Conservation of structural domains and biochemical activities of the MDM2 protein from Xenopus laevis.

The Mdm2 gene is the best known cellular regulator of p53 tumor suppressor activity. We report here the cloning and characterization of Xdm2, its homolog in Xenopus laevis. Human, mouse and Xenopus MDM2 proteins are more than 65% identical in several regions which are likely to be important for the biological activities of MDM2. Region I is sufficient for binding p53 and inhibiting its G1 arrest and apoptosis functions. Region II contains most of a central acidic region required for interaction with the L5 ribosomal protein and a putative C4 zinc finger. Region III is nearly identical from Xenopus to human and comprises the RING finger domain. We show that this structural conservation is associated with the conservation of three biochemical activities of MDM2; binding to the p53 and L5 proteins and specifically to RNA. Lastly, Xdm2 expression during early development is mainly restricted from the oocyte stage I/II to the blastula stage and is possibly independent of transcriptional activation by p53. These data as well as the utilization of Xenopus laevis to investigate the roles of MDM2 and p53 during early embryogenesis are discussed.