THE MDM2 ONCOPROTEIN BINDS SPECIFICALLY TO RNA THROUGH ITS RING FINGER DOMAIN

Background: The cellular mdm2 gene has transforming activity when over expressed and is amplified in a variety of human tumors. At least part of the transforming ability of the MDM2 protein is due to binding and inactivating the p53 tumor suppressor protein. Additionally, this pro tein forms a complex in vivo with the L5 ribosomal protein and its ass ociated 5S ribosomal RNA and may be part of a ribosomal complex. Mater ials and Methods: A RNA homopolymer binding assay and a SELEX procedur e have been used to characterize the RNA-binding activity of MDM2. Res ults: The MDM2 protein binds efficiently to the homopolyribonucleotide poly(G) but not to other homopolyribonucleotides. This binding is ind ependent of the interaction of MDM2 with the L5 protein, which occurs through the central acidic domain of MDM2. An RNA SELEX procedure was performed to identify specific RNA ligands that bind with high affinit y to the human MDM2 (HDM2) protein. After 10 rounds of selection and a mplification, a subset of RNA molecules that bound efficiently to HDM2 was isolated from a randomized pool. Sequencing of these selected lig ands revealed that a small number of sequence motifs were selected. Th e specific RNA binding occurs through the RING finger domain of the pr otein. Furthermore, a single amino add substitution in the RING finger domain, G446S, completely abolishes the specific RNA binding. Conclus ions: These observations, showing that MDM2 binds the L5/5S ribosomal ribonucleoprotein particle and can also bind to specific RNA sequences or structures, suggest a role for MDM2 in translational regulation in a cell.