Molecular Mechanism of Cell Cycle Regulation of Pathogenic Fungi Cryptococcus neoformans

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Cryptococcus neoformans is an opportunistic pathogen, belongs to basidiomycetous fungus, and has unique properties in cell cycle progression. In standard liquid condition, doubling time of exponentially growing C. neoformans was 132+-16 min, and duration time of G1, S, G2, and M phases were found to be about 71, 18, 25, and 18 min, respectively. DNA synthesis started before bud emergence, and completed by the stage that the size of bud became 1/4 of the mother cell. The doubling time of daughter cells was about twice as that of the mother cells. The spindle pole body was located on the outer nuclear envelop and showed duplicated form in G1 to G2 phase. Cyclin-dependent kinase (Ddk) has been known to control cell cycle by changing conformation and biological functions. Cdk has been also known to bind to or separate from the molecules, expecially cyclins, and to change substrate specificity of the enzyme. In C.neoformans, Cdk and cyclins were cloned by ourselves, and protein-protein interaction profile between the two proteins were analyzed and constructed in silico. We are further analyzing structure-function relationship between C. neoformans Cdk and cyclins.C. neoformans is killed by the bacterium Staphylococcus aureus and this death is inhibited by soluble capsular polysaccharides. To investigate the mechanism of killing, cells in co-culture were examined under scanning and transmission electron microscopy. S. aureus attached to the capsule of C. neoformans, and the ultrastructure of the attached C. neoformans cells was characteristic of dead cells. We identified the molecules that contributed to the fungal-bacterial interaction : triosephosphate isomerase (TPI) on S. aureus adheres to the capsule of C. neoformans by recognizing the structure of mannotriose units in the backbone of GXM ; we suggest that this contact is required for killing of C. neoformans.