Max activity is affected by phosphorylation at two NH2-terminal sites.

Max is a nuclear phosphoprotein that has a dose-dependent role in regulation of Myc function. The DNA-binding activity of Max homodimers, but not of Myc/Max heterodimers, has been reported to be inhibited by NH2-terminal phosphorylation. (S. J. Berberich and M. D. Cole, Genes & Dev., 6: 166-176, 1992). Here, we have mapped the NH2-terminal in vivo phosphorylation sites of Max to Ser2 and Ser11 and show that the NH2 termini of the two major alternatively spliced forms of Max (p21max and p22max) are equally phosphorylated despite differences in their amino acid sequences following Ser11. A Max mutant deficient in the NH2-terminal phosphorylation was found to inhibit both basal and Myc-induced transcription of a reporter gene more efficiently than the wild-type protein. Similarly, the ability of Myc and Ras to induce transformation was more severely impaired by the mutant. These results indicate that the NH2-terminal phosphorylation diminishes the ability of Max to negatively interfere with Myc function. However, we found no evidence that Max phosphorylation would be regulated during cell growth or differentiation. Similarly, we observed no major cell cycle-dependent changes in the extent of phosphorylation between cell populations fractionated by centrifugal elutriation or by cell cycle inhibitors.