Candida albicans Sfl1 Suppresses Flocculation and Filamentation

Abstract

Hyphal morphogenesis in Candida albicans is regulated by multiple pathways which act by either inducing or repressing filamentation. Most notably, Tup1, Nrg1, and Rfg1 are transcriptional repressors, while Efg1, Flo8, Cph1, and Czf1 can induce filamentation. Here, we present the functional analysis of CaSFL1, which encodes the C. albicans homolog of the Saccharomyces cerevisiae SFL1 (suppressor of flocculation) gene. Deletion of CaSFL1 results in flocculation (i.e., the formation of clumps) of yeast cells, which is most pronounced in minimal medium. The flocs contained hyphae already under noninducing conditions, and filamentation could be enhanced with hypha-inducing cues at 37°C. Expression of SFL1 in a heterozygous mutant under the control of the CaMET3 promoter was shown to complement these defects and allowed switching between wild-type and mutant phenotypes. Interestingly, increased expression of SFL1 using a MET3prom-SFL1 construct prior to the induction of filamentation completely blocked germ tube formation. To localize Sfl1 in vivo, we generated a SFL1-GFP fusion. Sfl1-green fluorescent protein was found in the nucleus in both yeast cells and, to a lesser extent, hyphal cells. Using reverse transcription-PCR, we find an increased expression of ALS1, ALS3, HWP1, ECE1, and also FLO8. Our results suggest that Sfl1 functions in the repression of flocculation and filamentation and thus represents a novel negative regulator of C. albicans morphogenesis.

A comprehensive functional portrait of two heat shock factor-type transcriptional regulators involved in Candida albicans morphogenesis and virulence

“…ScSfl1p has dual activator/repressor functions, acting as a transcriptional repressor of flocculation-related genes and as an activator of stress-responsive genes [35,36]. Interestingly, the C. albicans genome encodes two structural homologs of ScSfl1p, namely Sfl1p and Sfl2p [37][38][39][40]. Either SFL1 or SFL2 functionally complement an S. cerevisiae sfl1 mutation [38,39] and encode important regulators of morphogenesis and virulence in C. albicans [37][38][39][40].…”

“…Interestingly, the C. albicans genome encodes two structural homologs of ScSfl1p, namely Sfl1p and Sfl2p [37][38][39][40]. Either SFL1 or SFL2 functionally complement an S. cerevisiae sfl1 mutation [38,39] and encode important regulators of morphogenesis and virulence in C. albicans [37][38][39][40]. Intriguingly, although sharing structural homologies, Sfl1p and Sfl2p have antagonistic functions: while Sfl1p acts as a negative regulator of hyphal development, Sfl2p acts as a positive regulator of this process [37][38][39][40].…”

“…Either SFL1 or SFL2 functionally complement an S. cerevisiae sfl1 mutation [38,39] and encode important regulators of morphogenesis and virulence in C. albicans [37][38][39][40]. Intriguingly, although sharing structural homologies, Sfl1p and Sfl2p have antagonistic functions: while Sfl1p acts as a negative regulator of hyphal development, Sfl2p acts as a positive regulator of this process [37][38][39][40]. Functional analyses of C. albicans Sfl1p showed that deletion of SFL1 promoted filamentous growth and cell flocculation and correlated with induction of HSGs (ECE1, HWP1) and genes involved in cell adhesion (ALS1, ALS3), whereas its overexpression inhibited hyphal formation [37,38].…”

“…Intriguingly, although sharing structural homologies, Sfl1p and Sfl2p have antagonistic functions: while Sfl1p acts as a negative regulator of hyphal development, Sfl2p acts as a positive regulator of this process [37][38][39][40]. Functional analyses of C. albicans Sfl1p showed that deletion of SFL1 promoted filamentous growth and cell flocculation and correlated with induction of HSGs (ECE1, HWP1) and genes involved in cell adhesion (ALS1, ALS3), whereas its overexpression inhibited hyphal formation [37,38]. Consistent with a transcriptional regulatory function, an Sfl1p-GFP fusion localized to the nucleus, while one hybrid lacZ reporter analyses in C. albicans correlated with a repressor function [37].…”

“…Functional analyses of C. albicans Sfl1p showed that deletion of SFL1 promoted filamentous growth and cell flocculation and correlated with induction of HSGs (ECE1, HWP1) and genes involved in cell adhesion (ALS1, ALS3), whereas its overexpression inhibited hyphal formation [37,38]. Consistent with a transcriptional regulatory function, an Sfl1p-GFP fusion localized to the nucleus, while one hybrid lacZ reporter analyses in C. albicans correlated with a repressor function [37]. Importantly, either deletion or overexpression of SFL1 attenuated C. albicans virulence in a mouse model of systemic infection [38].…”