Biophysical characterization of the b-HLH-LZ of ΔMax, an alternatively spliced isoform of Max found in tumor cells: Towards the validation of a tumor suppressor role for the Max homodimers

• Published in: PloS one Vol. 12; no. 3; p. e0174413
• Main Authors: Maltais, Loïka, Montagne, Martin, Bédard, Mikaël, Tremblay, Cynthia, Soucek, Laura, Lavigne, Pierre
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 28.03.2017; Public Library of Science (PLoS)
• Subjects: Affinity; Alternative Splicing; Amino Acid Sequence; Basic Helix-Loop-Helix Leucine Zipper Transcription Factors - chemistry; Basic Helix-Loop-Helix Leucine Zipper Transcription Factors - genetics; Basic Helix-Loop-Helix Leucine Zipper Transcription Factors - metabolism; Binding; Biology and life sciences; Cancer; Cell growth; Deoxyribonucleic acid; Dimerization; DNA; DNA - chemistry; DNA - metabolism; E-Box Elements; Gene expression; Gene sequencing; Humans; Leucine; Medicine and Health Sciences; Models, Molecular; Mutation; Neoplasms - genetics; Neoplasms - metabolism; Oncogenes; Physical sciences; Physiology; Protein Multimerization; Proteins; Proto-Oncogene Proteins c-myc - metabolism; Repressor Proteins - chemistry; Repressor Proteins - genetics; Repressor Proteins - metabolism; Research and analysis methods; Transcription factors; Tumor Suppressor Proteins - chemistry; Tumor Suppressor Proteins - genetics; Tumor Suppressor Proteins - metabolism; Canada; Spain
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0174413

It is classically recognized that the physiological and oncogenic functions of Myc proteins depend on specific DNA binding enabled by the dimerization of its C-terminal basic-region-Helix-Loop-Helix-Leucine Zipper (b-HLH-LZ) domain with that of Max. However, a new paradigm is emerging, where the binding of the c-Myc/Max heterodimer to non-specific sequences in enhancers and promoters drives the transcription of genes involved in diverse oncogenic programs. Importantly, Max can form a stable homodimer even in the presence of c-Myc and bind DNA (specific and non-specific) with comparable affinity to the c-Myc/Max heterodimer. Intriguingly, alterations in the Max gene by germline and somatic mutations or changes in the gene product by alternative splicing (e.g. ΔMax) were recently associated with pheochromocytoma and glioblastoma, respectively. This has led to the proposition that Max is, by itself, a tumor suppressor. However, the actual mechanism through which it exerts such an activity remains to be elucidated. Here, we show that contrary to the WT motif, the b-HLH-LZ of ΔMax does not homodimerize in the absence of DNA. In addition, although ΔMax can still bind the E-box sequence as a homodimer, it cannot bind non-specific DNA in that form, while it can heterodimerize with c-Myc and bind E-box and non-specific DNA as a heterodimer with high affinity. Taken together, our results suggest that the WT Max homodimer is important for attenuating the binding of c-Myc to specific and non-specific DNA, whereas ΔMax is unable to do so. Conversely, the splicing of Max into ΔMax could provoke an increase in overall chromatin bound c-Myc. According to the new emerging paradigm, the splicing event and the stark reduction in homodimer stability and DNA binding should promote tumorigenesis impairing the tumor suppressor activity of the WT homodimer of Max.