Selective interference with P protein binding to paramyxovirus nucleocapsids

We previously observed that some anti-nucleoprotein monoclonal antibodies were able to displace P protein from Sendai virus (SV) nucleocapsid cores. The current work extends that observation by showing that such antibody-mediated P displacement is not unique to Sendai virus nucleocapsids, but can also occur with nucleocapsids of a related human respiratory pathogen. Anti-NP antibody prevents binding of SV P protein to nucleocapsids from human parainfluenza virus type 1 (PIV1) or from SV. Antibody also prevents binding of PIV1 P protein to PIV1 nucleocapsids, but not to SV nucleocapsids. We have also examined the stoichiometry of antibody interference with P binding, to determine how large a nucleocapsid region can be protected from P binding by a single antibody molecule. We found that approximately 40 antibody molecules per nucleocapsid complex can block attachment of most P protein. This indicates that a single antibody molecule can prevent P binding to a region representing about 65 nucleoprotein monomers on the nucleocapsid core.