UCSF

Candida albicans is an opportunistic fungal pathogen of humans. Among the many factors contributing to its virulence is the ability to switch between single cell (budding yeast) and filamentous (hyphal and pseudohyphal) forms. Hyphae are chains of long cells with parallel cell walls and are distinguished from pseudohyphae - chains of elongated ellipsoid cells - by highly polarized apical growth. Within hyphae, the tip (apical) cells are distinguished from sub-apical (non-tip) cells by a number of molecular and morphological features that contribute both to maintenance of hyphal polarity and to specialized hyphal function, including invasive and directional growth. The mechanisms that control this hyphal tip cell specification are incompletely understood.

This study addresses the role of RNA transport - a widespread mechanism for generating cellular polarity - in hyphal tip cell specification and hyphal development. Specifically, we characterized a She3-dependent RNA transport system in C. albicans. We used immunoprecipitation of She3-RNA complexes and whole genome microarray analysis to identify transported mRNAs, and we applied fluorescent in situ hybridization (FISH) techniques to assess these transcripts' sub-cellular localization. To assess the role of RNA transport in hyphal formation and function, we analyzed the phenotypes associated with loss of She3 and with loss of individual She3-associated transcripts.

We found that C. albicans She3 binds a set of 40 mRNAs and transports these transcripts to the buds of yeast form cells and to the tips of hyphae. The proteins encoded by She3-associated transcripts are predicted to act in a variety of cellular processes, including mitosis, cytoskeletal dynamics, transcription, and small molecule transport. Interestingly, there is little overlap between the sets of transcripts bound by She3 in C. albicans and Saccharomyces cerevisiae, a closely related fungus, suggesting that the transport system has undergone relatively rapid evolution. Inactivation of C. albicans She3 is associated with specific hyphal defects, including reduced invasive growth and decreased capacity to damage an epithelial cell monolayer. Disruption of individual cargo mRNAs produced a range of filamentation defects, suggesting that the associated genes make diverse contributions to hyphal development.