Differential expression of Myc1 and Myc2 isoforms in cells transformed by eIF4E: evidence for internal ribosome repositioning in the human c-myc 5'UTR.
eIF4E is essential for translation initiation, but its overexpression causes malignant transformation. Recent work demonstrated that eIF4E/F participates in exposing and locating alternate translation start codons during scanning. Translation initiation of several important protooncogenes and growth-regulators, such as Myc and FGF-2, can start at CUG start codon(s) upstream of the normal open reading frame (ORF). The resulting amino-terminal extension alters the properties of these proteins and their intracellular distribution. In cells overexpressing eIF4E, c-myc is overexpressed and particularly the larger, CUG-initiated form (Myc1). Recent reports suggest that synthesis of Myc2, the normally expressed AUG-initiated form, is mediated by an IRES. To determine what role eIF4E might play in c-myc expression, the c-myc 5' untranslated region (UTR) was fused in-frame to CAT reporters, and several more derivative constructs were made. In vitro translation experiments (with and without eIF4E/F); expression in CHO cells transformed with eIF4E; and deletion/mutation analysis demonstrated that Myc1 is translated by a scanning mechanism, while Myc2 is translated by Internal Ribosome Repositioning. Moreover, the existence of a true IRES in the 5'UTR was contradicted by its failure to direct translation of a circular transcript, in contrast to hsp70. The c-myc 5'UTR also failed to engage in translation in the absence of functional eIF4F, after cleavage of the eIF4G component with CVB4 protease-2A. The Internal Repositioning Element (IRPE) in c-myc 5'UTR was delimited to nucleotides (nt) 394-440 from the P1 transcription start site.
Carter
P S
PS
Department of Biochemistry and Molecular Biology, Louisiana State University Medical Center, Shreveport 71130-3932, USA.
Jarquin-Pardo
M
M
De Benedetti
A
A
D020121:5' Untranslated Regions / Q000235:genetics*; D000818:Animals; D016466:CHO Cells; D002461:Cell Line, Transformed; D015500:Chloramphenicol O-Acetyltransferase / Q000378:metabolism; D006224:Cricetinae; D004252:DNA Mutational Analysis; D039561:Eukaryotic Initiation Factor-4E; D017353:Gene Deletion; D015972:Gene Expression Regulation, Neoplastic; D017930:Genes, Reporter; D016259:Genes, myc / Q000235:genetics*; D006801:Humans; D008954:Models, Biological; D008957:Models, Genetic; D010448:Peptide Initiation Factors / Q000378:metabolism*; D014176:Protein Biosynthesis; D020033:Protein Isoforms; D012270:Ribosomes / Q000378:metabolism*
D020121:5' Untranslated Regions; D039561:Eukaryotic Initiation Factor-4E; D010448:Peptide Initiation Factors; D020033:Protein Isoforms; D015500:Chloramphenicol O-Acetyltransferase