An antirrhinum ternary complex factor specifically interacts with C-function and SEPALLATA-like MADS-box factors

The development of floral reproductive organs requires the activity of plant MADS-box transcription factors (MBFs) belonging to the C function. The C function can only operate within a floral context, specified by MBFs belonging to the SEPALLATA class of proteins. Here we describe the specific interaction between a novel protein, MIP1, and C-function and SEPALLATA (SEP)-like MBFs. MIP1 is the first member of a new class of proteins unique to plants. None of the family members have yet been assigned a function. Motif searches reveal a leucine zipper domain within a conserved N-terminal region of MIP1. The leucine zipper lies within a region sufficient for interaction with plant MBFs. MIP1 interacts with a domain of plant MBFs that is analogous to the domain of animal and yeast MBFs involved in ternary complex formation. The MIP1 protein is predicted to localise to the nucleus and activates yeast reporter genes in vivo. MIP1 is expressed in the fourth whorl of the flower, in an overlapping temporal and spatial expression pattern with the C-function and SEP-like genes. Taken together, this suggests that MIP1 acts as a ternary complex factor specifically with C-function and SEP-like MBFs.