INTERACTION BETWEEN 2 HOMEODOMAIN PROTEINS IS SPECIFIED BY A SHORT C-TERMINAL TAIL

TWO yeast homeodomain proteins, a1 and alpha 2, interact and cooperati vely bind the haploid-specific gene (Hsg) operator, resulting in the r epression of a set of genes involved in the determination of cell type (1-5). The cooperative binding of a1 and alpha 2 to DNA can be reconst ituted in vitro using purified fragments of a1 and alpha 2. Only the h omeodomain is needed for a1, but for alpha 2 a C-terminal 22-amino-aci d tail is required as well(4,6-9). As most of the specificity of DNA b inding appears to derive from a1, we proposed(4) that alpha 2 function s in the a1/alpha 2 heterodimer to contact a1 with its tail. By constr uction and analysis of several chimaeric proteins, we investigate how two DNA-binding proteins, one with low intrinsic specificity (alpha 2) and one with no apparent intrinsic DNA-binding ability (a1), can toge ther create a highly specific DNA-binding activity(4). We show that th e 22-amino-acid region of alpha 2 immediately C-terminal to the homeod omain, when grafted onto the a1 homeodomain, converts a1 to a strong D NA-binding protein. This alpha 2 tail can also be attached to the Dros ophila engrailed homeodomain, and the chimaeric protein now binds coop eratively to DNA with a1, showing how a simple change can create a new homeodomain combination that specifically recognizes a new DNA operat or.