Aberrant membrane insertion of a cytoplasmic tail deletion mutant of the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus

Morbillivirus and henipavirus attachment protein cytoplasmic domains differently affect protein expression, fusion support and particle assembly

The amino-terminal cytoplasmic domains of paramyxovirus attachment glycoproteins include trafficking signals that influence protein processing and cell surface expression. To characterize the role of the cytoplasmic domain in protein expression, fusion support and particle assembly in more detail, we constructed chimeric Nipah virus (NiV) glycoprotein (G) and canine distemper virus (CDV) haemagglutinin (H) proteins carrying the respective heterologous cytoplasmic domain, as well as a series of mutants with progressive deletions in this domain. CDV H retained fusion function and was normally expressed on the cell surface with a heterologous cytoplasmic domain, while the expression and fusion support of NiV G was dramatically decreased when its cytoplasmic domain was replaced with that of CDV H. The cell surface expression and fusion support functions of CDV H were relatively insensitive to cytoplasmic domain deletions, while short deletions in the corresponding region of NiV G dramatically decreased both. In addition, the first 10 residues of the CDV H cytoplasmic domain strongly influence its incorporation into virus-like particles formed by the CDV matrix (M) protein, while the co-expression of NiV M with NiV G had no significant effect on incorporation of G into particles. The cytoplasmic domains of both the CDV H and NiV G proteins thus contribute differently to the virus life cycle.

Role of the cytoplasmic tail amino acid sequences of Newcastle disease virus hemagglutinin-neuraminidase protein in virion incorporation, cell fusion, and pathogenicity

To determine the role of amino acid sequences of the hemagglutinin-neuraminidase (HN) cytoplasmic tail in Newcastle disease virus (NDV) replication and pathogenicity, we generated recombinant NDVs with a deletion or point mutation in the N-terminal cytoplasmic tail. The first 2-amino-acid deletion in the cytoplasmic tail did not affect the biological characteristics of NDV. However, a 4-amino-acid deletion and the substitution of alanine for serine at position 6 affected cell fusion, pathogenicity, and colocalization of the HN and M proteins of NDV, indicating that these residues of the HN cytoplasmic tail are critical for its specific incorporation into virions.

Importance of the cytoplasmic tails of the measles virus glycoproteins for fusogenic activity and the generation of recombinant measles viruses

The generation of replication-competent measles virus (MV) depends on the incorporation of biologically active, fusogenic glycoprotein complexes, which are required for attachment and penetration into susceptible host cells and for direct virus spread by cell-to-cell fusion. Whereas multiple studies have analyzed the importance of the ectodomains of the MV glycoproteins hemagglutinin (H) and fusion protein (F), we have investigated the role of the cytoplasmic tails of the F and H proteins for the formation of fusogenic complexes. Deletions in the cytoplasmic tails of transiently expressed MV glycoproteins were found to have varying effects on receptor binding, fusion, or fusion promotion activity. F tail truncation to only three amino acids did not affect fusion capacity. In contrast, truncation of the H cytoplasmic tail was limited. H protein mutants with cytoplasmic tails of <14 residues no longer supported F-mediated cell fusion, predominantly due to a decrease in surface expression and receptor binding. This indicates that a minimal length of the H protein tail of 14 amino acids is required to ensure a threshold local density to have sufficient accumulation of fusogenic H-F complexes. By using reverse genetics, a recombinant MV with an F tail of three amino acids (rMV-FcDelta30), as well as an MV with an H tail of 14 residues (rMV-HcDelta20), could be rescued, whereas generation of viruses with shorter H tails failed. Thus, glycoprotein truncation does not interfere with the successful generation of recombinant MV if fusion competence is maintained.