Viral hemagglutinins

Hemagglutinins Many animal viruses (e.g., ortho- and paramyxoviruses) agglutinate the red blood cells of certain animal species. This means that these red cells contain receptors for certain surface components of viral particles that act as cell attachment proteins which are glycoproteins and known as hemagglutinins. Viral hemagglutinins could be used in their quantitative measurement [7]. 

The jelly roll barrel is thus conceptually simple, but it can be quite puzzling if it is not considered in this way. Discussion of these structures will be exemplified in this chapter by hemagglutinin and in Chapter 16 by viral coat proteins. 

Progeny vims particles then bud from patches of the infected cell s plasma membrane that contain both the viral hemagglutinin and neuraminidase. The viral envelopes therefore contain both viral membrane proteins but no cellular membrane proteins. 

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. 

This fusogenic activity of influenza hemagglutinin is frequently exploited in the laboratory. If, for example, the virus is bound to cells at a temperature too low for endocytosis and then the pH of the external medium is lowered, the hemagglutinin causes direct fusion of the viral envelope with the plasma membrane infection is achieved without endocytosis. Similarly, artificial vesicles with hemagglutinin in their membrane and other molecules in their lumen can be caused to fuse with cells by first allowing the vesicles to bind to the plasma membrane via the hemagglutinin and then lowering the pH of the medium. In this way the contents of the vesicles are delivered to the recipient cell s cytoplasm. 

The influenza virus inhibitors, zanamivir, and oseltamivir, act outside the cell after virus particles have been formed. The dtugs have been designed to fit into the active site of the viral envelope enzyme neuraminidase, which is required to cleave sialic acid off the surface of the producing cells. When its activity is blocked, new virus particles stay attached to the cell surface through binding of the virus protein hemagglutinin to sialic acid and are prevented from spreading to other cells. 

Viruses don t have a reproductive system of their own and need to take over healthy cells by puncturing them with tiny spikes called hemagglutinin so that they can use the cells reproductive mechanism to make more viruses. These viral spikes are coated with an enzyme called neuraminidase, which helps to break down cellular walls. Flavonoids that occur in elderberries inhibit viral action and thereby improve immune response. It is thought that the flavoniods may also inhibit the action of neuraminidase. 

Immunopotentiating reconstituted influenza virosomes (IRTV) are spherical 150-nm sized particles consisting of a phospholipid bilayer in which influenza virus A/Singapore strain-derived hemagglutinin (HA) and neuraminidase (NA) are intercalated. As such, they resemble and mimic the influenza virus envelope. The difference from conventional liposome formulations lies in the inclusion of the viral envelope proteins HA and NA as well as viral phospholipids. Especially, the inclusion of influenza virus HA provides IRIV with delivery and immimogenic capacities. IRTV are licensed for human use as adjuvant in hepatitis A vaccination and as influenza subunit vaccine (1). 

Fusogenic peptides, derived from viral sources, are particularly well characterized (107 111). The amphipathic peptide from the A-terminal region of the hemagglutinin (HA)-2 subunit of HA was one of the first such peptides described, and, subsequently, a range of influenza-derived peptides... 

The contribution of resistance because of alterations in the viral hemagglutinin has not been fully evaluated. 

Figure 12.3 Colorimetric detection of influenza by sialoside bilayer assembly (2% sialoside lipid 2 and 98% matrix Upid 1). (a) The colorimetric response of the film, supported on a glass microscope slide, is readily visible to the naked eye for qualitative evaluation of the presence of vims. The film on the left (blue) has been exposed to a blank solution of PBS. The film on the right (ted) has heen exposed to 100 hemagglutinin units (HAU) of vims (CR = 77%). (b) The visible absorption spectmm of a bilayer assembly (—) prior to and (—) after viral incubation. Reprinted from Charych et al. (1993). Copyright 1993 American Association for the Advancement of Science. 

Their mechanism of action involves inhibition of the viral M2 protein, an integral membrane protein that acts as a H channel Blockade of the M2 protein prevents the acid-mediated dissociation of the ribonucleoprotein complex that occurs early in replication. In certain strains, the pH changes that result from M2 inhibition alter the conformation of hemagglutinin, hence inhibit viral assembly. 

Influenza virus resistant to oseltamivir has not been found in naturally acquired isolates but has been isolated from influenza patients who have undergone treatment with this drug. These resistant strains contain mutations in the active site of neuraminidase and are generally less virulent and infective than nonresistant virus. In vitro passage of influenza virus in the presence of oseltamivir carboxylate can produce mutations in hemagglutinin that decrease the overall dependence of viral replication on neuraminidase however, the clinical relevance of this resistance mechanism is unknown. 

Zanamivir (Relenza) is a neuraminidase inhibitor with activity against influenza A and B strains. Like oseltamivir, zanamivir is a reversible competitive antagonist of viral neuraminidase. It inhibits the release of progeny virus, causes viral aggregation at the cell surface, and impairs viral movement through respiratory secretions. Resistant variants with hemagglutinin and/or neuraminidase mutations have been produced in vivo however, clinical resistance to zanamivir is quite rare at present. 

A special example of cell-cell interaction is the adherence of group B streptococci to canine, epithelial cells that are infected with influenza A virus. However, this capacity was blocked in the presence of tunicamycin, and this result supports the concept that adherence of streptococci to mammalian cells involves recognition of viral hemagglutinin, or its carbohydrate complement, the synthesis of which is blocked by tunicamycin.556... 

FIGURE 11-24 Fusion induced by the hemagglutinin (FIA) protein during viral infection. HA protein is exposed on the membrane surface of the influenza virus. When the virus moves from the neutral pH of the interstitial fluid to the low-pH compartment (endosome) in the host cell, HA undergoes dramatic shape changes that mediate fusion of the viral and endosomal membranes, releasing the viral contents into the cytoplasm. 

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