Parainfluenza virus 5 (PIV5) is a member of the Paramyxoviridae family of 
membrane-enveloped viruses with a negative-sense RNA genome that is packaged and 
protected by long filamentous nucleocapsid-helix structures (RNPs). These RNPs, 
consisting of ∼2,600 protomers of nucleocapsid (N) protein, form the template 
for viral transcription and replication. We have determined the 3D X-ray crystal 
structure of the nucleoprotein (N)-RNA complex from PIV5 to 3.11-Å resolution. 
The structure reveals a 13-mer nucleocapsid ring whose diameter, cavity, and 
pitch/height dimensions agree with EM data from early studies on the 
Paramyxovirinae subfamily of native RNPs, indicating that it closely represents 
one-turn in the building block of the RNP helices. The PIV5-N nucleocapsid ring 
encapsidates a nuclease resistant 78-nt RNA strand in its positively charged 
groove formed between the N-terminal (NTD) and C-terminal (CTD) domains of its 
successive N protomers. Six nucleotides precisely are associated with each N 
protomer, with alternating three-base-in three-base-out conformation. The 
binding of six nucleotides per protomer is consistent with the "rule of six" 
that governs the genome packaging of the Paramyxovirinae subfamily of viruses. 
PIV5-N protomer subdomains are very similar in structure to the previously 
solved Nipah-N structure, but with a difference in the angle between NTD/CTD at 
the RNA hinge region. Based on the Nipah-N structure we modeled a PIV5-N open 
conformation in which the CTD rotates away from the RNA strand into the inner 
spacious nucleocapsid-ring cavity. This rotation would expose the RNA for the 
viral polymerase activity without major disruption of the nucleocapsid 
structure.