Identification of the P proteins and other disulfide-linked and phosphorylated proteins of Newcastle disease virus.

A unique abundant protein, designated P by analogy to the putative polymerase proteins of other paramyxoviruses, was identified in purified Newcastle disease virus. Under nonreducing conditions the P proteins could be separated from other viral proteins on sodium dodecyl sulfate-polyacrylamide gels. The P proteins were isolated from detergent-solubilized virions as 53,000- to 55,000-dalton monomers and disulfide-linked trimers. Distinct forms of P having four different isoelectric points and two different electrophoretic mobilities were resolved by two-dimensional electrophoresis. Two forms of P were phosphorylated, as were the nucleocapsid protein and non-glycosylated membrane protein. In addition to disulfide-linked forms of P, dimers of the hemagglutinin-neuraminidase glycoprotein and two disulfide-linked versions of the fusion glycoprotein were identified. Several electrophoretic variants of the nucleocapsid protein that were probably created by intrachain disulfide bonding were also isolated from virions under nonreducing conditions. The locations of the newly identified proteins were determined by detergent-salt fractionation of virions and by surface-selective radioiodination of the viral envelope. The P proteins were associated with nucleocapsids and were not detected at the surface of virions. Both forms of the fusion glycoproteins were on the exterior of the viral envelope. Herein the properties of the P proteins are compared with similar proteins of rhabdoviruses and other paramyxoviruses, and a role for multiple forms of proteins in the genetic economy of newcastle disease virus is discussed.