Regulation of CCAAT/enhancer-binding protein-beta isoform synthesis by alternative translational initiation at multiple AUG start sites

The mRNA of the intronless, single-copy CCAAT/ enhancer-binding protein-X ( C/EBP beta) gene encodes several isoforms that have truncated transcription activation domains. This occurs by the alternative translational initiatio n (ATI) at multiple AUG start sites. The C/EBP beta rnRNA has four in-frame AUGs and an internal out-of-frame AUG associated with a small open reading frame (sORF). Initiation of translation at the in-frame AUGs forms 40-kDa (AUG-1), 35-kDa (AUG-2), 20-kDa (AUG-3) and 8.5-kDa (AUG-4) isoforms. We sh ow that in COS-1 cells the 20-kDa isoform is not a product of proteolysis o f the higher molecular weight isoforms. The sORF contains an AUG and termin ation signal that may produce the oligopeptide MPPAAARRL. Our studies sugge st that ATI involves three mRNA structural features: (1) the cap structure, (ii) the context of the Kozak sequences that flank the AUG and (iii) the i ntegrity of the sORF. We propose that formation of C/EBP beta isoforms is a ccomplished by a leaky ribosomal scanning mechanism that facilitates ATI of multiple internal AUGs.