The myc gene product belongs to a family of basic, Helix-Loop-Helix, Leucine Zipper (b-HLH-LZ) proteins involved in cell growth, differentiation and tumorigenesis. The Max protein dimerizes with Myc to form a complex which binds to DNA and promotes transcription of target genes. Max exists in two major forms, Max-9 (long) and Max (short), which differ by 9 amino acids just amino to the basic region. We compared the in vivo behavior of the two forms in stable transfectants of long and short wild type (Lwt, Swt) and basic region mutants (Lbm, Sbm). While both Lwt-max and Swt-max clones exhibit delays in cell growth and differentiation, these delays are far more pronounced in the Lwt-max clones. In contrast, no difference is noted between Lbm-Max and Sbm-Max in their observed ability to delay growth, to accelerate HMBA-induced differentiation and to induce spontaneous differentiation. We suggest that a difference in affinity for DNA underlies the differential activities of Lwt- and Swt-Max.

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