CaRch1p does not functionally interact with the high-affinity Ca(2+) influx system (HACS) of Candida albicans

The plasma membrane protein CaRch1p of Candida albicans, homologous to the human solute carrier protein SLC10A7, is involved in the regulation of calcium homeostasis. C. albicans cells lacking CaRCH1 are hypersensitive to high extracellular Ca(2+) concentrations and show increased tolerance to ketoconazole (KCZ). We assume a higher basal Ca(2+) influx in the rch1/rch1 mutant strain at low extracellular Ca(2+) concentrations, which is not detrimental to C. albicans cells but may be sufficient to activate calcineurin, finally resulting in an increased tolerance to KCZ. However, at 8 µg/ml KCZ plus 3 mm Ca(2+) the rch1/rch1 mutant and the wild-type strains showed identical growth. By further increasing the Ca(2+) concentration to 30 mm, this phenotype was completely reversed and the rch1/rch1 mutant strain became extremely sensitive to 8 µg/ml KCZ, probably due to synergistic toxic effects of Ca(2+) and KCZ under these conditions. Furthermore, we aimed to clarify whether CaRch1p interacts with the Cch1p component of the voltage-gated calcium influx channel Cch1p/Mid1p in C. albicans cells. As disruption of the two alleles of CCH1 in the rch1/rch1 mutant strain did not alter its hypersensitivity to high extracellular Ca(2+) , and as this phenotype was completely abolished by low amounts of Mg(2+) in the rch1/rch1 mutant as well as in the cch1/cch1 rch1/rch1 double mutant, we conclude that CaRch1p is a functional component of the low-affinity calcium uptake system (LACS) system and does not functionally interact with Cch1p.