The Two-Component Signal Transduction Protein Chk1p Regulates Quorum Sensing in Candida albicans

Abstract

Regulation of hyphal morphogenesis in Candida albicans can occur through quorum sensing (QS). A QS signal, farnesol, is produced during high-density growth and inhibits morphogenesis. However, the signal transduction pathway that regulates QS is unknown. Here, we show that a C. albicans mutant lacking Chk1p but not either the Sln1p or the Nik1p histidine kinase is refractory to the inhibitory effect of farnesol both in cell suspension and during the formation of a biofilm. This study is the first to demonstrate a role for a two-component signal transduction protein in QS by a eukaryotic organism.Cell density is a critical factor in the regulation of Candida albicans hyphal morphogenesis. At a density of Ͼ10 6 cells/ml, yeast cells do not shift (germinate) to hyphae or do so at low frequencies, while at a density of Ͻ10 6 cells/ml, germination occurs (3). The relationship between cell density and new gene transcription (hyphal morphogenesis) resembles quorum sensing (QS) in some bacteria (14). Recent observations indicate that a QS system operates in C. albicans and that the isoprenoid farnesol is the QS autoinducer signal (12). Cells exposed to farnesol do not germinate, even at low cell densities. However, the regulatory and signal transduction events that direct QS are unknown, not only for C. albicans but for other fungi and eukaryotes in general. In some bacteria, two-component signaling regulates QS. Since C. albicans has several two-component signal proteins that are critical to a number of processes, including cell wall biosynthesis, adaptation to stress conditions, and virulence, our rationale was that farnesol sensing could be mediated through two-component proteins. C. albicans has three hybrid-histidine kinases, two of which have orthologues in Saccharomyces cerevisiae (Sln1p) and Neurospora crassa (Nik1p) that are presumed to play a role in an osmotic stress response (1,15,19,20). The third histidine kinase, Chk1p, has some similarity to two Schizosaccharomyces pombe proteins, Mak2p and Mak3p, which are known to function as sensors for oxidative stress (2, 5). In addition to the histidine kinases, C. albicans has two response regulator proteins, Ssk1p and Skn7p, whose S. cerevisiae homologs act downstream of the Sln1p histidine kinase (11). In C. albicans, Ssk1p and Skn7p function in the adaptation of cells to oxidant stress, while Ssk1p, in addition, regulates the expression of structural cell wall proteins and negatively regulates Chk1p expression (7, 18).The C. albicans strains used for this study have been described previously (4, 6, 9, 20). Unless noted, cells were routinely cultured in YPD (1% yeast extract, 2% dextrose, 2% peptone) or YNB (0.67% yeast nitrogen base [pH 7.0], 50 mM glucose) at 30°C. To assess whether the two-component signal transduction proteins of C. albicans play a role in QS, all strains (see Table 1) were first cultured overnight at 30°C in YPD. Subsequently, the cells were washed twice and then inoculated into 10 ml of prewarmed medium 199 (pH 7.5) with or without 250 M t...

Environmental Sensing and Signal Transduction Pathways Regulating Morphopathogenic Determinants of Candida albicans

“…How farnesol is sensed and how the signal is transmitted to this MAPK pathway remains to be determined. Interestingly, C. albicans mutants lacking the histidine kinase Chk1 (see below) are refractory to the inhibitory effect of farnesol both in cell suspension and during the formation of a biofilm (158). This indicates a role of two-component signal transduction proteins in quorum sensing and as upstream components of the Cek1 MAPK pathway.…”

Quorum sensing controls hyphal initiation in Candida albicans through Ubr1-mediated protein degradation

“…The addition of farnesol blocks hyphal formation by WT cells but not chk1 null mutants (7). However, farnesol could still repress SOK1 expression in a chk1 mutant, like in WT cells or the stp3 mutant (Fig.…”

Talking to Themselves: Autoregulation and Quorum Sensing in Fungi

“…The synthesis of biologically active fluorescent analogs of farnesol was used to show that farnesol derivatives localize to the cytoplasmic and nuclear membranes (84). Kruppa et al (40) found that C. albicans mutants defective in the Chk1 histidine kinase, which has similarity to Schizosaccharomyces pombe Mak2p and Mak3p oxidative stress response proteins, are resistant to farnesol suppression of filamentation and biofilm formation. Sato et al (79) showed that the mitogen-activated protein (MAP) kinase cascade that governs filamentation under nutrient-limiting conditions is repressed upon addition of farnesol.…”