The second STE12 homologue of Cryptococcus neoformans is MATa-specific and plays an important role in virulence

Cryptococcus neoformans STE12 alpha, a homologue of Saccharomyces cerevisiae STE12, exists only in MAT alpha strains. We identified another STE12 homologue, STE12a, which is MATa specific. As in the case with Delta ste12 alpha, the mating efficiency for Delta ste12a was reduced significantly. The Delta ste12a strains surprisingly still mated with Delta ste12 alpha strains. In MAT alpha strains, STE12a functionally complemented STE12 alpha for mating efficacy, haploid fruiting, and regulation of capsule size in the mouse brain. Furthermore, when STE12a was replaced with two copies of STE12 alpha, the resulting MATa strain produced hyphae on filament agar, STE12a regulates mRNA levels of several genes that are important for virulence including CNLAC1 and CAP genes. STE12a also modulates enzyme activities of phospholipase and superoxide dismutase. Importantly, deletion of STE12a markedly reduced the virulence in mice, as is the case with STE12 alpha. Brain smears of mice infected with the Delta ste12a strain showed yeast cells with a considerable reduction in capsule size compared with those infected with STE12a strains. When the disrupted locus of ste12a was replaced with a wild-type STE12a gene, both in vivo and in vitro mutant phenotypes were reversed. These results suggest that STE12a and STE12 alpha have similar functions, and that the mating type of the cells influences the alleles to exert their biological effects. C. neoformans, thus, is the first fungal species that contains a mating-type-specific STE12 homologue in each mating type. Our results demonstrate that mating-type-specific genes are not only important for saprobic reproduction but also play an important role for survival of the organism in host tissue.