Influenza virus ts61S hemagglutinin is significantly defective in polypeptide folding and intracellular transport at the permissive temperature

First isolation of a rhabdovirus from perch Perca fluviatilis in Switzerland

Perca fluviatilis is a fish species of increasing interest to the Swiss fish farming industry. In recent years, recirculation systems have been specifically set up to increase production. In one of these farms, abnormal spiral swimming associated with elevated mortalities occurred in repeated batches of imported perch shortly after stocking on several occasions. No bacterial or parasitic etiology was detected, but a virus grown in bluegill fry (BF-2) cells was identified as perch rhabdovirus. Subsequent investigations of other samples suggested a viral tropism for the central nervous system (CNS). Phylogenetic analysis of the partial N and entire G gene sequences positioned this isolate in genogroup C of the species Perch rhabdovirus, with high nucleotide and amino acid (aa) sequence identities with the DK5533 strain isolated in Denmark in 1989. Comparative studies using other closely related isolates allowed the distinction of 2 serological patterns among perch rhabdoviruses and the identification of a proline substitution by a serine in position 147 of the glycoprotein potentially involved in antigenic differentiation. Even if perch imported onto the farm tested negative by virus isolation prior to transport, they may have been the origin of this outbreak since CNS tissue was not included in the samples that were analyzed. Another possibility might be a sub-clinical infection with a viral load in resident fish too low to be detected. This study reports the first isolation of a perch rhabdovirus in Switzerland, and emphasizes the necessity of optimizing diagnostic tools that facilitate better control of the risks associated with fish translocation.

Transposon-mediated random insertions and site-directed mutagenesis prevent the trafficking of a mouse mammary tumor virus superantigen

Mouse mammary tumor viruses (MMTVs) encode superantigens (Sags) which are critical to the life cycle of infectious virus and can mediate extensive deletion of T lymphocytes when expressed by endogenous proviruses. Little is known about the structure, intracellular trafficking, or nature of Sag association with major histocompatibility (MHC) class II products. In order to gain a better understanding of Sag structure-function relationships, we extensively mutagenized this type II glycoprotein using two different approaches: transposon-mediated random in-frame insertion mutagenesis and site-directed mutagenesis targeting clusters of charged residues. We find that 31 codon insertions are infrequently tolerated in Mtv-7 Sag, with just 1 of 14 insertion mutants functionally presented on the surface of B cells. Surprisingly, similar effects were observed with Sag mutants with substitutions at pairs of charged residues; only 2 of 6 mutants trafficked to the plasma membrane and stimulated T cells, 1 with a temperature-sensitive phenotype. The data suggest that the nonfunctional Mtv-7 Sag mutants are stringently retained in the endoplasmic reticulum due to conformational defects rather than disrupted interactions with MHC class II, thus identifying charged amino acids critical to the structural stability of viral superantigens.

Effects of altering palmitylation sites on biosynthesis and function of the influenza virus hemagglutinin

Mutagenesis studies indicated that the three cytoplasmic cysteines of the influenza virus A/Japan/305/57 hemagglutinin (HA) are all palmitylated, but to an unequal extent. Replacement of all three cysteines abolished palmitylation, but affected neither HA biosynthesis nor function. Palmitate was not required for HA to be incorporated into virions.

Alterations to influenza virus hemagglutinin cytoplasmic tail modulate virus infectivity

The influenza virus hemagglutinin (HA) contains a cytoplasmic domain that consists of 10 to 11 amino acids, of which five residues have sequence identity for 10 of 13 HA subtypes. To investigate properties of these conserved residues, oligonucleotide-directed mutagenesis was performed, using an HA cDNA of influenza virus A/Udorn/72 (H3N2) to substitute the conserved cysteine residues with other residues, to delete the three C-terminal conserved residues, or to remove the entire cytoplasmic domain. The altered HAs were expressed in eukaryotic cells, and the rates of intracellular transport were examined. It was found that substitution of either conserved cysteine residue within the cytoplasmic domain did not affect the rate of intracellular transport, whereas deletion of residues within the C-terminal domain resulted in delayed cell surface expression. All the altered HAs were biologically active in hemadsorption and fusion assays. To investigate whether the wild-type HA and HAs with altered cytoplasmic tails could complement the influenza virus temperature-sensitive transport-defective HA mutant A/WSN/33 ts61S, the HA cDNAs were expressed by using a transient expression system and released virus was assayed by plaque analysis. The wild-type HA expression resulted in a release of approximately 10(3) PFU of virus per ml. Antibody neutralization of complemented virus indicated that the infectivity was due to incorporation of wild-type H3 HA into ts61S virions. Sucrose density gradient analysis of released virions showed that each of the HA cytoplasmic domain mutants was incorporated into virus particles. Virions containing HAs with substitution of the cysteine residues in the cytoplasmic domain were found to be infectious. However, no infectivity could be detected from virions containing HAs that had deletions in their cytoplasmic domains. Possible roles of the HA cytoplasmic domain in forming protein-protein interactions in virions and their involvement in the initiation of the infection process in cells are discussed.