Dimerization Specificity of Arabidopsis MADS Domain Homeotic Proteins APETALA1, APETALA3, PISTILLATA, and AGAMOUS

The MADS domain homeotic proteins APETALA1 (AP1), APETALA3 (AP3), PISTILLATA (PI), and AGAMOUS (AG) act in a combinatorial manner to specify the identify of Arabidopsis floral organs. The molecular basis for this combinatorial mode of action was investigated. Immunoprecipitation experiments indicate that all four proteins are capable of interacting with each other. However, these proteins exhibit ``partner-specificity'' for the formation of DNA-binding dimers; only AP1 homodimers, AG homodimers, and AP3/PI heterodimers are capable of binding to CArG-box sequences. Both the AP3/PI heterodimer and the AP1 or AG homodimers are formed when the three corresponding proteins are present together. The use of chimeric proteins formed by domain swapping indicates that the L region (which follows the MADS box) constitutes a key molecular determinant for the selective formation of DNA-binding dimers. The implications of these results for the ABC genetic model of flower development are discussed.