Radioimmunoassay of measles virus nucleocapsid antigen

A double antibody competitive binding radioimmunoassay (RIA) was developed as a tool for investigating the involvement of measles virus in persistent virus infections. The assay employs 125I-labelled measles virus nucleocapsids as the labelled antigen. Nucleocapsids were purified from cytoplasmic extracts of virus-infected Vero cells, treated with trypsin to prevent clumping and iodinated to a specific activity of about I5/zCi//zg. Electrophoretic analysis of iodinated trypsin-treated nucleocapsids revealed only labelled polypeptide with a relative mol. wt. (Mr) 4oooo, indicating that trypsin had cleaved the 6oooo mol. wt. native poly-peptide to a 4oooo mol. wt. subunit. The assay could detect as little as o.I ng of nucleocapsid protein. Either native (Mr 6o0oo) or cleaved (M, 4oooo) nucleo-capsid polypeptide was detected in the assay. As much as ioo/zg of protein from uninfected Vero cells did not react in the RIA. Another measure of specificity was the fact that 4oo ng of purified nucleocapsid protein from simian virus 5, Newcastle disease virus or canine distemper vi us did not react in the assay. In the RIA, nucleocapsid antigen of virus isolated from subacute sclerosing panencephalitis (SSPE) was indistinguishable from that of Edmonston strain measles virus, indi-cating antigenic homology between the 4oooo mol. wt. nucleocapsid polypeptide of the SSPE virus and that of measles virus