A strain of Fujinami sarcoma virus which is temperature-sensitive in protein phosphorylation and cellular transformation

Cells infected by one strain of Fujinami sarcoma virus (FSV) are transformed at 38°C but are phenotypically normal at 41.5°C. FSV encodes a 140,000 molecular weight protein (P140) with gag gene-related and FSV-specific peptide sequences. At 41.5°C, P140 is weakly phosphorylated at serine residues, and is inactive in the immune complex protein kinase assay. At 38°C, P140 is highly phosphorylated, contains phosphotyrosine in addition to phosphoserine, and in the immune complex kinase assay becomes phosphorylated at three tyrosine residues. Phosphorylation of cellular polypeptides at tyrosine residues in FSV-infected cells is also temperature-sensitive. These observations indicate that P140 is the transforming protein of FSV and that protein phosphorylation at tyrosine residues is involved in transformation by this virus.