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Members of all species in the genus Orthorubulavirus have hemagglutination and neuraminidase activities. They share greater sequence relatedness within the genus than with members of other genera. Some members (parainfluenza virus 5 [PIV-5] and mumps virus [MuV]) contain an extra transcriptional element (SH) between the fusion (F) and receptor binding protein (RBP, also designated haemagglutinin-neuraminidase protein, HN) loci (Figure 2.Paramyxoviridae). The unedited and edited versions of the mRNA from the P locus encode the zinc-binding cysteine-rich protein (V) and phosphoproteins (P), respectively. The intergenic sequences are of variable length. All members lack a non-structural (C) protein ORF. The P protein of orthorubulaviruses is substantially smaller than that of the respiroviruses or morbilliviruses. MuV and human parainfluenza virus 2 (HPIV-2) are significant human pathogens.
See discussion under family description.
Species demarcation is now entirely based on the distance in the phylogenic tree based on the comparison of complete large (L) protein amino acid sequences. Since the primary criterion is the tree topology, whether a virus belongs to the same species becomes a matter of branch length between the nearest node and the tip of the branch. This length is defined as 0.03 in the trees generated as described in the legend to Figure 3.Paramyxoviridae. This has resulted in the proposed, but not yet effected, removal of one hitherto recognized species Bat mumps rubulavirus, and the merger of human parainfluenza virus 4A and human parainfluenza virus 4B into a single species Human parainfluenza virus 4 as their branch lengths in the tree (Figure 3.Paramyxoviridae) did not fulfill the demarcation criterion.
In addition, human parainfluenza virus 2 (HPIV-2) and human parainfluenza virus 4 (HPIV-4) represent distinct serotypes that lack significant cross-neutralization and cross-protection. HPIV-2, parainfluenza virus 5 (PIV-5), and simian virus 41 (SV-41) exhibit considerable sequence relatedness and some antigenic relatedness, but these viruses can be distinguished on either basis. For example, the nucleoprotein (N) of HPIV-2 is 57% or 74% identical to the N proteins of PIV-5 or SV-41, respectively). Theses viruses are also distinguished by host range: HPIV-2, HPIV-4 infect humans, PIV-5 infect dogs, monkeys and humans, and SV41 infects monkeys. They also have differences in their gene maps: PIV-5 and MuV have an additional gene, SH, and SV-41 lacks a functional transcription termination signal for the M gene and thus does not express a monocistronic M mRNA. HPIV-4 contains two antigenic subgroups (a and b) that are distinguished by differences in reactivity with monoclonal antibodies but are highly related by sequence – 84% and 95% identity for the receptor binding protein (RBP) and fusion (F) protein, respectively – and are not considered to belong to distinct species. MuV also does not exhibit significant cross-neutralization and cross-protection with other paramyxoviruses, and it is distinguished by its gene map (it contains an SH gene, found within this group only in PIV-5), by sequence divergence (the MuV N protein shares 44% or less aa sequence identity with other rubulaviruses), and by its association with disease.
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