ABSTRACT
Mdm2 antagonizes the tumor suppressor p53. Targeting the Mdm2-p53 interaction represents an attractive approach for the treatment of cancers with functional p53. Investigating mechanisms underlying Mdm2-p53 regulation is therefore important. The scaffold protein ß-arrestin2 (ß-arr2) regulates tumor suppressor p53 by counteracting Mdm2. ß-arr2 nucleocytoplasmic shuttling displaces Mdm2 from the nucleus to the cytoplasm resulting in enhanced p53 signaling. ß-arr2 is constitutively exported from the nucleus, via a nuclear export signal, but mechanisms regulating its nuclear entry are not completely elucidated. ß-arr2 can be SUMOylated, but no information is available on how SUMO may regulate ß-arr2 nucleocytoplasmic shuttling. While we found ß-arr2 SUMOylation to be dispensable for nuclear import, we identified a non-covalent interaction between SUMO and ß-arr2, via a SUMO interaction motif (SIM), that is required for ß-arr2 cytonuclear trafficking. This SIM promotes association of ß-arr2 with the multimolecular RanBP2/RanGAP1-SUMO nucleocytoplasmic transport hub that resides on the cytoplasmic filaments of the nuclear pore complex. Depletion of RanBP2/RanGAP1-SUMO levels result in defective ß-arr2 nuclear entry. Mutation of the SIM inhibits ß-arr2 nuclear import, its ability to delocalize Mdm2 from the nucleus to the cytoplasm and enhanced p53 signaling in lung and breast tumor cell lines. Thus, a ß-arr2 SIM nuclear entry checkpoint, coupled with active ß-arr2 nuclear export, regulates its cytonuclear trafficking function to control the Mdm2-p53 signaling axis.
