Involvement of nucleophosmin (NPM1/B23) in assembly of infectious HPV16 capsids

We report that during assembly of HPV16 pseudovirus (PsV) the minor capsid protein, L2, interacts with the host nucleolar protein nucleophosmin (NPM1 /B23). Exogenously-expressed L2 colocalized with NPMI, a complex containing both proteins, could be immunoprecipitated, and L2 could redirect to the nucleus NPMI that was pharmacologically or genetically restricted to the cytoplasm. Coexpression of the major capsid protein, Ll, prevented both the colocalization and the biochemical association, and Ll pentamers could displace L2 from L2/NPM1 complexes attached to a nuclear matrix. HPV16 PsV that was produced in a cell line with reduced NPMI levels had significantly lower infectivity compared to PsV produced in the parental cell line, although the PsV preparations had comparable Ll and L2 ratios and levels of encapsidated DNA. The PsV produced in NPM1-deficient cells showed increased trypsin sensitivity and exhibited decreased L2 levels during endocytosis. These results suggest a critical role for NPM1 in establishing the correct interactions between L2 and Ll during HPV capsid assembly. A decrease in cellular levels of NPM1 results in the formation of seemingly normal, but unstable, capsids that result in a premature loss of L2, thus inhibiting successful infection. No role for NPMI in HPV infectious entry was found. Published by Elsevier B.V.