Oligosaccharides lipid carrier

Human-leukemic antigen (HLA-DR) has been found in remarkably low proportions on the plasma membrane of human, lymphoblastoid cells. In the presence of tunicamycin, both subunits of this cell-surface antigen showed lower apparent molecular weights. One of them completely lost its [3H]-labelled 2-acetamido-2-deoxy-D-glucose portion, implying that HLA-DR antigens possess oligosaccharides whose synthesis is lipid carrier-dependent.555... 

N-Linked Oligosaccharides Utilize a Lipid Carrier in the Early Stages of Synthesis... 

Initiation includes synthesis of the oligosaccharide-lipid precursor and transfer of the oligosaccharide to a protein. Evidence for a possible role for retinol (vitamin A) as a carrier in this step is discussed in Chapter 38. 

Oligosaccharide chains linked by 0-glycosidic bonds to seryl or threonyl residues show more structural variability than do the asparagine-linked oligosaccharides the serine (threonine)-linked oligosaccharides range in size from 1 to 20 or more monosaccharide residues. The residues are added one at a time, directly on the protein, rather than preassembled on a lipid carrier. Assembly is not random, however, and the glycosyltransferases involved have acceptor specificities that render certain structures preferable. 

At the Man5GlcNAc2-PP-Dol stage of synthesis, the lipid carrier undergoes a flip-flop, such that the oligosaccharide portion now becomes oriented in the lumen of the and four additional mannoses, as... 

The attachment of D-glucose to a D-manno-oligosaccharide lipid intermediate from Nil 8 fibroblasts seems to serve an important role in the transfer of the oligosaccharide from the lipid carrier to the protein acceptor, since a related D-glucose-free oligosaccharide is transferred from its lipid intermediate to protein at a significantly reduced rate. ... 

A central tenet of the above scheme of O-side chain biosynthesis, is that oligosaccharides linked to lipid carriers traverse the cytoplasmic membrane. There is little direct evidence that this occurs it being difficult to localise such an intermediate state. Some support for the hypothesis comes from the observation that the antibiotic bacitracin, which specifically interacts with the lipid carrier, inhibits O-side chain polymerisation. However, many consider the transfer of the oligosaccharide from one leaflet of the membrane to the other to pose insurmountable problems. To overcome this difficulty it has been postulated that nucleotide sugars are translocated through the membrane into the periplasmic space, where... 

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