1764 Identification of Tryptic Peptides

Several type C RNA-transforming viruses have been shown to contain acquired cellular sequences within preexisting viral genes (I-3). In many instances expression of proteins encoded by such sequences occurs by inphase read through of translation initiated at the 5' terminus of the viral genome. Resulting polyprotein translational products contain type C viral gag gene amino terminal structural components covalently linked to acquired sequence-encoded components with probable transforming function (4-10). The nonstructural components of polyproteins encoded by several such virus isolates have been shown to resemble the well-characterized avian sarcoma virus-encoded transforming protein (11-13), pp60 src, in that they are highly phosphorylated (14) and exhibit associated protein kinase activity (14, 15) with specificity for tyrosine acceptor sites (15-18). Transformation by viruses of this group is associated with elevated levels of total cellular phosphotyrosine (17, 18) and a reduction of epidermal growth factor (EGF) a binding capacity (16, 19). Staa[ et al. (20) have described the isolation of a weakly-transforming RNA type C virus from a thymoma suspension culture developed from a spontaneously lymphomatosis A K R / J mouse. This virus, designated T-8, is replication-defective (20), transforms mink cells in vitro (20), and encodes, as its major translational product, a highly phosphorylated 110,000 mol wt polyprotein containing AKR-mur ine leukemia virus (MuLV) amino terminal gag gene components, p 15 and p 12 (21). The functional significance and role, if any, of this protein in transformation is, as yet, unknown. In the present study, we have analyzed T-8 P 110 for possible acquired sequence encoded component(s) and have examined levels of tyrosine-specific protein kinase activity and epidermal growth factor EGF binding by mink cell lines nonproductively infected by T-8.