Cross-regulatory interaction between Stra13 and USF results in functional antagonism

Transcription factors belonging to the basic helix-loop-helix (bHLH) family are critical regulators of cellular proliferation and differentiation. The functional activity of these proteins can be regulated by heterodimerization through the HLH domain, as a result of formation of functional or non-functional heterodimers. The presence of a leucine zipper in bHLH-leucine zipper (bHLHZip) proteins, however, prevents such heterodimeric interactions via the HLH domain between bHLH and bHLHZip proteins. To identify cellular proteins that directly interact with and modulate transcriptional repression mediated by the bHLH protein Stra13, we carried out a yeast two hybrid screen. The bHLHZip protein USF (Upstream Stimulatory factor) was identified as a Stra13 interacting protein. We demonstrate a direct interaction between Stra13 and USF that is dependent upon the C-terminal repression domain of Stra13 and the DNA-binding domain of USF. Stra13 and USF also colocalize and functionally interact in mammalian cells. Co-expression of USF abrogates Stra13-mediated repression of target genes and conversely, Stra13 inhibits DNA-binding and USF-mediated transactivation. Taken together, our data demonstrate that Stra13 and USF interact physically and functionally, and identify a novel mode of cross regulatory interaction between members of the bHLH and bHLHZip families that abrogates their functional activity.