DNA binding and dimerisation determinants of Antirrhinum majus MADS-box transcription factors

Members of the MADS-box family of transcription factors are found in eukary otes ranging from yeast to humans; In plants, MADS-box proteins regulate se veral developmental processes including flower, fruit and root development. We have investigated the DNA-binding mechanisms used by four such proteins in Antirrhinum majus, SQUA, PLE, DEF and GLO. SQUA differs from the charac terised mammalian and yeast MADS-box proteins as it can efficiently bind tw o different classes of DNA-binding site. SQUA induces bending of these bind ing sites and the sequence of the site plays a role in determining the magn itude of these bends. Similarly, PLE and DEF/GLO induce DNA bending althoug h the direction of the resulting bends differ. Finally we demonstrate that the MADS-box and I-domains are sufficient for homodimer formation by SQUA, However, the K-box in SQUA can also act as an oligomerisation motif and in the full-length protein, the K-box plays a different role in;mediating dime risation in the context of SQUA homodimers or heterodimers with PLE, Togeth er these results contribute significantly to our understanding of the funct ion of SQUA and other plant MADS-box proteins at the molecular level.