Biochemical evidence that Semliki Forest virus obtains its envelope from the plasma membrane of the host cell

The data from chemical studies and electron microscopy suggest that Semliki Forest virus obtains its envelope by budding into the medium from the plasma membrane of the host cell. Biochemical evidence for this phenomenon, however, has not been published. Therefore, we undertook a series of pulse-chase studies so that we might quantitatively evaluate the importance of the budding mechanism in the morphogenesis of Semliki Forest virus. Baby hamster kidney cells (clone 13) were grown in culture and infected with Semliki Forest virus. The cells were exposed to [4,5-3H] leucine for 20 min and the subsequent incorporation of the label into virus proteins associated with cytoplasmic membranes and extracellular virus was determined. Initial experiments were conducted with microsomes and a precursor-product relationship was demonstrated between viral proteins in the microsomes and in extracellular virus. Further studies were performed with endoplasmic reticulum and plasma membrane preparations. Maximal incorporation of [3H] leucine was observed in the viral proteins located in the endoplasmic reticulum at the end of a 20-min pulse period; greater than 50% of this activity had disappeared within 2 h. The plasma membrane fraction contained no radioactivity at the end of the pulse period; subsequently, maximal labeling of the viral proteins in the plasma membrane occurred 4 h into the chase period and these labeled proteins had disappeared from this membrane 11 h after the pulse. At this time maximal incorporation of the labeled proteins into extracellular virus was observed. These data are consistent with a precursor-product relationship between the viral proteins in the endoplasmic reticulum which migrate to the plasma membrane and are subsequently incorporated into extracellular virus. All the radioactivity in the extracellular virus appears to have been derived from viral proteins associated with the plasma membrane of the cell. Therefore, mechanisms for the morphogenesis of Semliki Forest virus (in baby hamster kidney cells), other than budding from the plasma membrane, are unlikely to be of quantitative importance.