A novel Snail-related transcription factor Smuc regulates basic helix-loop-helix transcription factor activities via specific E-box motifs

Snail family proteins are zinc finger transcriptional regulators first iden tified in Drosophila which play critical roles in cell fate determination. We identified a novel Snail-related gene from murine skeletal muscle cells designated Smuc, Northern blot analysis showed that Smuc was highly express ed in skeletal muscle and thymus, Smuc contains five putative DNA-binding z inc finger domains in its C-terminal half. In electrophoretic mobility shif t assays, recombinant zinc finger domains of Smuc specifically bound to CAG GTG and CACCTG E-box motifs (CANNTG). Because basic helix-loop-helix transc ription factors (bHLH) bind to the same E-box sequences, we examined whethe r Smuc competes with the myogenic bHLH factor MyoD for DNA binding. Smuc in hibited the binding of a MyoD-E12 complex to the CACCTG E-box sequence in a dose-dependent manner and suppressed the transcriptional activity of MyoD- E12, When heterologously targeted to the thymidine kinase promoter as fusio n proteins with the GAL4 DNA-binding domain, the non-zinc finger domain of Smuc acted as a transcriptional repressor. Furthermore, overexpression of S muc in myoblasts repressed transactivation of muscle differentiation marker Troponin T, Thus, Smuc might regulate bHLH transcription factors by zinc f inger domains competing for E-box binding, and non-zinc finger repressor do mains might also confer transcriptional repression to control differentiati on processes.