Hemagglutinin virus

Other subtypes, such as H5, attack proteins in the digestive system of birds. Most of these are not dangerous to us, and do not typically threaten the lives of the birds, so they exist as an invisible reservoir of virus.

A potential danger occurs when these different strains start trading genes. The H5N1 avian influenza virus that is showing up in the news is decimating bird populations, but it is not currently a danger to us since it doesn't have the right hemagglutinin to attack human cells the N1 designation refers to the subtype of a second virus protein: neuraminidase.

However, there is the possibility that it could acquire a human-specific hemagglutinin, and then it could cause real problems. One way for this to happen involves pigs. Pigs are susceptible to viruses of several subtypes, both the types that attack birds and the types that attack us.

If a single pig gets infected with two different viruses at the same time, the viruses can shuffle and trade genes during the infection. This could potentially be a way to construct a virus with the virulence of the bird virus, combined with the ability to attack human cells. The hemagglutinin shown here is taken from an actual virus of the pandemic that killed so many people in The DNA encoding this hemagglutinin was isolated from preserved samples, and the hemagglutinin was made in the laboratory according to this genetic information.

Two crystal structures were obtained, the active form shown here PDB entry 1ruz , and a precursor form in PDB entry 1rd8. The protein is tethered to the virus membrane by a short segment of protein that is not seen in the crystal structure and is shown schematically here at the bottom. Conformational changes in hemagglutinin that lead to membrane fusion. Hemagglutinin is a deadly molecular machine that targets and attacks cells. This occurs in several steps.

First, the three binding sites at the top of the spike bind to sugars on cellular proteins, shown in green at the top left PDB entry 1hge. Then, the whole virus is carried inside the cell into the endosome and the cell adds acid, which normally digests the stuff inside the endosome. But in the case of the virus, the acidic environment serves to arm the attack mechanism.

In acid, hemagglutinin unfolds and then refolds into an entirely different shape. The portions shown in orange and red are normally folded against the protein, but in acid, they pop out and point upward, as shown in the center illustration PDB entries 1htm , 1ibn and 2vir. The red portion, termed the fusion peptide, has a strong affinity for membranes, so it inserts into the cell membrane and locks the virus to the cell.

Then, as shown on the left PDB entry 1qu1 , the yellow portions zip up the side of the protein, pulling the two membranes close together. Finally, the new conformation of hemagglutinin somehow causes the two membranes to fuse--that part is still not well understood--and the viral RNA flows into the cell, starting the process of infection.

Antibodies are our first line of defense against influenza virus. PDB entry 1qfu shows how one antibody attacks hemagglutinin, blocking it so that it cannot bind to cell surfaces. The structure includes hemagglutinin, shown in blue and orange, and three copies of a Fab fragment of antibody Fab fragments are one arm from the Y-shaped antibody. Of course, viruses find ways of evading attack by antibodies, creating new strains that infect us each year.

One way is to mutate the location of carbohydrate chains on the hemagglutinin surface. Several of these carbohydrates are shown in green here. If the virus adds a new carbohydrate at the location that the antibody binds, the antibody will no longer be effective. The numbers of drug-resistant isolates varied from year to year between and , but there was no consistently increased frequency in any strain or subtype, suggesting only low levels of transmission, if any However, despite little antiviral drug usage worldwide, in the winter of —, Tamiflu-resistant H1N1 viruses accounted for the vast majority of influenza isolates, in a season when H1N1 viruses dominated The reason for this occurrence is unknown, but it strikingly demonstrated the potential viability of drug-resistant viruses.

It happened in the year before the H1N1 pandemic of , in which, worldwide to date, H1N1 Tamiflu-resistant isolates have been made. Fortunately, in this case, there is no evidence of human-to-human transmission of resistant viruses. In addition, the same mutation was without effect on group 2 NAs.

By contrast, binding of Relenza, like sialic acid, involves hydrogen bond formation between the carboxyl group of Glu and the 8-OH and 9-OH groups of the sialic acid glycerol substituent. By contrast, the structure of the HY N1 NA-Relenza complex shows that Relenza is accommodated in the active site of mutant NA by a small movement in the Glu side chain and retains the hydrogen bonds made by wild-type NA.

The lack of effect of the HY substitution on group 2 NAs results from the substituted Tyr being able to adopt a different rotamer conformation because of an adjacent smaller residue, Thr in group 2 NA rather than Tyr in group 1 NA Fig.

