The E3 ubiquitin ligase Mdm2 is a focal regulator of p53 tumour suppressor activity. It binds p53, promoting its polyubiquitination and degradation, and also controls p53 synthesis. However, it is not known how this dual function of Mdm2 on p53 synthesis and degradation is achieved. Here we show that the p53 mRNA region encoding the Mdm2-binding site interacts directly with the RING domain of Mdm2. This impairs the E3 ligase activity of Mdm2 and promotes p53 mRNA translation. We also show that introduction of cancer-derived single silent point-mutations in the p53 mRNA weakens its binding to Mdm2 and results in reduced p53 activity. These data are consistent with a mechanism by which changes in silent nucleotides can affect the function of the encoded protein, and indicate that Mdm2-mediated control of p53 synthesis and degradation has evolved in the p53 mRNA sequence and its encoded amino acids.

The oncogenic Epstein-Barr virus (EBV) evades the immune system but has an Achilles heel: its genome maintenance protein EBNA1, which is essential for viral genome maintenance but highly antigenic. EBV has seemingly evolved a system in which the mRNA sequence encoding the glycine-alanine repeats (GAr) of the EBNA1 protein limits its expression to the minimal level necessary for function while minimizing immune recognition. Here, we identify nucleolin (NCL) as a host factor required for this process via a direct interaction with G-quadruplexes formed in GAr-encoding mRNA sequence. Overexpression of NCL enhances GAr-based inhibition of EBNA1 protein expression, whereas its downregulation relieves the suppression of both expression and antigen presentation. Moreover, the G-quadruplex ligand PhenDC3 prevents NCL binding to EBNA1 mRNA and reverses GAr-mediated repression of EBNA1 expression and antigen presentation. Hence the NCL-EBNA1 mRNA interaction is a relevant therapeutic target to trigger an immune response against EBV-carrying cancers.

The MHC class I antigen presentation pathway allows the immune system to distinguish between self and nonself. Despite extensive research on the processing of antigenic peptides, little is known about their origin. Here, we show that mRNAs carrying premature stop codons that prevent the production of full-length proteins via the nonsense-mediated decay pathway still produce a majority of peptide substrates for the MHC class I pathway by a noncanonical mRNA translation process. Blocking the interaction of the translation initiation factor eIF4E with the cap structure suppresses the synthesis of full-length proteins but has only a limited effect on the production of antigenic peptides. These results reveal an essential cell biological function for a class of translation products derived during the pioneer round of mRNA translation and will have important implications for understanding how the immune system detects cells harboring pathogens and generates tolerance.MHC class I-restricted antigen presentation | pioneer round of translation