A possible mechanism for partitioning between homo- and heterodimerization of the yeast homeodomain proteins MATa1 and MATalpha2.

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.     

On the origins of congenic MATalpha and MATa strains of the pathogenic yeast Cryptococcus neoformans.

The basidiomycetous yeast Cryptococcus neoformans infects humans and causes a meningoencephalitis that is uniformly fatal if untreated. The organism has a defined sexual cycle involving mating of haploid MATa and MATalpha strains, gene disruption by transformation and homologous recombination is now readily accomplished, and robust animal models for infection have been well established. In addition, a pair of congenic MATalpha and MATa haploid strains have been constructed that permit detailed studies on physiology and virulence by classical genetic approaches. These strains represent a valuable resource for further studies in this organism, and the genomic sequence of one of these strains, JEC21 (=B-4500), was recently chosen to be sequenced by an international consortium. Because of the importance of these strains for genetic studies in C. neoformans and the fact that the genomic sequence of one of these strains is in progress, we review here how these congenic strains were originally constructed.     

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.     

Isolation and characterization of the HO gene from the yeast Saccharomyces paradoxus

A DNA fragment homologous to the homothallism (HO) gene of Saccharomyces cerevisiae was isolated from Saccharomyces paradoxus and was found to contain an open reading frame that was 90.9% identical to the coding sequence of the S. cerevisiae HO gene. The putative HO gene was shown to induce diploidization in a heterothallic haploid strain from S. cerevisiae. Phylogenetic analysis revealed that the coding and 5'-upstream regulatory regions from five Saccharomyces sensu stricto HO genes have coevolved, and that S. paradoxus is phylogenetically closer to S. cerevisiae than to S. bayanus. Finally, heterothallic haploid strains were isolated from the original homothallic type strain of S. paradoxus by disrupting the S. paradoxus HO gene with the S. cerevisiae URA3 gene.