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 heterodimerizati on through the HLH domain, as a result of formation of functional or non-fu nctional heterodimers. The presence of a leucine zipper in bHLH-leucine zip per (bHLHZip) proteins, however, prevents such heterodimeric interactions v ia the HLH domain between bHLH and bHLHZip proteins. To identify cellular p roteins that directly interact with and modulate transcriptional repression mediated by the bHLH protein Stra13, we carried out a yeast two hybrid scr een. The bHLHZip protein USF (Upstream Stimulatory factor) was identified a s 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 an d 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 demo nstrate that Stra13 and USF interact physically and functionally, and ident ify a novel mode of cross regulatory interaction between members of the bHL H and bHLHZip families that abrogates their functional activity.