Proteins synthesized by Semliki Forest virus and its 16 temperature-sensitive mutants

The proteins synthesized in chicken embryo fibroblasts infected with wild-type Semliki Forest virus and 16 temperature-sensitive mutants derived from it were studied by polyacrylamide gel electrophoresis. In addition to the structural proteins, five nonvirion proteins (NVP) with molecular weight of 130,000, 97,000, 86,000, 78,000 and 62,000 were found varying amounts in cells infected with the different RNA+ mutants and also in the wild-type-infected cells. Pulse-chase experiments suggested that NVP 130, NVP 97, NVP 86, and NVP 62 are precursors presumably of the structural proteins. The amount of NVP 78 was not affected by the chase, and it may represent a translational product of the nonstructural part of the genome. The NVP 130 was shown to be a common precursor of the structural proteins by tryptic peptide mapping. Kinetic evidence from one of the mutants (ts-3) suggested that NVP 86 is one of the precursors of the capsid protein. A common feature of all the RNA+mutants was the inability to cleave the NVP 62 into E2 and E3, suggesting that this cleavage is a crucial reaction in the virus maturation.