Functional analysis of the AUG- and CUG-initiated forms of the c-Myc protein.
Activation of the c-myc proto-oncogene by chromosomal translocation or proviral insertion frequently results in the separation of the c-myc coding region from its normal regulatory elements. Such rearrangements are often accompanied by loss or mutation of c-myc exon 1 sequences. These genetic alterations do not affect synthesis of the major c-myc protein, p64, which is initiated from the first AUG codon in exon 2. However they can result in mutation or loss of the CUG codon located in exon 1 that normally serves as an alternative translational initiation codon for synthesis of an N-terminally extended form of c-Myc (p67). It has been hypothesized that p67 is a functionally distinct form of c-Myc whose specific loss during c-myc rearrangements confers a selective growth advantage. Here we describe experiments designed to test the functional properties of the two c-Myc protein forms. We introduced mutations within the translational initiation codons of a normal human c-myc cDNA that alter the pattern of Myc protein synthesis (p64 vs. p67). The functions of each of these proteins were experimentally addressed using co-transformation and transcriptional activation assays. Both the p64 and p67 c-Myc proteins were independently able to collaborate with bcr-abl in the transformation of Rat-1 fibroblasts. In addition, both the exon 1- and exon 2-initiated forms of the c-Myc protein stimulated transcription of a Myc/Max-responsive reporter construct to a similar level. Given the apparent absence of functional differences between p64 and p67, we conclude that the basis for c-Myc oncogenic activation lies primarily in the overall deregulation of its expression and not in alterations in the protein. The existence of the CUG translational initiator may reflect a mechanism for the continued synthesis of c-Myc protein under conditions where AUG initiation is inhibited.
Blackwood
E M
EM
Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104.
Lugo
T G
TG
Kretzner
L
L
King
M W
MW
Street
A J
AJ
Witte
O N
ON
Eisenman
R N
RN
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D018387:Codon, Initiator / Q000235:genetics*; D018076:DNA, Complementary / Q000235:genetics; D005786:Gene Expression Regulation; D017930:Genes, Reporter; D005822:Genetic Vectors; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010442:Peptide Chain Initiation, Translational / Q000235:genetics; D014176:Protein Biosynthesis; D000090063:Proto-Oncogene Mas; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis / Q000235:genetics*; D051381:Rats; D012190:Retroviridae / Q000235:genetics; D014170:Transformation, Genetic
D018387:Codon, Initiator; D018076:DNA, Complementary; C000715470:MAS1 protein, human; D000090063:Proto-Oncogene Mas; D016271:Proto-Oncogene Proteins c-myc