FC0067
Nucleoprotein Ntail (N)  -  Phosphoprotein (P)

Biological function
Nipah virus (NiV), the second known member of the genus Henipavirus, came to light as the etiologic agent of an outbreak of respiratory and central nervous system disease in pigs and humans.

Domain organization/sequence features
As in all Mononegavirales members, the negative-strand, non-segmented RNA genome of Henipavirus is encapsidated by the nucleoprotein (N) within a helical nucleocapsid that has the characteristic herringbone-like structure typically observed in other Paramyxoviridae members. This helical nucleocapsid, rather than naked RNA, is the substrate used by the polymerase complex during both transcription and replication. Minigenome replicon studies showed that in henipaviruses the nucleoprotein, the phosphoprotein (P) and the large protein (L) proteins are necessary and sufficient to sustain replication of viral RNA. By analogy with other Paramyxoviridae members, the polymerase complex is assumed to consist of the L protein and the P protein, with this latter serving as a tether for the recruitment of L onto the nucleocapsid template.

Structural evidence
NiV N_TAIL domains are disordered in the context of full-length nucleoproteins. P_XD triggers an increase in the α-helical content of N_TAIL. Using fluorescence spectroscopy, it was shown that P_XD has no impact on the chemical environment of a Trp residue introduced at position 527 of the NiV N_TAIL domain, thus arguing for the lack of stable contacts between the C termini of N_TAIL and P_XD. For both titrations, most resonances in the HSQC experienced no chemical shift changes, and only a few resonances underwent fast to intermediate exchange. P_XD binds to N_TAIL in close proximity of the α-MoRE predicted to encompass residues 473– 493 and is mediated by mostly hydrophobic contacts. Binding to P_XD triggers a gain of α-helicity for at least eight residues. Despite the α-helical transition in NiV N_TAIL undergo upon binding to P_XD, the experimentally determined RS of the NiV N_TAIL-P_XD complex suggests that binding to P_XD does not imply formation of a compact complex, with this latter rather retaining a considerable flexibility. In further support of this observation, the many observable and relatively sharp NMR resonances in NiV N_TAIL-P_XD complexes, displaying chemical shifts that are nearly unaltered, provide evidence that N_TAIL remains significantly disordered even in the bound form. Therefore the final complex is likely endowed with flexible appendages in a structural arrangement possibly reminiscent of that observed in the case of the MeV complex.

Structure/Mechanism
N_TAIL-P_XD interaction relies mainly on hydrophobic interactions.

Mechanism category
competitive binding

Significance
Considering that the contact between P_XD and N_TAIL within the replicative complex has to be dynamically made/broken to allow the polymerase to progress along the nucleocapsid template during both transcription and replication. A fuzzy N_TAIL-P_XD complex can deliver N monomers to the nascent RNA chain and enables this transition to occur efficiently at a high rate.

Further reading
24086133

Submitted by
Sonia Longhi   sonia.longhi@afmb.univ-mrs.fr