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

Members of the MADS-box family of transcription factors are found in eukaryotes ranging from yeast to humans; In plants, MADS-box proteins regulate several 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 characterised mammalian and yeast MADS-box proteins as it can efficiently bind two different classes of DNA-binding site. SQUA induces bending of these binding sites and the sequence of the site plays a role in determining the magnitude of these bends. Similarly, PLE and DEF/GLO induce DNA bending although 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 dimerisation in the context of SQUA homodimers or heterodimers with PLE, Together these results contribute significantly to our understanding of the function of SQUA and other plant MADS-box proteins at the molecular level.