Mutations in human ARF exon 2 disrupt its nucleolar localization and impair its ability to block nuclear export of MDM2 and p53

The mammalian ARF-INK4a locus uniquely encodes two cell cycle inhibitors by using separate promoters and alternative reading frames, p16(INK4a) maintains the retinoblastoma protein in its growth suppressive state while ARF stabilizes p53. We report that human ARF protein predominantly localizes to the nucleolus via a sequence within the exon 2-encoded C-terminal domain and is induced to leave the nucleolus by MDM2. ARF forms nuclear bodies with MDM2 and p53 and blocks p53 and MDM2 nuclear export. Tumor-associated mutations in ARF exon 2 disrupt ARF's nucleolus localization and reduce ARF's ability to block p53 nuclear export and to stabilize p53. Our results suggest an ARF-regulated MDM2-dependent p53 stabilization and link the human tumor-associated mutations in ARF with a functional alteration.