Plasma membrane carbohydrates

In addition to binding to sialic acid residues of the carbohydrate side chains of cellular proteins that the virus exploits as receptors, hemagglutinin has a second function in the infection of host cells. Viruses, bound to the plasma membrane via their membrane receptors, are taken into the cells by endocytosis. Proton pumps in the membrane of endocytic vesicles that now contain the bound viruses cause an accumulation of protons and a consequent lowering of the pH inside the vesicles. The acidic pH (below pH 6) allows hemagglutinin to fulfill its second role, namely, to act as a membrane fusogen by inducing the fusion of the viral envelope membrane with the membrane of the endosome. This expels the viral RNA into the cytoplasm, where it can begin to replicate. 

Glycolipids are widely distributed in every tissue of the body, particularly in nervous tissue such as brain. They occur particularly in the outer leaflet of the plasma membrane, where they contribute to cell surface carbohydrates. 

The plasma membrane contains a small amount of carbohydrate (2 to 10% of the mass of the membrane) on the outer surface. This carbohydrate is found attached to most of the protein molecules, forming glycoproteins, and to some of the phospholipid molecules (<10%), forming glycolipids. Consequently, the external surface of the cell has a carbohydrate coat, or glycocalyx. 

Human TNF-a is initially synthesized as a 233 amino acid polypeptide that is anchored in the plasma membrane by a single membrane-spanning sequence. This TNF pro-peptide, which itself displays biological activity, is usually proteolytically processed by a specific extracellular metallo-protease. Proteolytic cleavage occurs between residues 76 (Ala) and 77 (Val), yielding the mature (soluble) 157 amino acid TNF-a polypeptide. Mature human TNF-a appears to be devoid of a carbohydrate component, and contains a single disulfide bond. 

Eukaryotic cells have evolved a complex, intracellular membrane organization. This organization is partially achieved by compartmentalization of cellular processes within specialized membrane-bounded organelles. Each organelle has a unique protein and lipid composition. This internal membrane system allows cells to perform two essential functions to sort and deliver fully processed membrane proteins, lipids and carbohydrates to specific intracellular compartments, the plasma membrane and the cell exterior, and to uptake macromolecules from the cell exterior (reviewed in [1,2]). Both processes are highly developed in cells of the nervous system, playing critical roles in the function and even survival of neurons and glia. 

Carbohydrates related to membranes can be found as lipopolysaccharides or as parts of glycoproteins. Sugars are often characteristic determinants of cell surfaces (see below). The great majority of carbohydrates are found in the outer leaflet of a membrane, resulting in an asymmetrical structure. This is especially true for many plasma membranes and the outer membrane of Gram-negative bacterial cells (see below). 

Since in mammalian species metals first need to be assimilated from dietary sources in the intestinal tract and subsequently transported to the cells of the different organs of the body through the bloodstream, we will restrict ourselves in this section to the transport of metal ions across the enterocytes of the upper part of the small intestine (essentially the duodenum), where essentially all of the uptake of dietary constituents, whether they be metal ions, carbohydrates, fats, amino acids, vitamins, etc., takes place. We will then briefly review the mechanisms by which metal ions are transported across the plasma membrane of mammalian cells and enter the cytoplasm, as we did for bacteria, fungi and plants. The specific molecules involved in extracellular metal ion transport in the circulation will be dealt with in Chapter 8. 

Secretory glycoproteins are known to move from the ER to the Golgi complex where their carbohydrate side chains are trimmed and further modified (see Palade, 1975 Tartakoff, 1980 Farquhar and Palade, 1981). In most secretory cells the proteins are then concentrated into condensing vacuoles which store the secretory proteins until they are discharged by exocytosis through a fusion reaction between the vacuolar membrane and the plasma membrane. In other secretory cells, like plasma cells, proteins are condnuously secreted and they appear to leave the Golgi complex within vesicles without being concentrated before exocytosis. 

Figure19.1 A schematic diagram of a plasma membrane. Integral proteins are embedded in a bilayer composed of phospholipids (shown, for clarity, in much greater proportion than they have in biological membranes) and cholesterol. The carbohydrate components of glycoproteins and glycolipids occur only on the external face of the membrane. (Reproduced from D. Voet and J. G. Voet, Biochemistry, 3rd edn, 2004. 2004, Donald and Judith G Voet. Reprinted with permission of John Wiley and Sons, Inc.)...

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. 

Surrounding the outside of all cells is the plasma membrane (see Figure 1.89). It is composed primarily of lipids and is selectively permeable, limiting the exchange of molecules between the inside and outside of the cell. The outside of the plasma membrane contains all the carbohydrates and receptor sites. The cytoplasm includes everything inside the plasma membrane except for the nucleus. Energy is generated... 

Benchimol M, Elias CA, De Souza W (1982a) Tritrichomonas foetus ultrastructural localization of basic proteins and carbohydrates. Exp Parasitol 54 135-144 Benchimol M, Elias CA, De Souza W (1982b) Ultrastructural localization of calcium in the plasma membrane and in the hydrogenosome of Tritrichomonas foetus. Exp Parasitol 54 277-284... 

Selectins are plasma membrane lectins that bind carbohydrate chains in the extracellular matrix or on the surfaces of other cells, thereby mediating the flow of information between cell and matrix or between cells. 

Some membrane proteins are covalently linked to complex arrays of carbohydrate. For example, in gly-cophorin, a glycoprotein of the erythrocyte plasma membrane, 60% of the mass consists of complex oligosaccharide units covalently attached to specific amino acid residues. Ser, Thr, and Asn residues are the most common points of attachment (see Fig. 7-31). At the other end of the scale is rhodopsin of the rod cell plasma membrane, which contains just one hexasac-charide. The sugar moieties of surface glycoproteins influence the folding of the proteins, as well as their sta-... 

Experiments with such topology-specific reagents show that the erythrocyte glycoprotein glycophorin spans the plasma membrane. Its amino-terminal domain (bearing the carbohydrate chains) is on the outer sur-... 

The lipids and proteins of membranes are inserted into the bilayer with specific sidedness thus membranes are structurally and functionally asymmetric. Many membrane proteins contain covalently attached oligosaccharides. Plasma membrane glycoproteins are always oriented with the carbohydrate-bearing domain on the extracellular surface. 

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