Identification o f Proteins Specific to Friend Strain o f Spleen Focus forming Virus (

Introduction. The Friend leukemia virus (FLY) induces rapid neoplastic proliferation of erythroid precursor cells in susceptible mice.1 The FLY complex contains two distinct virus components,2)-4) spleen focus forming virus ( SFFV) , which is replication-defective and responsible for transformation of its target erythroid precursor cells, and replication-competent helper virus, inducing lymphoid leukemia after a long lalent period. Nucleic acid hybridization studies indicated that genomic RNA of SFFV contained a set of SFFV-specific sequences and sequences homologous to helper MuLY.5' 6> Therefore, SFFV-specific sequences may code for protein(s), which are responsible for transformation of erythroid precursor cells as the src gene of Rous sarcoma virus codes for the specific protein, which is responsible for transformation of mesenchymal cells. 7 Previously a glycoprotein of M.W. 55,000 was observed in several Friend erythroid leukemia lines.8~ The isolation of nonproducer cells infected with SFFY6> "° "2) or of those cells producing large excess of SFFV over helper MuLY6> has made it possible to analyze the protein products coded by the SFFV genome. As such cells contained a large amount of SFFV RNA, we can expect to detect SFFV-coded proteins, if the RNA is ever translated. This paper describes briefly the identification of M.W. 55,000 and 120,000 dalton proteins as SFFV-specific proteins, the former crossreacting with anti-Rauscher murine leukemia virus (MULY) gp70 serum, and was abundant in such an excess SFFV-producing or a nonproducing SFFV-infected erythroleukemia clone. Materials and methods. T3-K-1 cells, also called as K-1, a subclone of Friend leukemia cell line, T3-C1-2,11) released a large excess of SFFV over helper MuLY.6~ The helper component was isolated free from SFFV by repeated limiting dilutions of the T3-K-1 virus in SC-1 cells. FVTCT, a nonproducer Balb/c mouse cells infected with SFFV, was isolated and donated by Fieldsteel et al. and has been