neuraminic acid derivative with affinity to the HN site responsible for neuraminidase activity also interferes with HN-receptor binding. Strategies suggested by these findings are now being pursued to obtain information regarding the relative locations of the active sites of HN and to further elucidate the relationship between the receptor-binding and receptor-destroying activities of HN during the viral life cycle. The quantitative assay that we describe is of immediate applicability to large-scale screening for potential inhibitors of HPF3 infection in vivo.

Differences in the role of the cytoplasmic domain of human parainfluenza virus fusion proteins

We have investigated the roles of the cytoplasmic domains of the human parainfluenza virus type 2 (PI2) and type 3 (PI3) fusion (F) proteins in protein transport and cell fusion activity. By using the vaccinia virus-T7 transient expression system, a series of F protein cytoplasmic tail truncation mutants was studied with respect to intracellular and surface expression and the ability to induce cell fusion when coexpressed with the corresponding hemagglutinin-neuraminidase (HN) proteins. All of the cytoplasmic tail truncation mutants of PI2F were expressed at high levels intracellularly or on cell surfaces as measured by immunoprecipitation and cell surface biotinylation assays. In addition, when coexpressed with PI2HN, these truncation mutants of PI2F were all found to be essentially unimpaired in the ability to induce cell fusion as measured by a quantitative cell fusion assay. In contrast, surface expression and cell fusion activity were found to be eliminated by a mutant of PI3F in which the entire cytoplasmic tail was deleted, and the mutant protein appeared to be unable to assemble into a high-molecular-weight oligomeric structure. To further investigate whether there is a specific sequence requirement in the cytoplasmic tail of PI3F, a chimeric protein consisting of the PI3F extracellular and transmembrane domains and the PI2F cytoplasmic tail was constructed. This chimeric protein was detected on the surface, and it was capable of inducing cell fusion when expressed together with PI3HN, although the fusogenic activity was reduced compared with that of wild-type PI3F. These results demonstrate that although PI2 and PI3 viruses belong to the same parainfluenza virus genus, these viruses show marked differences with respect to functional requirements for the cytoplasmic tail of the F glycoprotein.

Crystals of hemagglutinin-neuraminidase of parainfluenza virus contain triple-stranded helices

When purified dimers of hemagglutinin-neuraminidase molecules released by protease digestion from three strains of human parainfluenza virus 1 were used in crystallization trials, long thin needle crystals formed. Electron microscopic analysis of these needle crystals revealed that they are composed of stacks of triple-stranded helices with each strand of the helix made up of subunits of hemagglutinin-neuraminidase. To our knowledge, this is the first direct demonstration of the assembly of protein subunits into large triple-stranded helices. An understanding of the organization of these triple helices may shed light on the structural properties of the hemagglutinin-neuraminidase molecules that cause them to form these helices.