Identification of casein kinase II phosphorylation sites in Max: effects on DNA-binding kinetics of Max homo- and Myc/Max heterodimers.

Myc proteins have been implicated in the regulation of cell growth and differentiation. The identification of Max, a basic region/helix-loop-helix/leucine zipper protein, as a partner for Myc has provided insights into Myc's molecular function as a transcription factor. Recent evidence indicates that the relative abundance of Myc and Max is important to determine the level of specific gene transcription. In this report we have identified two major in vivo phosphorylation sites in Max (Ser-e and -11) which can be modified in vitro by casein Kinase II (CKII). Phosphorylation of these sites modulates DNA-binding by increasing both the on- and off-rates of Max homo- as well as Myc/Max heterodimers