Biphasic effect of Max on Myc cotransformation activity and dependence on amino- and carboxy-terminal Max functions.

In Ras cotransformation assays, Max exhibited a biphasic effect on Myc transformation activity. Cotransfection of low levels of Max expression plasmid stimulated Myc transformation activity, but cotransfection of high levels suppressed it. Mutations in the functionally undefined Max amino- and carboxy-terminal regions outside of the B/HLH/LZ motif partly separated these activities, suggesting various modes of Max regulation. We demonstrate that the Max protein is a nuclear protein in vivo and identify a carboxy-terminal region similar to nuclear localization signals whose integrity is necessary for efficient localization. Two mutants that delete amino- or carboxy-terminal consensus signals for casein kinase II (CKII) exhibited altered gel mobility and DNA-binding potential in vitro and showed modified transforming potential in the Ras cotransformation assay, suggesting that CKII or a CKII-related enzyme may regulate Max function in vivo. Our data suggest that both the ratio of Myc/Max hetero-oligomers to Max homo-oligomers and Max-specific regulation can contribute to determining the biological activity of Myc in vivo.