Protein metabolism during the steady state of Newcastle disease virus infection. I. Kinetics of amino acid and protein accumulation.

Pulse-chase experiments and studies on the effects of varying pulse lenghts on radioactive amino acid and protein accumulation have been carried out to evaluate several possible mechanisms for the inhibition in cellular protein accumulation during infection of chicken embryo cells by Newcastle disease virus. The inhibition is probably at the level of synthesis of cellular protein since no evidence for either increased degradation of protein or alterations in cellular permeability to protein was found in infected cultures. The magnitude of the reduction in the rate of cellular protein accumulation and consequently total protein accumulation depend upon the length of the radioisotopic labeling period. In contrast, the rate of viral protein accumulation is independent of the length of the labeling period. A double-label difference analysis of polyacrylamide gels was used in all of the kinetics studies to distinguish between viral and cellular protein accumulation. An unstable fraction which could be labeled with radioactive amino acids was detected in both infected and uninfected cultures. This material migrated mainly in the 50,000- to 60,000-dalton region of polyacrylamide gels, exhibited saturation kinetics during accumulation studies, and turned over rapidly during a chase. The relative amount of this fraction was not affected by infection. Gel analysis of the radioactive protein recovered from the medium from both infected and uninfected cultures revealed a major component with an apparent molecular weight of 33,000. None of the major viral polypeptides could be detected in the medium after a 30-min chase following a brief labeling period.