Role of Actin-Bundling Protein Sac6 in Growth of Cryptococcus neoformans at Low Oxygen Concentration

Cryptococcus neoformans , the etiologic agent of cryptococcosis, is an obligately aerobic yeast that inhabits an environmental niche exposed to ambient air. The cell doubling time was significantly prolonged under 1% O2 relative to that under normoxic conditions. No apparent cell cycle arrest occurred following a shift from ambient air to 1% O2 . However, yeast cells became hypersensitive to the actin monomer-sequestering agent latrunculin A at 1% O2 , indicating that proper actin function is critical for growth at low oxygen concentrations. We showed that Sac6, an actin-binding protein, played an important role in cell growth under low oxygen conditions. Sac6 colocalized with cortical actin patches and with the ring structures between mother cells and buds. Under low oxygen conditions, thesac6 deletion mutant grew poorly, and accumulation of the actin capping protein Cap1 was observed in the vacuole of thesac6Δ strain. Furthermore, endocytic processes were hampered in thesac6Δ mutant, but cell polarity and cytokinesis were not visibly disturbed. The deficiency of endocytosis in thesac6Δ strain could be rescued by 1 M sorbitol under 1% O2 , but growth remained retarded. These results suggest an absence of a direct link inC. neoformans between endocytosis and coping with the stress of low oxygen conditions. This interpretation is further supported by the observation that deletion of three conserved genes,ABP1 ,CRN1 , andSLA2 , which play important roles in endocytosis, had no effect on growth under 1% O2 . Interestingly, deletion ofSAC6 inC. neoformans had no effect on virulence in mice.