Ubiquitin-Mediated Binding and Stabilization of β-Catenin by p210 BCR/ABL.

Abstract 3278

Poster Board III-1

Chronic myelogenous leukemia (CML) is a malignant hematopoietic stem cell disorder that is invariably associated with a balanced reciprocal translocation between chromosomes 22 and 9. The major product of the rearrangement is a 210 kD in-frame fusion protein (p210 BCR/ABL) that contains amino-terminal sequences from BCR, and carboxy-terminal sequences from the non-receptor tyrosine kinase ABL. This fusion event leads to the constitutive activation of the ABL-encoded kinase activity, which is the principal driving force behind CML. Thus, inhibitors that target the kinase activity produce durable clinical remissions. Although the ABL-encoded kinase activity is essential for disease progression, several studies have shown that BCR encoded sequences are also necessary for BCR/ABL-mediated leukemogenesis. In this current study we have identified ubiquitin as a binding partner for BCR, and demonstrated that binding is retained in p210 BCR/ABL. We have localized the docking site for ubiquitin to residues 180-191 which is present in all fusion variants of BCR/ABL (p190, p210 and p230). This putative ubiquitin binding domain (UBD) does not conform to any known consensus sequence, and is unique to BCR. Deletion of the UBD does not impair the auto-or trans-kinase activity of p210 BCR/ABL, nor does it impair the interaction between p210 BCR/ABL and GRB2, or the ability of p210 BCR/ABL to activate ERK1/2. A mutation at residue tyr-177 of p210 BCR/ABL also does not impair the interaction with ubiquitin suggesting that the GRB2 and ubiquitin binding sites are adjacent, but separable. β-catenin has been identified as a binding partner for BCR and BCR/ABL, and the docking site has been mapped to a region of BCR/ABL that contains the UBD. Over-expression of p210 BCR/ABL, but not the ubiquitin binding mutant, leads to an accumulation of β-catenin that is phosphorylated on the serine residues that normally trigger ubiquitin-mediated turnover. This difference cannot be attributed to a difference in the activation status of GSK-3β or Pin-1. These results suggest that either β-catenin and ubiquitin share a docking site on p210 BCR/ABL, or that the interaction between β-catenin and p210 BCR/ABL is ubiquitination dependent. The latter possibility is consistent with a previous study which showed that glycine residue in β-catenin is required for Bcr interaction. We propose a model in which p210 BCR/ABL may influence the Wnt signaling pathway by binding ubiquitinated β-catenin, and stabilizing it against degradation.