A possible mechanism for partitioning between homo- and heterodimerization of the yeast homeodomain proteins MATa1...

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.