A LINK BETWEEN INCREASED TRANSFORMING ACTIVITY OF LYMPHOMA-DERIVED MYC MUTANT ALLELES, THEIR DEFECTIVE REGULATION BY P107, AND ALTERED PHOSPHORYLATION OF THE C-MYC TRANSACTIVATION DOMAIN

The c-Myc protein is a transcription factor with an N terminal transcr iptional regulatory domain and C terminal oligomerization and DNA-bind ing motifs. Previous studies have demonstrated that p107, a protein re lated to the retinoblastoma protein, binds to the c-Myc transcriptiona l activation domain and suppresses its activity. We sought to characte rize the transforming activity and transcriptional properties of lymph oma-derived mutant MYC alleles. Alleles encoding c-Myc proteins with m issense mutations in the transcriptional regulatory domain were more p otent than wild-type c-Myc in transforming rodent fibroblasts. Althoug h the mutant c-Myc proteins retained their binding to p107 in in vitro and in vivo assays, p107 failed to suppress their transcriptional act ivation activities. Many of the lymphoma-derived MYC alleles contain m issense mutations that result in substitution for the threonine at cod on 58 or affect sequences flanking this amino acid. We observed that i n vivo phosphorylation of Thr-58 was absent in a lymphoma cell line wi th a mutant MYC allele containing a missense mutation flanking codon 5 8. Our in vitro studies suggest that phosphorylation of Thr-58 in wild -type c-Myc was dependent on cyclin A and required prior phosphorylati on of Ser-62 by a p107-cyclin A-CDK complex. In contrast, Thr 58 remai ned unphosphorylated in two representative mutant c-Myc transactivatio n domains in vitro. Our studies suggest that missense mutations in MYC may be selected for during lymphomagenesis, because the mutant MYC pr oteins have altered functional interactions with p107 protein complexe s and fail to be phosphorylated at Thr-58.