Structures of MADS-domain proteins and their functions in determining floral organ identity. (a) Crystal structure of a dimer of the MADS-domain of human serum response factor (SRF) bound to DNA (PDB 1SRS; note that no crystal structure exists for plant MADS-domain proteins). DNA is shown in ball-and-stick representation and colored in gray, while the two MADS domains of the dimer are colored blue and red, respectively. The α-helix is represented by a spring-like structure whereas the β-strands are shown as darker colored arrows. (b) Structures of 'floral quartets'. According to the floral quartet model, multimeric complexes of MIKCC-group proteins, bound to two DNA sequence elements (CArG-boxes) present in numerous target genes, determine floral organ identity. Specifically, quartet formation involving two dimers of AG and SEP proteins (Table 1) determines carpel identity; complex formation involving a dimer of AG and SEP with a dimer of AP3 and PI determines stamen identity; quartet formation involving a dimer of AP1 and SEP with a dimer of AP3 and PI determines petal identity; and complex formation involving two dimers of AP1 and SEP determines sepal identity. CArG1-3, different CArG-boxes.