SEQUENCES OUTSIDE THE HOMEODOMAIN OF BICOID ARE REQUIRED FOR PROTEIN-PROTEIN INTERACTION

The Drosophila morphogenetic protein Bicoid (Bcd) is required for the development of anterior structures of the embryo. Bcd, a homeodomain p rotein, is distributed as an anterior-to-posterior gradient ill the em bryo. It stimulates the expression of the hunchback (hb) gene in the a nterior half in an all-or-none fashion. We have recently shown that Bc d binds cooperatively to a hb enhancer element and proposed that coope rative DNA binding is facilitated by an interaction between Bcd molecu les. In this report, we further analyze the interaction between Bcd mo lecules and define regions important for protein-protein interaction. We show that the homeodomain of Bcd alone fails to interact with anoth er Bcd molecule efficiently. The protein sequence flanking either side of the homeodomain restores the protein-protein interaction function. Mutations in the homeodomain that affect DNA binding do not adversely affect the protein-protein interaction function, suggesting that the surfaces for DNA binding and protein-protein interaction are separable . Finally, we demonstrate that the homeodomain of Bcd alone, unlike th e intact Bcd, fails to bind DNA cooperatively. These results further s upport the notion that cooperative DNA binding is facilitated by the i nteraction between Bcd molecules. They strongly suggest that protein-p rotein interaction is an important property of Bcd for its biological activities.