The 36K polypeptide synthesized in Newcastle disease virus-infected cells possesses properties predicted for the hypothesized 'V' protein

Newcastle disease virus (NDV) virions possess two proteins which react strongly with monoclonal antibody 688 following separation by high resolution two-dimensional (isoelectric focusing/SDS) PAGE and detection by Western blotting. One is the phosphorylated nucleocapsid-associated 53K [P (NAP)] protein, the other comigrates with the 36K protein detected by radiolabelling NDV-infected chick embryo fibroblasts. [³⁵S]Cysteine/[³H]leucine dual-labelling experiments show that the 36K protein is very rich in cysteine compared to the P (NAP) protein. In the Beaudette C strain it comigrates on one-dimensional SDS-polyacrylamide gels with the matrix protein (M); however, it is resolved from the slower migrating M protein from the Ulster strain of NDV. The size, strain-specific isoelectric point, high cysteine content and antigenic relatedness to the P (NAP) protein suggest that the 36K protein is the V protein of NDV, the counterpart of which is found in other Paramyxoviridae.