Influenza virus-induced proteins in nuclei and cytoplasm of infected cells

Nuclear entry and nucleolar localization of the Newcastle disease virus (NDV) matrix protein occur early in infection and do not require other NDV proteins

A large proportion of the Newcastle disease virus (NDV) matrix (M) protein is found in the nuclei of infected chicken embryo cells. Kinetic analysis indicated that much of the M protein enters the nucleus early in infection, concentrating in discrete regions of the nucleus and remaining there throughout infection. The M protein was found in localized regions of the nuclei of a variety of cell lines infected with NDV. Immunostaining for both M protein and nucleolar antigens indicated that most of these regions represent nucleoli. Moreover, this nucleolar localization of the M protein was observed in chicken embryo cells infected with 11 different strains of NDV. Only the M protein of strain HP displayed a modified pattern, concentrating in the nucleolus early in infection but in the cytoplasm late in infection. M protein transiently expressed in COS-1 cells also localized to the nucleus and nucleolus, indicating that the M protein does not require other NDV proteins for this localization.

The phosphorylation of the integral membrane (M1) protein of influenza virus

The phosphorylation of the internal and integral membrane (M1) protein of influenza virus was studied. Four points can be made based on the data: (1) The M1 contains at least two moles of phosphate per mole of M1. (2) Phosphorylation of M1 is conserved between influenza A, B and C viruses. Other characteristics of the M1 are also conserved, such as solubility in organic solvent, heterogeneity and ability to partition into lipid vesicles. (3) M1 is phosphorylated in cells infected with a vaccinia recombinant (vP273) containing only the gene of M1, either as a result of a vaccinia virus associated kinase or a cellular one. (4) The phosphate is located within or in close proximity to the major stretch of neutral and hydrophobic amino acids found in M1, as determined by analyzing cyanogen bromide fragments.

M protein (M1) of influenza virus: antigenic analysis and intracellular localization with monoclonal antibodies

A panel of 16 monoclonal antibodies recognizing M protein (M1) of influenza virus was generated. Competition analyses resulted in localization of 14 monoclonal antibodies to three antigenic sites. Three monoclonal antibodies localized to site 1B recognized a peptide synthesized to M1 (residues 220 to 236) with enzyme-linked immunosorbent assay titers equivalent to or greater than that seen with purified M1; therefore, site 1B is located near the C terminus of M1. Sites 2 and 3 localize to the N-terminal half of M1. Antigenic variation of M proteins was seen when the monoclonal antibodies were tested against 14 strains of type A influenza viruses. Several monoclonal antibodies showed specific recognition of A/PR/8/34 and A/USSR/90/77 M proteins and little or no reactivity for all other strains tested. Immunofluorescence analysis with the monoclonal antibodies showed migration of M protein to the nucleus during the replicative cycle and demonstrated association of M protein with actin filaments in the cytoplasm. Use of a vaccinia virus recombinant containing the M-protein gene demonstrated migration of M protein to the nucleus in the absence of synthesis of gene products from other influenza virus RNA segments.

Electroelution for purification of influenza A matrix protein for use in immunoassay

A new preparative method for isolation of matrix protein from type A influenza virus was developed. Commercially available whole virus or split virus vaccines were lysed, and the soluble proteins separated by electrophoresis on polyacrylamide gel. The matrix protein was located on the gel by precipitation with KCl, and recovered by electroelution. The method was technically simple and required little direct supervision during the two-step recovery process. Yields of A matrix were consistently high, averaging 68.1% in five trials with A/Brazil/X-71. The method was also successful with other A viruses, although not with influenza B virus. Isolated A matrix had less than 0.5% contamination by hemagglutinin or nucleoprotein, as determined by immunoblotting and ELISA. Matrix protein was immunoreactive in Western blots and was detectable in concentrations as low as 1 ng/ml with ELISA. The isolated matrix provided a suitable standard for detection of matrix protein in nasal washes from patients with influenza A virus infection, and could also be used to detect anti-matrix antibodies, including monoclonal antibodies in tissue culture supernatants. The advantages of electroelution for separation of matrix protein compared to other methods were its technical simplicity, applicability to formalin-fixed influenza virus in commercially available vaccines, its consistently high yield, and its very high level of purification.

