Abstract
Smad proteins are transcriptional regulators activated by TGF-β. They are known to bind to two distinct Smad-responsive motifs, namely the Smad-binding element (SBE) (5'-GTCTAGAC-3') and CAGA motifs (5'-AGCCAGACA-3' or 5'-TGTCTGGCT-3'). However, the mechanisms by which these motifs promote Smad activity are not fully elucidated. In this study, we performed DNA CASTing, binding assays, ChIP sequencing, and quantitative RT-PCR to dissect the details of Smad binding and function of the SBE and CAGA motifs. We observed a preference for Smad3 to bind CAGA motifs and Smad4 to bind SBE, and that either one SBE or a triple-CAGA motif forms a cis-acting functional half-unit for Smad-dependent transcription activation; combining two half-units allows efficient activation. Unexpectedly, the extent of Smad binding did not directly correlate with the abilities of Smad-binding sequences to induce gene expression. We found that Smad proteins are more tolerant of single bp mutations in the context of the CAGA motifs, with any mutation in the SBE disrupting function. CAGA and CAGA-like motifs but not SBE are widely distributed among stimulus-dependent Smad2/3-binding sites in normal murine mammary gland epithelial cells, and the number of CAGA and CAGA-like motifs correlates with fold-induction of target gene expression by TGF-β. These data, demonstrating Smad responsiveness can be tuned by both sequence and number of repeats, provide a compelling explanation for why CAGA motifs are predominantly used for Smad-dependent transcription activation in vivo.