A Suppressor Mutation in the β-Subunit Kis1 Restores Functionality of the SNF1 Complex in Candida albicans snf4Δ Mutants

The heterotrimeric protein kinase SNF1 is a key regulator of metabolic adaptation in the pathogenic yeast Candida albicans, and mutants with a defective SNF1 complex cannot grow on carbon sources other than glucose. We identified a novel type of suppressor mutation in the β-subunit Kis1 that rescued the growth defects of cells lacking the regulatory γ-subunit Snf4 of the SNF1 complex. Unlike wild-type Kis1, the mutated Kis1(A396T) could bind to the catalytic α-subunit Snf1 in the absence of Snf4. Binding of Kis1(A396T) did not enhance phosphorylation of Snf1 by the upstream activating kinase Sak1, which is impaired in snf4Δ mutants. Nevertheless, the mutated Kis1(A396T) reestablished SNF1-dependent gene expression, confirming that SNF1 functionality was restored. The repressor proteins Mig1 and Mig2 were phosphorylated even in the absence of Snf1, but their phosphorylation patterns were altered, indicating that SNF1 regulates Mig1 and Mig2 activity indirectly. In contrast to wild-type cells, mutants lacking Snf4 were unable to reduce the amounts of Mig1 and Mig2 when grown on alternative carbon sources, and this deficiency was also remediated by the mutated Kis1(A396T). These results provide novel insights into the regulation of SNF1 and the repressors Mig1 and Mig2 in the metabolic adaptation of C. albicans. IMPORTANCE The highly conserved protein kinase SNF1 plays a key role in the metabolic adaptation of the pathogenic yeast Candida albicans, but it is not clear how it regulates its downstream targets in this fungus. We show that the repressor proteins Mig1 and Mig2 are phosphorylated also in cells lacking the catalytic α-subunit Snf1 of the SNF1 complex, but the amounts of both proteins were reduced in wild-type cells when glucose was replaced by alternative carbon sources, pointing to an indirect mechanism of regulation. Mutants lacking the regulatory γ-subunit Snf4 of the SNF1 complex, which cannot grow on alternative carbon sources, were unable to downregulate Mig1 and Mig2 levels. We identified a novel type of suppressor mutation, an amino acid substitution in the β-subunit Kis1, which enabled Kis1 to bind to Snf1 in the absence of Snf4, thereby restoring Mig1 and Mig2 downregulation, SNF1-dependent gene expression, and growth on alternative carbon sources. These results provide new insights into the SNF1 signaling pathway in C. albicans.