Initiation of encapsidation as evidenced by deoxycholate-treated Nucleocapsid protein in the Chandipura virus life cycle.

Encapsidation of nascent genome RNA into an RNase-resistant form by nucleocapsid protein, N is a necessary step in the rhabdoviral life cycle. However, the precise mechanism for viral RNA specific yet processive encapsidation remains elusive. Using Chandipura virus as a model system, we examined RNA binding specificity of N protein and dissected the biochemical steps involved in the rhabdoviral encapsidation process. Our analysis suggested that N protein in its monomeric form specifically binds to the first half of the leader RNA in a 1:1 complex, whereas, oligomerization imparts a broad RNA binding specificity. We also observed that viral P protein and dissociating detergent deoxycholate, both were able to maintain N in a monomeric form and thus promote specific RNA recognition. Finally, use of a minigenome length RNA in an in vitro encapsidation assay revealed the monomeric N and not its oligomeric counterpart, to be the true encapsidating unit. Based on our observations, we propose a model to explain encapsidation that involves two discrete biochemically separable steps, initiation and elongation.