The high and low affinity alpha-Pal binding sites of the consensus sequence CG CAT GCG CA contain a non-canonical E-box sequence CA NN TG. The question posed is whether alpha-Pal competes with Max for binding sites and the functional consequences of such a competition. Max is an important E-box binding factor that lacks an activation domain. Thus Max:Max homodimers behave as transcriptional repressors. However, Max can serve as a transcriptional activator if it heterodimerizes with another B-zip protein containing an activation domain. In most cells Max is constitutively expressed and exists as a homodimer which inhibits transcription of the target genes. Activation of c-myc transcription rapidly increases the bio-availability of c-Myc thus favoring Myc:Max heterodimer formation. The c-Myc:Max heterodimer can then bind to and activate transcription of E-box target genes. Since alpha-Pal and Max share overlapping binding sequences and are both B-zip proteins, we wanted to see if alpha-Pal bound as a homodimer or as an alpha-Pal:Max heterodimer. Whereas alpha-Pal bound to both of the tandem direct repeat sequences, Max only binds to the low affinity site. At physiological concentrations neither protein could form stable complexes. If identical amounts of these proteins were preincubated with each other, however, a strong complex was formed. Both immunoprecipitation and supershift experiments using anti-Max or anti-alpha-Pal indicate that these proteins bind as higher affinity heterodimers. Thus the bioavailability of both alpha-Pal and Max can have a large effect on the regulation of target genes. The heterodimerization of alpha-Pal and Max is the first reported dimerization of Max with a transcription factor that has affinity for DNA in the presence or absence of Max. In addition, a null mutation of the murine myc gene had little or no effect on the phenotype. Thus it appears that other related factors, such as alpha-Pal, can functionally replace the myc gene.
