The
isoleucine and
valine biosynthetic
enzyme acetolactate synthase (Ilv2p) is an attractive antifungal
drug target, since the
isoleucine and
valine biosynthetic pathway is not present in mammals, Saccharomyces cerevisiae ilv2Delta mutants do not survive in vivo, Cryptococcus neoformans ilv2 mutants are avirulent, and both S. cerevisiae and Cr. neoformans ilv2 mutants die upon
isoleucine and
valine starvation. To further explore the potential of Ilv2p as an antifungal
drug target, we disrupted Candida albicans ILV2, and demonstrated that Ca. albicans ilv2Delta mutants were significantly attenuated in virulence, and were also profoundly
starvation-cidal, with a greater than 100-fold reduction in viability after only 4 h of
isoleucine and
valine starvation. As fungicidal
starvation would be advantageous for
drug design, we explored the basis of the
starvation-cidal phenotype in both S. cerevisiae and Ca. albicans ilv2Delta mutants. Since the mutation of ILV1, required for the first step of
isoleucine biosynthesis, did not suppress the ilv2Delta
starvation-cidal defects in either species, the cidal phenotype was not due to alpha-ketobutyrate accumulation. We found that
starvation for
isoleucine alone was more deleterious in Ca. albicans than in S. cerevisiae, and
starvation for
valine was more deleterious than for
isoleucine in both species. Interestingly, while the target of
rapamycin (TOR) pathway inhibitor
rapamycin further reduced S. cerevisiae ilv2Delta
starvation viability, it increased Ca. albicans ilv1Delta and ilv2Delta viability. Furthermore, the recovery from
starvation was dependent on the
carbon source present during recovery for S. cerevisiae ilv2Delta mutants, reminiscent of
isoleucine and
valine starvation inducing a viable but non-culturable-like state in this species, while Ca. albicans ilv1Delta and ilv2 Delta viability was influenced by the
carbon source present during
starvation, supporting a role for
glucose wasting in the Ca. albicans cidal phenotype.