Abelson murine leukaemia virus transformation involves loss of epidermal growth factor-binding sites

Abstract

Malignant transformation by mammalian RNA sarcoma viruses has previously been shown to involve a reduction in receptor sites for a well characterized 6,000-molecular weight (MW) growth-promoting substance, designated epidermal growth factor (EGF)^1–3. Although Abelson murine leukaemia virus (AbLV) resembles sarcoma viruses in its ability to transform embryo fibroblasts in cell culture⁴, AbLV induces a rapid B-cell lymphoid leukaemia rather than fibrosarcomas in vivo^5,6. The major translational product of AbLV is a highly phosphorylated polyprotein^7,8 of MW 120,000 which exhibits an associated tyrosine-specific protein kinase activity^9,10 and probable transforming function^11–14. We show here that AbLV transformation resembles transformation by RNA sarcoma viruses with respect to the abolition of EGF-binding sites. EGF binding is restored to control levels following loss of polyprotein expression in morphological revertants of AbLV-transformed clones and remains uninfluenced in cell lines infected with transformation-defective (td) AbLV mutants encoding polyproteins deficient in protein kinase activity. These findings indicate that AbLV transformation involves a polyprotein-associated, tyrosine-specific protein kinase activity which mediates its effect through a mechanism resulting directly or indirectly in the abolition of EGF-binding sites.