Nucleotide binding by the Mdm2 RING domain facilitates Arf-independent Mdm2 nucleolar localization.

The RING domain of Mdm2 contains a conserved Walker A or P loop motif that is a characteristic of nucleotide binding proteins. We found that Mdm2 binds adenine-containing nucleotides preferentially and that nucleotide binding leads to a conformational change in the Mdm2 C terminus. Although nucleotide binding is not required for Mdm2 E3 ubiquitin ligase activity, we show that nucleotide binding-defective P loop mutants are impaired in p14(ARF)-independent nucleolar localization both in vivo and in vitro. Consistent with this, ATP-bound Mdm2 is preferentially localized to the nucleolus. Indeed, we identify a unique amino acid substitution in the P loop motif (K454A) that uncouples nucleolar localization and E3 ubiquitin ligase activity of Mdm2 and leads to upregulation of the E3 activity both in human cells and in Caenorhabditis elegans. We propose that nucleotide binding-facilitated nucleolar localization of Mdm2 is an evolutionarily conserved regulator of Mdm2 activity.