Characterization of the Interactions between the Nucleoprotein and the Phosphoprotein of Henipavirus

The Henipavirus genome is encapsidated by the nucleoprotein (N) within a helical nucleocapsid that recruits the polymerase complex via the phosphoprotein (P). In a previous study, we reported that in henipaviruses, the N-terminal domain of the phosphoprotein and the C-terminal domain of the nucleoprotein (N-TAIL) are both intrinsically disordered. Here we show that Henipavirus N-TAIL domains are also disordered in the context of full-length nucleoproteins. We also report the cloning, purification, and characterization of the C-terminal X domains (P-XD) of Henipavirus phosphoproteins. Using isothermal titration calorimetry, we show that N-TAIL and P-XD form a 1:1 stoichiometric complex that is stable under NaCl concentrations as high as 1 M and has a K-D in the mu M range. Using far-UV circular dichroism and nuclear magnetic resonance, we show that P-XD triggers an increase in the alpha-helical content of N-TAIL. Using fluorescence spectroscopy, we show that P-XD has no impact on the chemical environment of a Trp residue introduced at position 527 of the Henipavirus N-TAIL domain, thus arguing for the lack of stable contacts between the C termini of N-TAIL and P-XD. Finally, we present a tentative structural model of the interaction in which a short, order-prone region of N-TAIL (alpha-MoRE; amino acids 473 493) adopts an alpha-helical conformation and is embedded between helices alpha 2 and alpha 3 of P-XD leading to a relatively small interface dominated by hydrophobic contacts. The present results provide the first detailed experimental characterization of the N-P interaction in henipaviruses and designate the N-TAIL-P-XD interaction as a valuable target for rational antiviral approaches.