The B-box dominates SAP-1-SRF interactions in the structure of the ternary complex

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Abstract
The serum response element (SRE) is found in several immediate‐early gene promoters. This DNA sequence is necessary and sufficient for rapid transcriptional induction of the human c‐ fos proto‐oncogene in response to stimuli external to the cell. Full activation of the SRE requires the cooperative binding of a ternary complex factor (TCF) and serum response factor (SRF) to their specific DNA sites. The X‐ray structure of the human SAP‐1–SRF–SRE DNA ternary complex was determined (Protein Data Bank code 1hbx). It shows SAP‐1 TCF bound to SRF through interactions between the SAP‐1 B‐box and SRF MADS domain in addition to contacts between their respective DNA‐binding motifs. The SAP‐1 B‐box is part of a flexible linker of which 21 amino acids become ordered upon ternary complex formation. Comparison with a similar region from the yeast MATα2–MCM1–DNA complex suggests a common binding motif through which MADS‐box proteins may interact with additional factors such as Fli‐1.