Orchestrated actin nucleation by the Candida albicans polarisome complex enables filamentous growth

Abstract

Candida albicans is a dimorphic fungus that converts from a yeast form to a hyphae form during infection. This switch requires the formation of an actin cable to coordinate polarized cell growth. It’s known that nucleation of this cable requires a multiprotein complex localized at the tip called the polarisome, but the mechanisms underpinning this process were unclear. Here, we found that C. albicans Aip5, a homolog of polarisome component ScAip5 in S. cerevisiae that nucleates actin polymerization and synergizes with the formin ScBni1, regulates actin assembly and hyphae growth synergistically with other polarisome proteins Bni1, Bud6, and Spa2. The C-terminus of Aip5 binds directly to G-actin, Bni1 and the C-terminal of Bud6, which form the core of the nucleation complex to polymerize F-actin. Based on insights from structural biology and molecular dynamic simulations, we propose a possible complex conformation of the actin nucleation core, which provides cooperative positioning and supports the synergistic actin nucleation activity of a tri-protein complex Bni1-Bud6-Aip5. Together with known interactions of Bni1 with Bud6 and Aip5 in S. cerevisiae, our findings unravel molecular mechanisms of C. albicans by which the tri-protein complex coordinates the actin nucleation in actin cable assembly and hyphal growth, which is likely a conserved mechanism in different filamentous fungi and yeast.

The actin networks of chytrid fungi reveal evolutionary loss of cytoskeletal complexity in the fungal kingdom

“…48 We find Bni-type formins in all other dikaryotic fungi included in our analyses ( Figure 5A), as well as references to their assembly of actin cables. [49][50][51][52] We also find that some chytrids have at least one bni1-type formin, including Sp, whose sporangia we recently found can build actin cables within the cell body. 2 Other chytrids, including Bd and Bs, are missing these formins, an interesting finding given the lack of actin cables in the cell bodies of Bd zoospores and sporangia ( Figure 1).…”

Polarisome assembly mediates actin remodeling during polarized yeast and fungal growth

“…Furthermore, bni1Δ79-988 mutant cells, which lack the N-terminal Spa2-binding domain in Bni1, are synthetic lethal when Spa2 is also deleted, suggesting that Spa2 plays additional roles besides binding to Bni1 (Liu et al, 2012). Spa2 possibly also regulates the function of Bud6 and Aip5, the two polarisome-resident nucleation-promoting factors (NPFs), both of which synergistically promote the activation of the NF Bni1, which localizes to the barbed end of F-actin to initiate polymerization (Glomb et al, 2019;Moseley et al, 2004;Sheu et al, 1998;Xie et al, 2020Xie et al, , 2019. Apart from the Spa2-regulated localization of polarisome proteins, physical interactions between other polarisome members could also influence their localization.…”

“…Moreover, Spa2 SHD-V is also critical for the maintenance of a tip-localized pool of the NPF Aip5 through direct binding to the IDR of Aip5 (1-271 aa) (Glomb et al, 2019;Xie et al, 2019). The NPFs Bud6 and Aip5 promote the actin-nucleating activity of Bni1 without affecting F-actin elongation induced by profilin (Graziano et al, 2011;Moseley and Goode, 2005;Moseley et al, 2004;Xie et al, 2020Xie et al, , 2019. The Bni1 C-terminus (1767-1953 aa; Bni1-C) is an IDR that associates, with strong affinity, with the core domain of Bud6 (550-688 aa) and Upon receiving the polarization signal from GTP-Cdc42, the effectors Gic1 and Gic2 activate polarisome components, which then localize to the future bud tip in late G1.…”

“…bni1Δ cells show more substantial defects in polarity and bud emergence than bnr1Δ cells, suggesting that Bni1 is the major formin involved in these processes (Chen et al, 2012;Imamura et al, 1997;Vallen et al, 2000). Bni1mediated actin-cable assembly is also crucial for hyphal growth in filamentous fungi, such as C. albicans (Sudbery, 2011;Xie et al, 2020). Hyphal development and progressive elongation of filamentous fungi are highly associated with their pathogenicity during tissue invasion in mammals and plants, such as seen with C. albicans and Ashbya gossypii (Desai, 2018;Kohli et al, 2008;Noble et al, 2017;Sudbery, 2011).…”

“…Hyphal development and progressive elongation of filamentous fungi are highly associated with their pathogenicity during tissue invasion in mammals and plants, such as seen with C. albicans and Ashbya gossypii (Desai, 2018;Kohli et al, 2008;Noble et al, 2017;Sudbery, 2011). Hyphae formation is mainly driven by the assembly of the polarized actin cable and exocytosis (Steinberg, 2007;Xie et al, 2020). During subsequent maintenance of filamentous fungal growth, secretion vesicles are deposited at a region close to the hyphae tip that is rich in membrane compartments, named the Spitzenkörper (Jones and Sudbery, 2010).…”