Casein kinase II inhibits the DNA-binding activity of Max homodimers but not Myc/Max heterodimers.

Max is a heterodimeric partner of the Myc oncoprotein with sequence-specific DNA-binding activity. We found that the DNA-binding activity of bacterially expressed Max homodimers was inhibited in an ATP-dependent reaction by phosphorylation in vitro with purified bovine casein kinase II (CKII). In contrast, phosphorylation of Max and/or Myc by CKII had no inhibitory or stimulatory effect on the DNA-binding activity of Myc/Max heterodimers. By deletion analysis and site-directed mutagenesis, the inhibitory domain was localized to a CKII phosphorylation site in the amino terminus of Max. Finally, extracts prepared from NIH-3T3 cell lines that overexpress Max contained a phosphorylated Max protein which, following phosphatase treatment or heterodimerization with Myc, was capable of sequence-specific DNA-binding activity. Immunoprecipitation experiments confirmed that Max was also phosphorylated in NIH-3T3 cells, demonstrating that Max phosphorylation may have an important physiological function.