The structure of a complex containing the homeodomain repressor protein MATα2 and the MADS-box transcription factor MCM1 bound to DNA has been determined by X-ray crystallography at 2.25 Å resolution. It reveals the protein-protein interaction responsible for cooperative binding of MATα2 and MCM1 to DNA. The otherwise flexible amino-terminal extension of the MATα2 homeodomain forms a β-halrpin that grips the MCM1 surface through parallel β-strand hydrogen bonds and close-packed, predominantly hydrophobic, side chains. DNA bending induced by MCM1 brings the two proteins closer together, facilitating their interaction. An unusual feature of the complex is that an eight-amino-acid sequence adopts an α-helical conformation in one of two copies of the MATα2 monomer and a β-strand conformation in the other. This 'chameleon' sequence of MATα2 may be important for recognizing natural operator sites.
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