The intracellular distribution and the kinetics of association of Sendai virus polypeptides with cytoplasmic fractions and plasma membranes have been studied. The viral surface glycoproteins HN and F0 have been found in pulse-chase experiments to migrate from rough to smooth membranes and to the plasma membrane. The M protein was found in varying amounts in most cell fractions but was predominantly associated with smooth membranes; there was no evidence of its migration from rough to smooth membranes. The L, P, and NP polypeptides were found with the rough membrane and free ribosome fractions. Polypeptides B and C, which were previously found in extracts of whole infected cells, were found to be unstable during cell fractionation. In the presence of protease inhibitors, polypeptide C, which is thought to be a virus-specific nonstructural protein, was found with the rough membrane fraction. Thus its instability in cell fractions appears to be due to proteolytic digestion. Polypeptide B was not found in cell fractions even in the presence of protease inhibitors. Evidence reported in the following paper has indicated that B is a phosphorylated form of polypeptide M and that its instability is presumably due to loss of phosphate. Polypeptides I-IV, which the available evidence suggests are cellular polypeptides whose synthesis may be enhanced in infected cells, were found in significant amount in soluble form after cell fractionation, although IV was also associated with most membrane fractions.
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