Abstract
This chapter summarizes the properties of the mammalian cell mutant ldlD. It reviews the recent experiments that exploit its novel characteristics and provide suggestions for the experimental manipulation of these cells. Classic biochemical genetics combined with recombinant-DNA technology provides a powerful approach for identifying and characterizing the genes, gene products, and biochemical and cellular functions that underlie the multicompartmental pathways. Seven recessive complementation groups (ldlA–ldlG) are identified, which define genes required for the normal expression of LDL receptor activity. The ldlB, ldlC, and ldlD mutants exhibit global, pleiotropic glycosylation defects affecting protein- and lipid-linked oligosaccharides. ldlD cells were initially isolated in selections and screens were designed to identify Chinese hamster ovary (CHO) mutants expressing defects in the endocytosis of LDL. The defective synthesis of N-linked, 0-linked, and lipid-linked glycoconjugates and the LDL receptor deficiency of ldlD cells are because of a virtual absence of UDP-Gal/UDP-GalNAc 4-epimerase enzymatic activity. This enzyme catalyzes the reversible isomerizations of UDP-glucose to UDP-galactose (Gal) and UDP-N-acetylglucosamine to UDP-N-acetylgalactosamine (GalNAc).