Reversible defects in O-linked glycosylation and LDL receptor expression in a [formula omitted] 4-epimerase deficient mutant

Abstract

We previously isolated an unusual hamster cell mutant ( IdID) that does not express LDL receptor activity unless it is cocultivated with other cells or grown in high concentrations of serum. We now show that IdID cells are deficient in the enzyme UDP-galactose and UDP-N-acetylgalactosamine (GaINAc) 4-epimerase. When IdID cells are grown in glucose-based media, they cannot synthesize enough UDP-galactose and UDP-GaINAc to allow normal synthesis of glycolipids and glycoproteins. The 4-epimerase deficiency accounts for all glycosylation defects previously observed in IdID cells, including production of abnormal LDL receptors. All abnormal phenotypes of IdID cells can be fully corrected by exogenous galactose and GaINAc. The separate effects of these sugars on LDL receptor activity suggest that O-linked carbohydrate chains are crucial for receptor stability. IdID cells may be useful for structural and functional studies of many proteins, proteoglycans, and glycolipids containing galactose or GaINAc.