Physical and Functional Interactions of the Arf Tumor Suppressor Protein with Nucleophosmin/B23

Abstract

ABSTRACT The Arf tumor suppressor inhibits cell cycle progression through both p53-dependent and p53-independent mechanisms, including interference with rRNA processing. Using tandem-affinity-tagged p19 Arf , we purified Arf-associated proteins from mouse NIH 3T3 fibroblasts undergoing cell cycle arrest. Tagged p19 Arf associated with nucleolar and ribosomal proteins, including nucleophosmin/B23 (NPM), a protein thought to foster the maturation of preribosomal particles. NPM is an abundant protein, only a minor fraction of which binds to p19 Arf ; however, a significant proportion of p19 Arf associates with NPM. The interaction between p19 Arf and NPM requires amino acid sequences at the Arf amino terminus, which are also required for Mdm2 binding, as well as the central acidic domain of NPM and an adjacent segment that regulates NPM oligomerization. The interaction between p19 Arf and NPM occurs in primary mouse embryonic fibroblasts, including those lacking both Mdm2 and p53 . In an NIH 3T3 derivative cell line (MT-Arf) engineered to conditionally express an Arf transgene, induced p19 Arf associates with NPM and colocalizes with it in high-molecular-weight complexes (2 to 5 MDa). An NPM mutant lacking its carboxyl-terminal nucleic acid-binding domain oligomerizes with endogenous NPM, inhibits p19 Arf from entering into 2- to 5-MDa particles, and overrides the ability of p19 Arf to retard rRNA processing.