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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.

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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 alpha1 and alpha2, respectively, and the a/alpha diploid which expresses both a1 and alpha2 proteins. In a/alpha cells, a1-alpha2 heterodimers repress haploid-specific genes and MATalpha1, whereas alpha2 homodimers repress a-specific genes, indicating dual regulatory functions for alpha2 in mating-type control. We previously demonstrated that the two leucine zipper-like coiled-coil motifs, called alpha2A and alpha2B, in the alpha2 N-terminal domain are important to a1-alpha2 heterodimerization. A unique feature of alpha2B is the occurrence of three atypical amino acid residues at a positions within the hydrophobic core. We have conducted mutational analyses of alpha2B peptides and the full-length protein. Our data suggest that these residues may play a critical role in partitioning of the alpha2 protein between heterodimerization with a1 and homodimerization with itself.