Sterol-rich plasma membrane domains in fungi.

Concepts regarding the eukaryotic plasma membrane have been evolving in light of growing evidence that it is segregated into distinct lateral domains known as lipid rafts. These steroland sphingolipid-rich raft domains are thought to play important roles in dynamic processes, including protein sorting, cell polarity, and signal transduction. Because of this, it has been very intriguing that sterol-rich domains (SRDs) that are much larger than lipid rafts have been identified in the plasma membranes of fungi. The larger SRDs can be readily observed in cells stained with filipin, a naturally fluorescent sterol-binding antibiotic. These large SRDs are located at sites of active morphogenesis and are regulated in a developmental or cellcycle-regulated manner. For example, SRDs were detected at the tips of mating projections in Saccharomyces cerevisiae and hyphae in Candida albicans, but not in budding cells. In the fission yeast Schizosaccharomyces pombe, an SRD forms in a cell-cycle regulated manner in the medial zone of the rodshaped cells at the future site of cell division. SRDs have also been reported at sites of polarized morphogenesis in Cryptococcus neoformans and Aspergillus nidulans. These domains may be formed by the clustering of lipid rafts and may therefore also function as organizing centers in the membrane. In this review, we assess recent progress in the analysis of lipid rafts and large SRDs in fungi. Comparisons with other plasma membrane domains, including eisosomes and septin boundary domains, are made to examine how these specialized regions work together to contribute to polarized morphogenesis and cytokinesis.