During fungal cell division, cytokinesis is followed by the formation of a cross-wall, the septum, which is made of chitin and acts as a stabilising barrier between the mother and bud in yeast cells, and between compartments in hyphae. There is an absolute requirement for chitin in the synthesis of the septum for growth and viability of all fungi. However, surprisingly little is known about the mechanism of septum formation in fungi. A thorough understanding of the process of fungal septation and how it is regulated may therefore lead to the identification of tractable drug targets for future antifungal chemotherapeutic strategies.
Candida albicans is the major fungal pathogen of humans and can grow in both a yeast and hyphal form. During cell division, a primary chitinous septum is synthesised by four chitin synthase (Chs) enzymes, Chs1, Chs2, Chs3 and Chs8, all four of which are localised to sites of septation prior to cytokinesis. To understand how the four Chs enzymes work together to synthesise chitin at division sites, strains expressing pairs of fluorescently tagged Chs enzymes were constructed. These were imaged by live-cell fluorescence microscopy to elucidate the temporal and spatial distribution of the chitin synthases in relation to each other. GFP pull-downs were carried out to identify proteins which interact with the Chs enzymes and contribute to septum formation.
We demonstrate that the timing of recruitment to and the configuration of the Chs enzymes at septation sites is different in C. albicans yeast and hyphal cells, and that the Chs enzymes may interact with different sets of proteins in yeast and hyphal cells. Based on this information, we propose a new model for chitin synthesis during septation in C. albicans yeasts and hyphae which may be used to inform our understanding of septation in all fungi and allow us to exploit these essential processes in order to combat fungal infections.