Fusogenic Variants of a Noncytopathic Paramyxovirus

Abstract

SER virus is a type 5 parainfluenza virus that does not exhibit syncytium formation, in contrast to most other paramyxoviruses. This property has been attributed, at least in part, to the presence of an extension of the cytoplasmic tail (CT) of the SER F protein, as truncations or mutations of this region resulted in enhanced fusion. In this study we used repeated passage to select for mutant SER viruses, which were found to be fusogenic. The mutant viruses replicated at levels comparable to or higher than the wild-type SER virus and caused plaque formation, in contrast to the wild-type virus which does not form plaques. The mutants differed strikingly in their plaque sizes. The F genes of mutant viruses were cloned and sequenced and shared some mutations, including a proline-to-leucine change at position 22 and an isoleucine-to-leucine substitution at position 191; other changes that were specific to each mutant were also found. The HN proteins of mutant viruses also showed mutations spanning the length of the protein whereas the M protein showed a consistent mutation, threonine to isoleucine, at position 129. The structure of the F protein was used to identify residues involved in the mutant phenotypes in terms of their location and proximity to heptad repeat domains.SER virus, an isolate of parainfluenza virus type 5, belongs to the family Paramyxoviridae and was isolated from aborted pigs. Simian virus 5 (SV5) is a prototype type 5 parainfluenza virus which has been shown to induce syncytium formation (fusion) in most cell types. SER virus is closely related to SV5 serologically and in its protein profile and its nucleotide sequence but replicates without causing cytopathology or syncytium formation (31). Sequence comparisons of SER with SV5 revealed the presence of an extended cytoplasmic tail (CT) in the SER F protein. Truncations or mutations in this domain resulted in enhanced syncytium formation and indicated that the elongated CT interferes with membrane fusion in a sequence-dependent manner (28, 31).

Residues of the Human Metapneumovirus Fusion (F) Protein Critical for Its Strain-Related Fusion Phenotype: Implications for the Virus Replication Cycle

“…The F proteins of these viruses do not require cooperation of the attachment protein for syncytium formation (25,33), because, presumably, they require lower activation energy to initiate membrane fusion. Furthermore, paramyxovirus variants carrying F proteins with destabilizing substitutions replicate faster in cell culture (34). In agreement with this observation, we have reported that NL/1/99 B1 bearing a pH-independent F protein displays faster multistep growth kinetics in Vero-118 cells than NL/ 1/00 A1 , with a low-pH-dependent F protein (6,14), although the final titers achieved by the two viruses were almost the same.…”

Review on Newcastle disease in poultry

“…This fusion takes place after activation by the attachment of the HN protein to cellular receptors. When activated, the F protein also fuses contiguous cells during the passage of virions from cells to syncytiumforming cells (Hernandez et al, 1996;Seth et al, 2007;Yamakawa et al, 2007).…”