In Saccharomyces cerevisiae rafts localize to the tips of pheromone-induced mating projections and might contribute to their growth, so a role for rafts in the polarized growth of other fungi seemed likely. C. albicans is a superb model for cell growth and morphogenesis because it has different cell morphologies — buds, pseudohyphae and hyphae — that are readily manipulated in vitro. Switching between buds and hyphae is also clinically relevant because hyphae can invade tissues and C. albicans virulence factors are hyphal-specific.
Using fluorescence microscopy and a fluorescent antibiotic (filipin) that binds to the main membrane sterol (ergosterol) Martin and Konopka observed intense staining at the tips of C. albicans hyphae, indicating that membrane lipids are enriched to form rafts. Tip morphology does not have a role in providing a scaffold to recruit lipids into rafts because the rafts formed even when tip shape was altered by the inhibition of hyphal growth. Germ-tube switching — the sequential formation of germ tubes that abort growth before a mature hypha is produced — can occur in C. albicans. Rafts were only found at the actively growing hyphal tip after germ-tube switching, so rafts only participate in active polar growth. Detergent-resistant membranes could be isolated from buds and hyphae, but raft polarization was again only detected at hyphal tips. Finally, monitoring raft location during hyphal growth from unbudded cells confirmed that rafts were only present in actively growing sites, including regions of hyphal growth initiation, the hyphal tip and at the presumptive septum just prior to septum formation.
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