Candida albicans hyphae have a Spitzenkörper that is distinct from the polarisome found in yeast and pseudohyphae

Abstract

Fungi grow with a variety of morphologies: oval yeast cells, chains of elongated cells called pseudohyphae and long, narrow, tube-like filaments called hyphae. In filamentous fungi, hyphal growth is strongly polarised to the tip and is mediated by the Spitzenkörper, which acts as a supply centre to concentrate the delivery of secretory vesicles to the tip. In the budding yeast Saccharomyces cerevisiae, polarised growth is mediated by the polarisome, a surface cap of proteins that nucleates the formation of actin cables delivering secretory vesicles to the growing tip. The human fungal pathogen, Candida albicans, can grow in all three morphological forms. Here we show the presence of a Spitzenkörper at the tip of C. albicans hyphae as a ball-like localisation of secretory vesicles, together with the formin Bni1 and Mlc1, an ortholog of an S. cerevisiae myosin regulatory light chain. In contrast, in C. albicans yeast cells, pseudohyphae and hyphae Spa2 and Bud6, orthologs of S. cerevisiae polarisome components, as well as the master morphology regulator Cdc42, localise predominantly, but not exclusively, to a surface cap resembling the polarisome of S. cerevisiae yeast cells. A small amount of Cdc42 also localises to the Spitzenkörper. Furthermore, we show differences in the genetic and cytoskeletal requirements, and cell cycle dynamics of polarity determinants in yeast, pseudohyphae and hyphae. These results, together with the cytological differences between the cell types, suggest that the Spitzenkörper and polarisome are distinct structures, that the polarisome and Spitzenkörper coexist in hyphae, and that polarised growth in hyphae is driven by a fundamentally different mechanism to that in yeast and pseudohyphae.

Yeast and fungal morphogenesis from an evolutionary perspective

“…As this study did not take recycling processes into account this may even be an underestimation [65]. Markers localized to the spk in C. albicans include the endocytic dye FM4-64, the myosin light chain Mlc1 and the formin Bni1 [66]. The integrity of the spk is dependent on the actin cytoskeleton and on several proteins that interact with Bni1 [66].…”

“…Markers localized to the spk in C. albicans include the endocytic dye FM4-64, the myosin light chain Mlc1 and the formin Bni1 [66]. The integrity of the spk is dependent on the actin cytoskeleton and on several proteins that interact with Bni1 [66]. One possible interpretation of the spk is simply as the terminal site of secretory vesicle transport.…”

Regulation of polarised growth in fungi

“…In C. albicans hyphae Mlc1p, the regulatory light chain of Myo2p that provides the motive force for secretory vesicle transport, accumulates in a Spitzenkörper-like structure at the tip (Crampin et al, 2005). This suggests that the arrival of vesicles at the tip is more rapid than the rate of onward transport from the Spitzenkörper to the cell surface.…”

“…This suggests that the arrival of vesicles at the tip is more rapid than the rate of onward transport from the Spitzenkörper to the cell surface. Both microtubules and microfilaments appear to be required for polarised growth in C. albicans (Akashi et al, 1994;Crampin et al, 2005). However, they have different roles.…”

“…So presumably, the components that generate bud growth in S. cerevisiae are also capable of generating hyphae. In the case of the polymorphic pathogen Candida albicans, its genome encodes components that support yeast, pseudohyphal or hyphal growth, including the formation of a Spitzenkörper-like structure in hyphae (Berman and Sudbery, 2002;Sudbery et al, 2004;Crampin et al, 2005) (Fig. 3).…”