Description:
<jats:title>ABSTRACT</jats:title>
<jats:p>
Adaptation to alternating CO
<jats:sub>2</jats:sub>
concentrations is crucial for all organisms. Carbonic anhydrases—metalloenzymes that have been found in all domains of life—enable fixation of scarce CO
<jats:sub>2</jats:sub>
by accelerating its conversion to bicarbonate and ensure maintenance of cellular metabolism. In fungi and other eukaryotes, the carbonic anhydrase Nce103 has been shown to be essential for growth in air (~0.04% CO
<jats:sub>2</jats:sub>
). Expression of
<jats:italic>NCE103</jats:italic>
is regulated in response to CO
<jats:sub>2</jats:sub>
availability. In
<jats:italic>Saccharomyces cerevisiae</jats:italic>
,
<jats:italic>NCE103</jats:italic>
is activated by the transcription factor ScCst6, and in
<jats:italic>Candida albicans</jats:italic>
and
<jats:italic>Candida glabrata</jats:italic>
, it is activated by its homologues CaRca1 and CgRca1, respectively. To identify the kinase controlling Cst6/Rca1, we screened an
<jats:italic>S. cerevisiae</jats:italic>
kinase/phosphatase mutant library for the ability to regulate
<jats:italic>NCE103</jats:italic>
in a CO
<jats:sub>2</jats:sub>
-dependent manner. We identified ScSch9 as a potential ScCst6-specific kinase, as the
<jats:italic>sch9</jats:italic>
Δ mutant strain showed deregulated
<jats:italic>NCE103</jats:italic>
expression on the RNA and protein levels. Immunoprecipitation revealed the binding capabilities of both proteins, and detection of ScCst6 phosphorylation by ScSch9
<jats:italic>in vitro</jats:italic>
confirmed Sch9 as the Cst6 kinase. We could show that CO
<jats:sub>2</jats:sub>
-dependent activation of Sch9, which is part of a kinase cascade, is mediated by lipid/Pkh1/2 signaling but not TORC1. Finally, we tested conservation of the identified regulatory cascade in the pathogenic yeast species
<jats:italic>C. albicans</jats:italic>
and
<jats:italic>C. glabrata</jats:italic>
. Deletion of
<jats:italic>SCH9</jats:italic>
homologues of both species impaired CO
<jats:sub>2</jats:sub>
-dependent regulation of
<jats:italic>NCE103</jats:italic>
expression, which indicates a conservation of the CO
<jats:sub>2</jats:sub>
adaptation mechanism among yeasts. Thus, Sch9 is a Cst6/Rca1 kinase that links CO
<jats:sub>2</jats:sub>
adaptation to lipid signaling via Pkh1/2 in fungi.
</jats:p>
<jats:p>
<jats:bold>IMPORTANCE</jats:bold>
All living organisms have to cope with alternating CO
<jats:sub>2</jats:sub>
concentrations as CO
<jats:sub>2</jats:sub>
levels range from very low in the atmosphere (0.04%) to high (5% and more) in other niches, including the human body. In fungi, CO
<jats:sub>2</jats:sub>
is sensed via two pathways. The first regulates virulence in pathogenic yeast by direct activation of adenylyl cyclase. The second pathway, although playing a fundamental role in fungal metabolism, is much less understood. Here the transcription factor Cst6/Rca1 controls carbon homeostasis by regulating carbonic anhydrase expression. Upstream signaling in this pathway remains elusive. We identify Sch9 as the kinase controlling Cst6/Rca1 activity in yeast and demonstrate that this pathway is conserved in pathogenic yeast species, which highlights identified key players as potential pharmacological targets. Furthermore, we provide a direct link between adaptation to changing CO
<jats:sub>2</jats:sub>
conditions and lipid/Pkh1/2 signaling in yeast, thus establishing a new signaling cascade central to metabolic adaptation.
</jats:p>