Heterodimerization of the yeast MATa1 and MAT alpha 2 proteins is mediated by two leucine zipper-like coiled-coil motifs.

The yeast Saccharomyces cerevisiae has three cell types distinguished by the proteins encoded in their mating type (MAT) loci: the a and alpha haploids, which express the DNA binding proteins a1 and alpha 1 and alpha 2, respectively, and the a/alpha diploid which expresses both a1 and alpha 2 proteins. In a/alpha cells, a1-alpha 2 heterodimers repress haploid-specific genes, while alpha 2 homodimers repress a-specific genes, indicating a dual regulatory function for alpha 2 in mating type control. a1 does not form homodimers. We have identified two sequences in the alpha 2 N-terminal domain which contain the 3,4-hydrophobic heptad repeat pattern characteristic of coiled-coils. Mutational analyses show that both sequences are important to a1-alpha 2 heterodimerization. We propose that these two sequences associate in a coiled-coil-like manner with a sequence within a1 which bears two adjacent, overlapping 3,4-hydrophobic heptad repeats. This model, which describes a novel dimerization motif for homeodomain proteins, also provides a mechanism by which a1-a1 homodimerization is prevented.