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 inositolphosphorylceramide (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 alpha-1, 6-mannosyltransferase encoded by OCH1. The SUR1/CSG1 gene is a dose-dependent suppressor of the Ca(2+)-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 Ca(2+)-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.

Inactivation of CaMIT1 Inhibits Candida albicans Phospholipomannan β-Mannosylation, Reduces Virulence, and Alters Cell Wall Protein β-Mannosylation

“…Mass spectrometry confirmed these results and showed that the biosynthetic pathway was interrupted upstream between IPC and MIPC. Despite slight differences between S. cerevisiae and C. albicans phytosphingosine and fatty acid chains already observed with another C. albicans strain, these results confirm that the C. albicans gene MIT1, like its homologue SUR1 in S. cerevisiae (18), is responsible for the addition of mannose to inositol of IPCs. As expected, inactivation of MIT1 prevented further addition of inositol phosphate leading to the synthesis of M(IP) 2 C. This inactivation also prevented the addition of mannose phosphate, the first step in the extensive mannosylation of sphingolipids deduced from complete elucidation of the…”

“…Morphogenesis was not affected by the deletion, including the yeast-to-hyphal transition considered to be a pathogenic trait of C. albicans. However, as observed for sur1⌬ in S. cerevisiae (18), the mutant was more sensitive to calcium. This result, suggesting a plasma membrane or cell wall defect, was confirmed by increased sensitivity of the strain to SDS.…”

“…1, A and B). Among the genes involved in the S. cerevisiae MIPC biosynthetic pathway, SUR1 is responsible for mannosylation of the IPC core (18), and its inactivation in S. cerevisiae results in viable mutants (18). It was considered that biosynthesis of PLM in C. albicans uses the MIPC biosynthetic pathway described in S. cerevisiae and that inactivation of the C. albicans homologue of SUR1 could lead to C. albicans viable mutants defective in PLM biosynthesis (Fig.…”

“…In this study, elucidation of the structure of PLM and complete sequencing of the S. cerevisiae and C. albicans genomes enabled the identification of a candidate gene in the MIPC pathway, an analogue of SUR1 in S. cerevisiae (18), which was named MIT1 in C. albicans. Inactivation of this gene prevented PLM ␤-mannosylation.…”

Biochemistry, cell biology and molecular biology of lipids ofSaccharomyces cerevisiae

“…This reaction requires two genes, SUR1 24 and CSG2. 303 IPC accumulates in the absence of either gene.…”