Separate domains of Sendai virus P protein are required for binding to viral nucleocapsids

Characterization of the Sendai virus V protein with an anti-peptide antiserum

The Sendai virus V protein, which is a fusion of the P and V ORFs of the P gene, was characterized with antisera to a portion of the V ORF and compared to the P protein. The only property found in common with P is that V is also highly phosphorylated, and this is so even when these proteins are expressed independently of the other viral proteins. Otherwise, V was not found in virions, was not strongly associated with viral nucleocapsids like P, and anti-V had no effect on viral RNA synthesis in vitro under conditions where anti-P was highly inhibitory. The available evidence suggests that V may play a role in RNA synthesis, but it is not an essential one like that of the P protein.

Rescue of a Sendai virus DI genome by other parainfluenza viruses: implications for genome replication

Using a defective interfering Sendai virus stock (DIH4) freed of nondefective helper virus, we found that the closely related parainfluenza viruses 1 and 3 could substitute for the Sendai virus helper in replicating DIH4, creating chimeric nucleocapsids. The morbillivirus measles and the rhabdovirus VSV could not substitute. When DIH4 is incubated intracellularly for 5 days in the absence of help, the ability of PIV3 to rescue DIH4 at this time depended on fresh Sendai virus polymerase. The PIV3 polymerase apparently can only copy the chimeric template, but not that wrapped in the homologous Sendai NP protein. These results suggest that the cis-acting RNA sequences important for genome replication, e.g., the promoter and the encapsidation site, have been conserved among these viruses, but that the interactions between the polymerase and the template protein NP are unique for each virus.

The P gene of bovine parainfluenza virus 3 expresses all three reading frames from a single mRNA editing site

The P gene of bovine parainfluenza virus 3 (bPIV3) contains two downstream overlapping ORFs, called V and D. By comparison with the mRNA editing sites of other paramyxoviruses, two editing sites were predicted for bPIV3; site a to express the D protein, and site b to express the V protein. Examination of the bPIV3 mRNAs, however, indicates that site b is non-functional whereas site a operates frequently. Insertions at site a give rise to both V and D protein mRNAs, because a very broad distribution of Gs is added when insertions occur. This broad distribution is very different from the editing sites of Sendai virus or SV5, where predominantly one form of edited mRNA containing either a one or two G insertion respectively is created, to access the single overlapping ORF of these viruses. A model is proposed to explain how paramyxoviruses control the range of G insertions on that fraction of the mRNAs where insertions occur. The bPIV3 P gene is unique as far as we know, in that a sizeable portion of the gene expresses all 3 reading frames as protein. bPIV3 apparently does this from a single editing site by removing the constraints which control the number of slippage rounds which take place.

Sendai virus protein-protein interactions studied by a protein-blotting protein-overlay technique: mapping of domains on NP protein required for binding to P protein

Proteins from Sendai virus particles and from infected cells were analyzed in a protein-blotting protein-overlay assay for their interaction with in vitro-synthesized, [35S]methionine-labeled viral proteins NP, P, and M. After separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, transfer onto polyvinylidene difluoride membranes, and renaturation, the immobilized proteins were found to interact specifically with radiolabeled proteins. NP proteins from virus particles and from infected cells retained 35S-P protein equally well. Conversely, P protein from virus particles and from infected cells retained 35S-NP protein. 35S-M protein was retained mainly by NP protein but also by several cellular proteins. To determine the domains on NP protein required for binding to immobilized P protein, a series of truncated and internally deleted 35S-NP proteins was constructed. The only deletion that did not affect binding resides between residues 426 and 497. The carboxyl-terminal 27 residues (positions 498 to 524) contribute significantly to the binding affinity. Removal of 20 residues (positions 225 to 244) in the hydrophobic middle part of NP protein completely abolished its binding to P protein.

Two noncontiguous regions of Sendai virus P protein combine to form a single nucleocapsid binding domain

Binding of Sendai virus P protein to viral nucleocapsids requires amino acids in two separate regions of P protein. Both required regions are near the carboxyl terminus, and they are separated by a region which is expendable for binding (K. W. Ryan and A. Portner, 1990, Virology 174, 515-521). To examine the topography of these regions in the folded P protein molecule we mapped the epitopes present in several undenatured P proteins with overlaping deletions near their carboxyl termini. The epitopes recognized by two monoclonal antibodies were each composed of both protein regions necessary for binding, indicating that these two regions are each required at some point during the folding of P protein. To determine if these protein regions interact directly in forming the nucleocapsid binding domain, we constructed a deleted P gene which encodes a protein comprising only these two regions with all other P protein sequences deleted. This protein was able to bind to nucleocapsids, demonstrating that these two regions alone are sufficient to form the nucleocapsid-binding domain. In addition, this protein formed the folded epitopes comprising the two nucleocapsid-binding regions, indicating that the two regions interact directly with each other to form a single folded structure. The involvement of this binding domain in viral mRNA synthesis was examined by testing the ability of each monoclonal antibody to inhibit the in vitro transcription activity of full-size P protein. Several antibodies to epitopes near the binding domain were found to be potent inhibitors of viral transcription, showing that these regions contribute to P protein's role in mRNA synthesis.