A novel protein, CSG2p, is required for Ca²⁺ regulation in Saccharomyces cerevisiae

Nineteen mutants that lost the ability to grow in 100 mM Ca²⁺ (but remained insensitive to 50 mM Sr²⁺) were identified in a screen of approximately 60,000 mutagenized yeast colonies. Cells carrying mutations in the CSG2 gene grow normally in low Ca²⁺ medium but have decreased growth rates when the Ca²⁺ concentration is above 10 mM. The csg2 mutant cells accumulate much higher levels of Ca²⁺ in a compartment that is exchangeable with extracellular Ca²⁺ but the nonexchangeable Ca²⁺ pool which predominates in wild-type cells is not influenced. Sr²⁺ influx is not increased in the csg2 mutant cells. Mg²⁺ decreases the amount of Ca²⁺ in the non-exchangeable pool without influencing the csg2-induced exchangeable Ca²⁺ pool. The data indicate that the csg2 mutation causes a selective increase in Ca²⁺ accumulation into a pool which is distinct from the vacuolar pool. The CSG2 protein consists of 410 amino acids, contains nine putative transmembrane segments, four potential sites for N-linked glycosylation, and a sequence with homology to the EF-hand Ca²⁺-binding site.