Cytoplasmic membrane oligosaccharides

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... 

The presence and biological importance of oligosaccharide structures, usually as components of glycolipids and glycoproteins, in bacterial capsular and cell-wall polysaccharides, in mammalian cell membranes, in cytoplasm, and in extracellular fluids, are now well documented. They are important constituents in... 

Many proteins on the surface of the plasma membrane, and the majority of secreted proteins, contain oligosaccharide residues that are post-translationally added to the endoplasmic reticulum and in the Golgi apparatus (see p.230). By contrast, cytoplasmic proteins are rarely glycosylated. Glycoproteins can contain more than 50% carbohydrate however, the proportion of protein is generally much greater. 

Cell fractionation studies of five strains of cyanobacteria indicate that MAAs are located primarily (>90%) within the cytoplasm and not the cell sheaths, walls, or membranes.132 Extracellular placement of MAAs does occur in some cyanobacterial species that posses cellular sheath layers.134135 Extracellular MAAs are covalently bonded to oligosaccharide molecules embedded in the cyanobacterial sheath matrix and provide substantial protection to prevent photobleaching of chlorophyll within the cell. Intracellular or extracellular distributions of MAAs in eukaryotic cells have not been investigated. Based on the high MAA concentrations of Phaeocystis antarctica colonies, it has been suggested that MAAs are associated with the extracellular mucopolysaccharide matrix of the colony.125 This may be a more common phenomenon than currently recognized, and future research efforts will be necessary to further document extracellular occurrence of MAAs in cyanobacteria and algae. 

Fig. 1.2 Export of LPS and its precursors in E. coli. O-antigen oligosaccharides is assembled separately on undecaprenyl diphosphate, flipped from the cytoplasmic face to the periplasmic face of the inner membrane by the transporter Wzx, and polymerized on the periplasmic face of the inner membrane by Wzy and Wzz. The ABC transporter MsbA flips the core-lipid A from the inner surface to the outer surface of the inner membrane. Then the polymerized O-antigen is transferred to the core-lipid A in the periplasm by WaaL. The protein LptA, LptB, LptC, LptF and LptG might shuttle the nascent LPS from the periplasmic face of the inner membrane to the inner layer of the outer membrane. The outer membrane proteins LptD and LptE are required for the assembly of LPS into the outer surface of the outer membrane (Ma et al., 2008 Wu et al., 2006)...

The terminal phosphate group is the site of attachment of the achvated oligosaccharide, which is subsequently transferred to the protein acceptor. Dolichol phosphate resides in the ER membrane with its phosphate terminus on the cytoplasmic face. 

Figure 11.23. Assembly of an -linked oligosaccharide precursor on dolichol phosphate. The first stage of oligosaccharide synthesis takes place in the cytoplasm on the exposed phosphate of a membrane-embedded dolichol molecule. Synthesis of the precursor is completed in the lumen of the ER after flipping of the dolichol phosphate and attached oligosaccharide.

---