P140 C-ABL THAT BINDS DNA IS PREFERENTIALLY PHOSPHORYLATED AT TYROSINE RESIDUES/

EP is a DNA element found in the enhancer and promoter regions of seve ral cellular and viral genes. Previously, we have identified the DNA b inding p140/c-Abl protein that specifically recognizes this element. H ere we show that phosphorylation is essential for the p140/c-Abl DNA b inding activity and for the formation of DNA-protein complexes. Furthe rmore, by P-32 labeling of cells and protein purification, we demonstr ate that in vivo the EP-DNA-associated p140/c-Abl is a tyrosine phosph oprotein. By employing two different c-Abl antibodies, we demonstrate the existence of two distinct c-Abl populations in cellular extracts. p140/c-Abl is quantitatively the minor population, is heavily phosphor ylated at both serine and tyrosine residues, and is active in autophos phorylation reactions.