Apoptin Nuclear Accumulation Is Modulated by a CRM1-Recognized Nuclear Export Signal that Is Active in Normal but not in Tumor Cells

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Abstract
Tumor cell–specific activity of chicken anemia virus viral protein 3 (VP3 or apoptin) is believed to be dependent on its ability to localize in the nucleus of transformed but not of primary or nontransformed cells. The present study characterizes the signals responsible for the novel nucleocytoplasmic trafficking properties of VP3 using two isogenic tumor/nontumor cell pairs. In addition to the tumor cell–specific nuclear targeting signal, comprising two stretches of basic amino acids in the VP3 COOH terminus which are highly efficient in tumor but not in normal cells, we define the CRM1-recognized nuclear export sequence (NES) within the VP3 tumor cell–specific nuclear targeting signal for the first time. Intriguingly, the NES (amino acids 97-105) is functional in normal but not in tumor cells through the action of the threonine 108 phosphorylation site adjacent to the NES which inhibits its action. In addition, we characterize a leucine-rich sequence (amino acids 33-46) that assists VP3 nuclear accumulation by functioning as a nuclear retention sequence, conferring association with promyelocytic leukemia nuclear bodies. This unique combination of signals is the basis of the tumor cell–specific nuclear targeting abilities of VP3.