The formin family protein CaBni1p has a role in cell polarity control during both yeast and hyphal growth inCandida albicans

Abstract

Formins are conserved eukaryotic proteins playing key roles in regulating cell polarity. We have characterized the roles of a formin CaBni1p in the polymorphic fungus Candida albicans. CaBni1p localized persistently to hyphal tips during hyphal growth but to distinct growth sites at different cell cycle stages during yeast growth. Cabni1Δ yeast cells exhibited several morphological defects, such as round and enlarged cells, widened bud necks and a random budding pattern. Although Cabni1Δ cells could still undergo yeast-hypha growth switch, the hyphae were markedly swollen. Cabni1Δ also showed defects in spindle and cytoplasmic microtubule orientation and positioning. Coincidentally, the spindle orientation protein CaKar9p in Cabni1Δ yeast cells appeared as multiple random cortical spots, in contrast to the single spot at the bud tip of many wild-type cells. Interestingly, several defects manifested in Cabni1Δ yeast cells were partially corrected during hyphal growth. We found that the second formin CaBnr1p was recruited to hyphal tips, while it localized only to the bud neck during yeast growth. This behavior of CaBnr1p may play a key role in correcting Cabni1Δ defects during hyphal growth. Cabni1Δ exhibited reduced virulence in mice. These results indicate that the formins play an important role in Candida albicans polarized growth and CaBni1p's function is required for virulence.

Yeast-to-Hyphal Transition Triggers Formin-dependent Golgi Localization to the Growing Tip inCandida albicans

“…However, spatial differences cannot fully explain the different hyphal phenotypes of C. albicans bni1⌬ and bnr1⌬ null mutants. CaBni1 performs unique functions during hyphal formation, because bnr1⌬ null mutants do not display severe hyphal phenotypes, but bni1⌬ cells do (Li et al, 2005;Martin et al, 2005). Moreover, like Bni1, Bnr1 is also localized at the hyphal tip in C. albicans and actin cables emanating from the hyphal tip persist in the bni1 null mutant (Martin et al, 2005).…”

“…Bni1 and its partially redundant homologue Bnr1 are required for actin nucleation and linear cable assembly at sites of polarized growth Sagot et al, 2002). Unlike bnr1 mutants, in which hyphal formation is normal, C. albicans bni1 mutants still undergo the yeast-tohyphal switch, but they exhibit severe polarity defects, forming short, swollen true hyphae that are kinetically delayed (Li et al, 2005;Martin et al, 2005). Despite these hyphal defects, bni1 null mutants are unaffected in budding and germination.…”

“…As in S. cerevisiae, C. albicans encodes two partially redundant formins, CaBni1 and CaBnr1. Simultaneous loss of both formins leads to lethality, and even though both proteins can be found at the hyphal tip, Bni1, but not Bnr1, is required for normal hyphal progression (Li et al, 2005;Martin et al, 2005).…”

“…As in S. cerevisiae, C. albicans encodes two partially redundant formins, CaBni1 and CaBnr1. Simultaneous loss of both formins leads to lethality, and even though both proteins can be found at the hyphal tip, Bni1, but not Bnr1, is required for normal hyphal progression (Li et al, 2005;Martin et al, 2005).Although budding in S. cerevisiae has proved to be a useful paradigm for studying polarized growth, several observations suggest that there are some fundamental differences between the regulation of budding in S. cerevisiae and hyphal formation in C. albicans (for review, see Sudbery et al, 2004). First, budding in S. cerevisiae is tightly coupled to the cell cycle (Lew and Reed, 1995), whereas in C. albicans the initiation and elongation of hyphae is regulated independently of the cell cycle (Hazan et al, 2002).…”

From Function to Shape: A Novel Role of a Formin in Morphogenesis of the FungusAshbya gossypii

“…After submission of this manuscript, two other fungal formins, CaBni1p and CaBnr1p, were shown to play an important but nonessential role in yeast and hyphal growth of Candida albicans (Li et al, 2005).…”