Apoptin is modified by SUMO conjugation and targeted to promyelocytic leukemia protein nuclear bodies

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Abstract
Apoptin, a protein of the chicken anemia virus (CAV), represents a novel potential anticancer therapeutic, because it induces apoptotic death specifically in tumor but not normal cells. The cellular localization appears to be crucial for apoptin's selective toxicity. In normal cells apoptin remains in the cytoplasm, whereas in transformed cells it migrates into the nucleus and kills the cell. However, the manner by which apoptin is able to distinguish between tumor and normal cells is unknown. Here, we report for the first time that apoptin interacts directly with the promyelocytic leukemia protein (PML) in tumor cells and accumulates in PML nuclear bodies (NBs), which are involved in apoptosis induction and viral replication. We also demonstrate that apoptin is sumoylated and that a sumoylation-deficient apoptin mutant is no longer recruited to PML-NBs, but localizes in the nuclear matrix. This mutant fails to bind PML, but can still induce apoptosis as efficiently as wild-type apoptin. Moreover, apoptin kills also PML−/− cells and promyelocytic leukemia cells with defective PML expression. Our results therefore suggest that apoptin kills tumor cells independently of PML and sumoylation, however, the interaction of apoptin with PML and small ubiquitin-like modifier (SUMO) proteins might be relevant for CAV replication.