Activity of MDM2, a ubiquitin Ligase, toward p53 or itself is dependent onthe RING finger domain of the ligase

We previously showed that oncoprotein MDM2 has ubiquitin ligase activity to ward tumor suppressor p53, In that paper, we showed very weak homology in t he carboxyl terminal portion between MDM2 and E6AP (HECT domain). We mutate d the cysteine residue (C464) corresponding to the residue essential for th e ubiquitin ligase activity of E6AP and this mutation diminished the ligase activity of MDM2. The cysteine residue described above is also one of the cysteine residues that form the RING finger domain of MDM2. We tried to fin d out whether the diminishing of the activity by the mutation is attributab le to the disruption of the RING finger domain or not. When the ring finger domain of MDM2 was deleted, the truncation mutant did not have the ubiquit in ligase activity. When we mutated the seven cysteine residues of RING fin ger domain of MDM2 in the carboxyl terminus, the disruption of each residue in the RING finger completely diminished the ubiquitin ligase activity of MDM2 toward MDM2 itself and toward tumor suppressor p53. These data indicat e that the RING finger domain in MDM2 is essential for its ubiquitin ligase activity toward p53 and itself.