A NOVEL PROTEIN, CSG2P, IS REQUIRED FOR CA2-CEREVISIAE( REGULATION INSACCHAROMYCES)

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