CrkL is a key signaling protein that mediates the leukemogenic activity of Bcr-Abl and is thought to adopt a structure that is similar to that of its CrkII homolog. The two proteins share high sequence identity and indistinguishable ligand binding preferences, yet they have distinct physiological roles. Here we show that the structures of CrkL and phosphorylated CrkL are markedly different than the corresponding structures of CrkII. As a result, the binding activities of the Src homology 2 and

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... rc homology 3 domains in the two proteins are regulated in a distinct manner and to a different extent. The different structural architecture of CrkL and CrkII may account for their distinct functional roles. The data show that CrkL forms a constitutive complex with Abl,explaining the strong preference of Bcr-Abl for CrkL. The results also highlight how the structural organization of the modular domains in adaptor proteins can control signaling outcome. iii Cyclophilin A (CypA) is a polyprolyl isomerase that is ubiquitously expressed in all human cells. It catalyzes the cis-trans isomerization of X-P motifs. Recent studies have shown that CypA is overexpressed in many human cancers. The role of CypA in this area is not well understood. We have identified a novel binding site for CypA on CrkII, an adaptor protein. Although several binding partners of CypA are known, the underlying mechanism of the CypA action and the physiological implications of the interactions have remained in most cases unknown. It is unclear whether CypA acts as an enzyme or a binding partner in mediating the biological processes. Here we show that CypA binds specifically to CrkII and modulates its level of phosphorylation by the Abl kinase. We show that the conserved proline residue in CrkII next to the tyrosinephosphorylation site undergoes cis−trans isomeriza on. CypA is recruited to the site and prevents the tyrosine from becoming phosphorylated by Abl. The interaction between CypA and CrkII occurs specifically both in vitro and in vivo. This is a novel role for CypA which appears to act as a selective switch to modulate the level of phosphorylation of a signaling protein. iv