Bcr‐Abl oncoproteins bind directly to activators of the Ras signalling pathway.

The cytosolic 185 and 210 kDa Bcr‐Abl protein tyrosine kinases play important roles in the development of Philadelphia chromosome positive (Ph+) chronic myelogenous leukemia (CML) and acute lymphoblastic leukemia (Ph+ ALL). p185 and p210 Bcr‐Abl contain identical abl‐encoded sequences juxtaposed to a variable number of bcr‐derived amino acids. As the mitogenic and transforming activities of tyrosine kinases involve stimulation of the Ras pathway, we analyzed Bcr‐Abl oncoproteins for interactions with cytoplasmic proteins that mediate Ras activation. Such polypeptides include Grb2, which comprises a single Src homology 2 (SH2) domain flanked by two SH3 domains, and the 66, 52 and 46 kDa Shc proteins which possess an SH2 domain in their carboxy‐terminus. Grb2 associates with tyrosine phosphorylated proteins through its SH2 domain, and with the Ras guanine nucleotide releasing protein mSos1 through its SH3 domains. mSos1 stimulates conversion of the inactive GDP‐bound form of Ras to the active GTP‐bound state. In bcr‐abl‐transformed cells, Grb2 and mSos1 formed a physical complex with Bcr‐Abl. In vitro, the Grb2 SH2 domain bound Bcr‐Abl through recognition of a tyrosine phosphorylation site within the amino‐terminal bcr‐encoded sequence (p.Tyr177‐Val‐Asn‐Val), that is common to both Bcr‐Abl proteins. These results suggest that autophosphorylation within the Bcr element of Bcr‐Abl creates a direct physical link to Grb2‐mSos1, and potentially to the Ras pathway, and thereby modifies the target specificity of the Abl tyrosine kinase.(ABSTRACT TRUNCATED AT 250 WORDS)