Max: A Helix-Loop-Helix Zipper Protein That Forms a Sequence-Specific DNA-Binding Complex with Myc

Abstract

The myc protooncogene family has been implicated in cell proliferation, differentiation, and neoplasia, but its mechanism of function at the molecular level is unknown. The carboxyl terminus of Myc family proteins contains a basic region helix-loop-helix leucine zipper motif (bHLH-Zip), which has DNA-binding activity and has been predicted to mediate protein-protein interactions. The bHLH-Zip region of c-Myc was used to screen a complementary DNA (cDNA) expression library, and a bHLH-Zip protein, termed Max, was identified. Max specifically associated with c-Myc, N-Myc, and L-Myc proteins, but not with a number of other bHLH, bZip, or bHLH-Zip proteins. The interaction between Max and c-Myc was dependent on the integrity of the c-Myc HLH-Zip domain, but not on the basic region or other sequences outside the domain. Furthermore, the Myc-Max complex bound to DNA in a sequence-specific manner under conditions where neither Max nor Myc exhibited appreciable binding. The DNA-binding activity of the complex was dependent on both the dimerization domain and the basic region of c-Myc. These results suggest that Myc family proteins undergo a restricted set of interactions in the cell and may belong to the more general class of eukaryotic DNA-binding transcription factors.

Lack of transcriptional repression by max homodimers

“…Max(USF) was also unable to mediate repression of either transiently expressed or endogenous c-myc-responsive genes. Our observations thus strongly suggest that, despite the ease with which max homodimerization and DNA binding normally occur in vitro (Blackwood and Eisenman, 1991;Prendergast et al, 1991;Berberich and Cole, 1992;Prochownik and VanAntwerp, 1993), these attributes are not su cient to allow transcriptional repression in vivo. Rather, this activity seems to be mediated solely by the various heterodimeric associations described above.…”

“…Max is a bHLH-ZIP protein that plays a central role in both the positive and negative transcriptional regulation of myc oncoproteins (Blackwood and Eisenman, 1991;Prendergast et al, 1991;Mukherjee et al, 1992;Kretzner et al, 1992a,b;Amin et al, 1993;Gu et al, 1993). In the ®rst case, positive regulation is achieved by binding of the myc-max heterodimer to a consensus E-box sequence (Blackwood and Eisenman, 1991;Prendergast et al, 1991).…”

“…In the ®rst case, positive regulation is achieved by binding of the myc-max heterodimer to a consensus E-box sequence (Blackwood and Eisenman, 1991;Prendergast et al, 1991). Transcriptional activation is then mediated by a TAD encoded by the amino terminus of myc (Kato et al, 1990).…”

“…Max readily dimerizes with the three major myc proteins (c-myc, N-myc, and L-myc) both in vitro and in vivo, and greatly augments their binding to consensus DNA sites termed E-boxes (Blackwood and Eisenman, 1991;Prendergast et al, 1991;Mukherjee et al, 1992;Blackwood et al, 1992). This leads to the transcription of E-box-containing genes (Kretzner et al, 1992b;Amin et al, 1993;Gu et al, 1993), an activity mediated by transactivation domains (TADs) in the amino termini of the myc proteins (Kato et al, 1990).…”

“…In addition to its ability to heterodimerize with myc proteins, max also homodimerizes and, at least in vitro binds to the same E-boxes as myc-max heterodimers (Blackwood and Eisenman, 1991;Prendergast et al, 1991;Berberich and Cole, 1992;Prochownik and Van Antwerp, 1993). Although the co-expression of c-myc and max in vivo stimulates transcription at low max : myc ratios, transcriptional repression is seen at higher ratios or in the absence of exogenous c-myc (Prendergast et al, 1991;Kretzner et al, 1992a;Amin et al, 1993;Gu et al, 1993;Lee et al, 1995;Zhang et al, 1997).…”