LOCATION OF THE INTERNAL RIBOSOME ENTRY SITE IN THE 5' NONCODING REGION OF THE IMMUNOGLOBULIN HEAVY-CHAIN BINDING-PROTEIN (BIP) MESSENGER-RNA - EVIDENCE FOR SPECIFIC RNA-PROTEIN INTERACTIONS

The 220 nucleotide 5' non-coding region (5'NCR) of the human immunoglo bulin heavy chain binding protein (Rip) mRNA contains an internal ribo some entry site (IRES) that mediates the translation of the second cis tron in a dicistronic mRNA in cultured mammalian cells. In this study, experiments are presented that locate the IRES immediately upstream o f the start-site AUG codon in the BiP mRNA, Furthermore, crosslinking of thiouridine-labeled BiP IRES-containing RNA to cellular proteins id entified the specific binding of two proteins, p60 and p95, to the 3' half of the BiP 5'NCR. Interestingly, both p60 and p95 bound also spec ifically to several viral IRES elements. This correlation suggests tha t p60 and p95 could have roles in internal initiation of cellular and viral IRES elements.