Fungal homoserine kinase (thr1δ) mutants are attenuated in virulence and die rapidly upon threonine starvation and serum incubation

Abstract

The fungally conserved subset of amino acid biosynthetic enzymes not present in humans offer exciting potential as an unexploited class of antifungal drug targets. Since threonine biosynthesis is essential in Cryptococcus neoformans, we further explored the potential of threonine biosynthetic enzymes as antifungal drug targets by determining the survival in mice of Saccharomyces cerevisiae homoserine kinase (thr1δ) and threonine synthase (thr4δ) mutants. In striking contrast to aspartate kinase (hom3δ) mutants, S. cerevisiae thr1and thr4δmutants were severely depleted after only 4 h in vivo. Similarly, Candida albicans thr1δmutants, but not hom3δmutants, were significantly attenuated in virulence. Consistent with the in vivo phenotypes, S. cerevisiae thr1δand thr4δmutants as well as C. albicans thr1δ mutants were extremely serum sensitive. In both species, serum sensitivity was suppressed by the addition of threonine, a feedback inhibitor of Hom3p. Because mutation of the HOM3 and HOM6 genes, required for the production of the toxic pathway intermediate homoserine, also suppressed serum sensitivity, we hypothesize that serum sensitivity is a consequence of homoserine accumulation. Serum survival is critical for dissemination, an important virulence determinant: thus, together with the essential nature of C. neoformans threonine synthesis, the cross-species serum sensitivity of thr1δ mutants makes the fungus-specific Thr1p, and likely Thr4p, ideal antifungal drug targets. Copyright © 2010, American Society for Microbiology.