The use of ultrathin cryosections for localisation of influenza virus antigens in infected vero cell cultures

The localisation of influenza virus antigens in infected Vero cell monolayer cultures by post embedding immunoelectron microscopy requires both good resolution and the retention of antigenicity in the tissue sections. Ultrathin cryosections are superior to ultrathin resin sections for this purpose. The colloidal gold probe was used in conjunction with specific antibody preparations to localise three viral proteins. Antibody raised against haemagglutinin glycoproteins labelled the host cell membrane and the virus fringe without contamination of the host cell nucleus, whereas antibody raised against viral nuclear protein labelled throughout the host cell cytoplasm and nucleus. Matrix protein was localised within the nucleus and was associated with the host cell membrane of the infected cell. The appearance of all these proteins was maximal 24 h post infection.

The membrane (M1) protein of influenza virus occurs in two forms and is a phosphoprotein

The membrane (M1) protein of influenza virus was found to be heterogenous and to occur in two forms in the virus particle. The two forms of M1 were found in virus which was produced both early and late after infection and in infected cells. The two forms could be separated on polyacrylamide gels under specific conditions. The two components of M1 contained similar tryptic peptides. However, a small proteolytic difference between the two proteins could not be ruled out. Both M1 proteins were present in phosphorylated form in the virus particle. The phosphorylated M1 components were not readily distinguished from phosphorylated nonstructural protein (NS1) when cytoplasm of infected cells was analyzed on polyacrylamide gels. The two phosphorylated M1 components could, however, be detected in infected cells by immunoprecipitation. One M1 component contained only phosphoserine whereas the second contained phosphoserine and a small amount of phosphothreonine as well. In addition to the phosphorylated nucleoprotein and M1, a third phosphorylated protein was routinely detected in virus particles. It was a surface component of the virus, since it could be removed from whole virus with chymotrypsin and contained phosphate at serine residues. The identity of this component was not known.

Insertion of influenza M protein into the viral lipid bilayer and localization of site of insertion

Recent studies with isolated M protein from influenza virus have shown that the protein has a high affinity for lipid. The ability of M to partition into lipid vesicles merely by shaking vesicles and M together is suggestive evidence that the protein could be interacting with the lipid in the virus particle. A more direct analysis was carried our here to determine whether M is in contact with the viral lipid in situ, by using the photoactivatable hydrophobic probe, pyrenesulfonyl azide. Covalent linkage of this probe to M indicated that a segment of M residues with in the virus membrane in contact with the lipid bilayer. M inserted into lipid vesicles at two locations on the molecule. A major insertion into lipid occurred in the middle of the molecule where a large cluster of 20 hydrophobic and neutral amino acids occurs. A second insertion occurred approximately one fourth in from the amino terminus, where a smaller segment of 13 uncharged amino acids is found. Confirmation that M inserted into lipid at these locations came also from results with cyanogen bromide fragments of M. Of the 12 to 13 fragments produced, 3 specifically bound to lipid vesicles. These were the first, second, and third contiguous segments beginnings at the amino terminus and containing the two hydrophobic areas noted above.

Interaction of influenza M protein with viral lipid and phosphatidylcholine vesicles

The M protein of influenza is the predominant structural component of the virus. The interactions of this protein with the viral lipid or with other proteins are not known. The ability of M to interact with viral or other lipids was investigated. Purified M was mixed with viral lipid or egg phosphatidylcholine and was incorporated into vesicles (i) by addition of sodium deoxycholate followed by dialysis or (ii) by sonication. Between 90 and 100% of the M became firmly associated with the lipid by either of these two methods, whereas nucleoprotein failed to associate with the vesicles. From association also occurred if M was mixed with performed vesicles. Most of the M attached to the vesicles could be hydrolyzed with proteolytic enzymes such as trypsin or thermolysin, except for a small fragment of about 5,000 daltons which remained associated with the lipid vesicles. The ability of fragments of M to interact with lipids was also investigated. Of 13 fragments produced by cleavage with cyanogen bromide, 3 specifically associated with lipid vesicles. The data indicate that a specific portion of the M molecule has a high affinity for lipid bilayers of various origins.