Members of the subfamily Orthoparamyxovirinae have 6–8 transcriptional elements in their genomes. Within the subfamily Orthoparamyxovirinae, the sequence relatedness between corresponding proteins is greatest for nucleocapsid (N), matrix (M) and large (L) proteins. The fusion protein (F) and receptor binding protein (RBP, designated variably as HN, haemagglutinin-neuraminidase protein, H, haemagglutinin or G, glycoprotein) are less conserved and the non-structural protein (C) and phosphoprotein (P) are poorly conserved. However, the unique region of the zinc-binding cysteine-rich protein (V) that is not shared with P contains highly conserved sequence elements. The nucleocapsids of the orthoparamyxoviruses have diameters of 18 nm and a pitch of 5.5 nm; the length of the surface F and RBP spikes is 8 nm. The genome length must be a multiple of 6 nucleotides for efficient genome replication (the “rule of six”), perhaps reflecting the precise packing of 6 nucleotides by a nucleocapsid protein subunit (Calain and Roux 1995). RNA editing of the P/V transcriptional element occurs for all members except human parainfluenza virus 1 (HPIV-1 (Matsuoka et al., 1991)) and Cedar virus (Marsh et al., 2012). Members of one genus (Morbillivirus) lack neuraminidase activity and some viruses (canine distemper virus, phocine distemper virus and rinderpest virus) lack a detectable hemagglutinating activity. Henipaviruses lack both neuraminidase and hemagglutinating activities (Table 1. Paramyxoviridae).
Although cellular receptors have been identified for viruses of the genera Respirovirus, Henipavirus (except Mòjiāng virus), and Morbillivirus, receptors have not yet been identified for members of the other 5 genera: Aquaparamyxovirus, Ferlavirus, Jeilongvirus, Narmovirus and Salemvirus. The data in Table 2. Paramyxoviridae suggest that the viruses in the first three genera (Aquaparamyxovirus, Ferlavirus, Jeilongvirus) may use neuraminic acid residues as part of the receptors while narmoviruses and Salem virus may use protein receptors like the morbilli- and henipaviruses.
Genus demarcation criteria
The division of this subfamily into the eight genera is consistent with phylogenetic grouping based on L protein amino acid sequence relationships (Figure 3. .Paramyxoviridae).