INHIBITION OF BCR SERINE KINASE BY TYROSINE PHOSPHORYLATION

The first exon of the BCR gene encodes a novel serine/threonine protei n kinase, Abnormal fusion of the BCR and ABL genes, resulting from the formation of the Philadelphia chromosome (Ph), is the hallmark of Ph- positive leukemia, We have previously demonstrated that the Bcr protei n is tyrosine phosphorylated within first-exon sequences by the Bcr-Ab l oncoprotein, Here we report that in addition to tyrosine 177 (Y-177) , Y-360 and Y-283 are phosphorylated in Bcr and Bcr-Abl proteins in vi tro, Moreover, Bcr tyrosine 360 is phosphorylated in vivo within both Bcr-Abl and Bcr. Bcr mutant Y360F had a greatly reduced ability to tra nsphosphorylate casein and histone H1, whereas Bcr mutants Y177F and Y 283F had wild-type activities, In contrast, the Y360F mutation had lit tle effect on Bcr's autophosphorylation activity, Tyrosine phosphoryla ted Bcr, phosphorylated in vitro by Bcr-Abl, was greatly inhibited in its serine/threonine kinase activity, impairing both auto- and transki nase activities of Bcr. Similarly, the isolation of Bcr from cells exp ressing Bcr-Abl under conditions that preserve phosphotyrosine residue s also reduced Bcr's kinase activity, These results indicate that tyro sine 360 of Bcr is critical for the transphosphorylation activity of B cr and that in Ph-positive leukemia, Bcr serine/threonine kinase activ ity is seriously impaired.