Sialic acids receptor function

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. 

Polythiophenes functionalized with monosaccharides have been evaluated for their ability to detect the influenza virus and E. coli (Baek et al. 2000). Copolymers of thiophene acetic acid 10 and carbohydrate-modified thiophenes 11 have been prepared via iron(III) chloride mediated polymerization. Addition of influenza virus to a sialic acid containing copolymer resulted in a blue shift of the polymer absorption maximum, resulting in an orange to red chromatic transition. Mannose-containing polythiophenes underwent color changes upon the addition of the lectin ConA or E. coli cells that contain cell surface mannose-binding receptors. A similar biotinylated pol5hhiophene afforded a streptavidin responsive material (Paid and Leclerc 1996). 

The receptor for insulin in liver has also been shown to be a glyco-conjugate, although involvement of sialic acid in the function of this receptor has not yet been unequivocally elucidated.545,546 D-Gal seems to be an essential constituent of this receptor.547 Evidence was obtained for the binding of interferon to cell membranes by way ofgang-liosides.541,5411... 

Further examples of the involvement of sialic acids in the function of receptors are (a) guinea-pig macrophages, which hind the migration-... 

As mentioned in Section 17.2.1, HA is a spike glycoprotein anchored to the virus lipid membrane [50], This glycoprotein functions as a receptor-binding protein and is responsible for the first step of viral infection when it binds to sialic acid residues of receptor glycoproteins on host cells [18]. When the virus is endocytosed into the cell, the low pH (5-6) changes the structure of HA, and this new fusion-active state triggers the fusion of the viral membrane and the endosome membrane, ultimately allowing entry of the viral nucleocapsid into the cytosol of the host cell [18]. 

In addition to the lipid bilayer, enveloped viruses generally have two or more distinct layers of protein that are organized across the membrane. Thus, most viruses have an outer layer of proteins, usually glycoproteins, which are anchored in the membrane as integral membrane proteins. These proteins function to attach the virion to target host cell receptors and facilitate the entry or fusion of the viral membrane with that of the host cell. In addition, some viruses also contain enzymatic activities associated with this outer layer of protein. For example, influenza virus carries with it a neuraminidase that is responsible for cleaving sialic acid residues on host cells. 

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