HRX LEUKEMIC FUSION PROTEINS FORM A HETEROCOMPLEX WITH THE LEUKEMIA-ASSOCIATED PROTEIN SET AND PROTEIN PHOSPHATASE 2A

One of the most common chromosomal abnormalities in acute leukemia is a reciprocal translocation involving the HRX gene at chromosome locus 11q23, resulting in HRX fusion proteins. Using the yeast two-hybrid sy stem, in vitro binding studies, and human cell culture coimmunoprecipi tation experiments, we show here that a region of the HRX protein that is consistently retained in HRX leukemic fusion proteins interacts di rectly with SET, another protein implicated in leukemia. We have ident ified the binding sites on HRX for SET and show that these sequences a re clustered near the A.T hooks that have been shown to bind DNA. We a lso show that carboxyl-terminal SET sequences, possibly the acidic tai l of SET, bind to HRX. We have also found serine/threonine-specific pr otein phosphatase activity in anti-HRX coimmunoprecipitates. Using the phosphatase inhibitor okadaic acid and Western blotting, the phosphat ase was identified as protein phosphatase 2A (PP2A). Mutation of a sin gle amino acid in one of the SET binding sites of HRX resulted in lowe r amounts of both coimmunoprecipitated SET protein and coimmunoprecipi tated PP2A. These results suggest that the leukemogenic effects of HRX fusion proteins may be related to interactions with SET and PP2A.