Expression of two distinct homologues of Xenopus Max during early development

The Max protein belongs to the basic region-helix-loop-helix-leucine zipper family of transcriptional regulators. Max heterodimerizes with Myc family proteins to form sequence-specific DNA-binding complexes. In order to elucidate the potential role of Myc and Max during amphibian embryogenesis, we have isolated and analyzed the expression of two Xenopus Max complementary DNAs: XMax1 and XMax2. Comparison of XMax1 and XMax2 with their mammalian counterparts demonstrates a strikingly high degree of conservation at both the nucleotide and amino acid levels, with the exception of a 24-residue deletion in both XMax proteins within their COOH termini. In addition, the two Xenopus Max proteins differ in that XMax2 contains a unique 27-amino acid insertion that interrupts the COOH-terminal end of the zipper domain; XMax1 lacks this insertion. Despite these differences, both XMax1 and XMax2 can form complexes with either Xenopus or human c-Myc proteins. Analysis of XMax expression during embryogenesis reveals that both mRNA and protein are expressed throughout early development, including the egg, 32-cell stage, and midblastula transition. Although the expression of XMax1 RNA appears to predominate at all stages examined, the ratios of XMax1 to XMax2 protein vary during development as well as between different tissue culture cell lines, suggesting a potential for cell type-specific regulation. Our results demonstrate the presence of Xenopus Max throughout frog development, raising the possibility that Myc and Max could function as a complex even during early embryogenesis.