Inhibitor binding to the active site of NA. Thr of group 2 NA is light blue. In the lower panel , the structures of sialic acid, zanamivir, and oseltamivir are shown in blue , gray , and yellow , respectively, with selected carbon atoms associated with the hydrophobic moiety at C-6 of oseltamivir numbered.

The N1 NA mutation HY was also dominant among drug-resistant mutant viruses selected in vitro ; several other mutations were also identified, and the structures of the mutant NAs were analyzed 78 , 85 , The results of these drug selection experiments included the important finding that the majority of mutations occurred not in NA but in HA 81 , These mutations apparently decreased the affinity of the mutant HAs for sialic acid, and as a consequence, the newly made mutant viruses were less dependent on NA activity to release them from infected cells.

In some instances such as the SN mutation, the mutant viruses were dependent on anti-NA drugs for infectivity, an indication that NA can act in the early stages of infection and that viable viruses must contain HAs of sufficient affinity to balance NA activity.

Interdependence of HA affinity and NA activity has been concluded from numerous genetic studies 88 , — 92 particularly involving co-variation of virus NA stalk length and HA affinity for the receptor, with HA decreases in affinity often resulting from extra glycosylation of HA near the receptor-binding site.

The H1N1 pandemic exemplifies the unpredictability of human influenza and has emphasized the importance of the virus membrane glycoproteins in our response to new viruses. Both glycoproteins are important immunogens in anti-influenza vaccines, and the NA active site is the target of the available anti-influenza drugs. The importance of immune recognition of HA and NA is evidenced by the extent of amino acid sequence variation, with time, during a pandemic period. This is much greater than for other influenza virus proteins despite the fact that the RNA-associated nucleoprotein, for example, is a very powerful immunogen.

The regions of the glycoproteins that are recognized by antibodies that block virus infection are on their upper surfaces, in positions where binding of antibodies could prevent receptor binding in the case of HA 93 or the enzyme activity of NA Details of the binding to HA or NA of specific monoclonal antibodies have been determined by x-ray crystallography to show the likely way in which they function and, in the case of HA, the relative efficiencies of virus infectivity neutralization that result from binding to different positions on the molecule.

As a result of antibody-mediated selection of antigenic variants during pandemics, antibodies produced following infection are virus strain-specific. This is largely the case also for vaccine-induced antibodies, hence the need for frequent, almost yearly updates of the viruses used to prepare vaccines, chosen on the basis of the results of international surveillance for antigenically distinguishable new viruses.

An ideal vaccine would induce immune responses that would cross-neutralize either all viruses in a subtype or, better, all influenza viruses. A number of cross-reactive antibodies against HA that block virus infection have been prepared 95 , — Complexes that some of them form with HA have been analyzed by x-ray crystallography, and they are seen to bind relatively near to the region of HA that associates with the virus membrane 96 , Their binding is also reported to prevent the low pH-induced conformational changes in HA required for membrane fusion, and this may be the way that they influence virus infection.

As an alternative, they may function to prevent virus assembly at the membranes of infected cells, at the time in infection when anti-NA antibodies have their effect They would, however, have the additional attribute to anti-NA antibodies of being cross-reactive. If such antibodies can be induced by vaccination, they could be very valuable.

As therapeutic antibodies, they could also join the anti-NA drugs in combination therapy. For treatment of infections with highly pathogenic viruses such as the H5N1 avian virus, they could be very valuable in this role. The unexplained worldwide spread of Tamiflu-resistant H1N1 viruses in is a strong stimulus to the development of other anti-influenza drugs and therapies that could be used, together with Tamiflu, Relenza, or both drugs, like anti-human immunodeficiency virus drug cocktails, to combat the risk of the development and spread of drug-resistant influenza viruses Their availability would add confidence to the tactic, in many pandemic plans, of stockpiling the anti-NA drugs.

We are grateful to Philip Walker for preparation of figures. We thank our many colleagues, past and present, for valuable discussions. This minireview will be reprinted in the Minireview Compendium, which will be available in January, National Center for Biotechnology Information , U.

Journal List J Biol Chem v. J Biol Chem. Published online Jun Steven J. Gamblin 1 and John J. Skehel 2. John J. Author information Copyright and License information Disclaimer. E-mail: ku. Author's Choice —Final version full access. This article has been cited by other articles in PMC. Introduction The two glycoproteins of the influenza virus membrane, hemagglutinin HA 3 and neuraminidase NA , both recognize sialic acid 1 , — 3. Open in a separate window. Hemagglutinin HA is a trimer of identical subunits, each of which contains two polypeptides that result from proteolytic cleavage of a single precursor Acknowledgments We are grateful to Philip Walker for preparation of figures.

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