SUR1 (CSG1 / BCL21), a gene necessary for growth of Saccharomyces cerevisiae in the presence of high Ca²⁺ concentrations at 37° C, is required for mannosylation of inositolphosphorylceramide

Abstract

Saccharomyces cerevisiae cells require two genes, CSG1/SUR1 and CSG2, for growth in 50 mM Ca²⁺, but not 50 mM Sr²⁺. CSG2 was previously shown to be required for the mannosylation of inositol-phosphorylceramide (IPC) to form mannosylinositolphosphorylceramide (MIPC). Here we demonstrate that SUR1/CSG1 is both genetically and biochemically related to CSG2. Like CSG2, SUR1/CSG1 is required for IPC mannosylation. A 93–amino acid stretch of Csg1p shows 29% identity with the α-1, 6-mannosyltransferase encoded by OCH1. The SUR1/CSG1 gene is a dose-dependent suppressor of the Ca²⁺-sensitive phenotype of the csg2 mutant, but overexpression of CSG2 does not suppress the Ca²⁺ sensitivity of the csg1 mutant. The csg1 and csg2 mutants display normal growth in YPD, indicating that mannosylation of sphingolipids is not essential. Increased osmolarity of the growth medium increases the Ca²⁺ tolerance of csg1 and csg2 mutant cells, suggesting that altered cell wall synthesis causes Ca²⁺-induced death. Hydroxylation of IPC-C to form IPC-D requires CCC2, a gene encoding an intracellular Cu²⁺ transporter. Increased expression of CCC2 or increased Cu²⁺ concentration in the growth medium enhances the Ca²⁺ tolerance of csg1 mutants, suggesting that accumulation of IPC-C renders csg1 cells Ca²⁺ sensitive.