Precursor protein for Newcastle disease virus.

The course of viral protein synthesis during infection of chicken embryo fibroblasts with Newcastle disease virus (NDV) L. Kansas has been followed by using sodium dodecyl sulfate polyacrylamide gel electrophoresis. Of the three major virion polypeptide molecular weight classes, I (78,400 daltons), II (53,500 daltons), and III (37,600 daltons), only II, having the same electrophoretic mobility as nucleocapsid polypeptide, appears to be the cleavage product of a precursor polypeptide PII (64,800 daltons) detected in NDV-infected cells after brief labeling with radioactive amino acids. Nucleocapsids were isolated from NDV-infected cells which had been pulse-labeled with radioactive amino acids or pulse-labeled and further incubated with unlabeled amino acids. Gel electrophoretic analysis of proteins derived from nucleocapsids showed that an increase in the period of incubation with unlabeled amino acids resulted in an increase in the amount of radioactivity in nucleocapsid protein. Polypeptide PII was not detected as a transient component of the isolated nucleocapsid fraction. These results are consistent with two interpretations. The product of PII cleavage is (i) nucleocapsid polypeptide, or (ii) a nonvirion or minor envelope polypeptide having the same electrophoretic mobility as nucleocapsid polypeptide.