Differential effects of the widely expressed dMax splice variant of Max on E-box vs initiator element-mediated regulation by c-Myc.

dMax, a naturally occurring splice variant of the Myc binding protein Max, lacks the DNA binding basic region and helix 1 of the Helix-Loop-Helix domain; dMax interacts with c-Myc in vitro and in vivo, and inhibits E-box Myc site driven transcription in transient transfection assays. Here we have investigated the expression, function and interactions of dMax. RT/PCR analyses detected dmax mRNA in multiple tissues of the developing, newborn and adult mouse. Functionally, dMax reduced the ability of c-Myc to cooperate with the progression factor A-Myb to promote S phase entry of quiescent smooth muscle cells. In contrast, dMax failed to ablate inhibition of initiator element (Inr)-mediated transcription by c-Myc in Jurkat T cells. In in vitro protein:protein association assays, dMax interacted with c-Myc, N-Myc, L-Myc, Mad1, Mxi1, Mad3 and Mad4, but not with itself or wild-type Max. These interactions required an intact leucine zipper. Inhibition of E-box-mediated transactivation by induction of dMax overexpression resulted in apoptosis of WEHI 231 B cells. Thus, dMax is a widely expressed, naturally occurring protein, with the capacity to bind most members of the Myc/Max superfamily; dMax has little effect on Inr-mediated repression by c-Myc, but can significantly decrease E-box-mediated events promoting proliferation and cell survival.