Transcriptive complex of Newcastle disease virus. II. Structural and functional assembly associated with the cytoskeletal framework.

The transcriptive complex, the nucleocapsid with the viral RNA-synthesizing activity, of Newcastle disease virus (NDV) contains three protein components, the major structural subunit (NP) and two associated proteins (P and L) involved in the RNA synthesis. We studied the pathway of these proteins from synthesis to assembly into the complex by pulse-chase labeling of infected cells followed by detergent extraction of the cells to separate soluble and cytoskeletal fractions. Most molecules of NP and P (and probably L) became associated with the cytoskeletal framework immediately after their synthesis. Most of the remaining molecules were initially found in the soluble fraction, but joined the cytoskeletal framework within several minutes. Once attached, none of the proteins left the cytoskeleton, and it was here that they assembled with 50 S viral RNA into nucleocapsids. The nucleocapsids thus formed remained bound to the cytoskeletal framework and were found by in situ autoradiography to exhibit viral RNA synthesis on the framework. These results suggested that the cytoskeletal framework could actively participate in the structural and functional assembly of NDV transcriptive complex.