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Hemagglutinin (HA) - Cell Binding protein in Avian Influenza


Jack Cerchiara,'06 and Brendan Holsberry, 07


I. Introduction

The Influenza A virus is an orthomyxovirus, and its receptor binding complex is comprised of two primary structural proteins, Hemagglutinin (HA) and Neuraminidase (NA). It has been determined that Hemagglutinin is the primary protein responsible for binding to receptor sites on the cell membrane, allowing the virion to enter the cell (Subbarao 2000). Hemagglutinin is species specific binding protein that binds only to matched sialic acid receptors in host cells (Subbarao 2000). The molecule under study here is the HA protein extracted from the H1-human influenza strain.

The structure of Hemagglutinin is very similar between strains, and differs in only a few structural difference. What makes this strain differ from the H5N1 influenza now posing a pandemic threat is the speficity of its binding regions, which will be the basis of this study. What makes this strain of influenza so dangerous is both its rapid ability to evolve and the fact that mammals, specifically humans contain to immune defenses against this avian strain (Suarez 2000). In 1918, a strain of influenza A virus killed 20-40million people worldwide and the World Health Organization estimates nearly 7 million dead and 1 billion ill from an outbreak of the current H5N1 strain (Horimoto 2001).

Why look at Hemagglutinin?

Researchers have been able to determine that Hemagglutinin (HA) is a species specific binding protein that allows for the virus to bind to the cell membrane of host respiratory cells and propagate through cellular processes. By examining this process, medical researchers hope to determine a vaccine that may prevent this binding from occurring, thus preventing host infection.

II. General Structure

Hemagglutinin is a trimer protein composed of a globular domain and a stem domain, divided along the longitudinal axis of the protein . As was stated, HA protein is made up of three monomers; HA1, HA2, and HA3 domains . Each of these monomers is comprised to two subdomains, in the stem domain the two helix are bonded at Phe-88 to Phe-63 a clear difference between influenza strains, in the avian H5N1, it displays a inward facing Phenylalanine ring, while H1- human strains display an outward facing ring shown here. Along the longitudinal axis, the protein is comprised of structural alpha-helices and beta sheets are seen especially in forming the "bonding depression" in the globular region which will be discussed in the following section.

Note the beta-sheets are primarily present in the globular head where the binding region to sialic acid resides and the alpha helices make up the stem region of the HA monomers .

III. Receptor Binding Sites

The hemagglutinin protein, as was previously stated, is a trinomer protein, primarily responsible for the binding of the Influenza A virion to cell surface receptors, membrane fusion and intracellular infection, which is the first stage of viral infection. Hemaggluntin recognizes sialic acid components of cell-surface glycoproteins and glycolipids (Gamblin et al., 2004; Ha et al., 2000). Hemagglutinin contains a shallow depression at its "head" which allows the sialic acid sequence to move into like a "lock and key." According to Ha et al. (2000), species specific sailic acid receptor analogs do not have a binding affect on the orientation of the sialic acid domain into the base of the hemagglutinin depression. One side of the sialic acid's pyranose ring faces the base of the binding depression. The axial carboxylate, acetamido nitrogen, and the 8- and 9-hydroxyl groups face the site and form bonds.

The binding depression surrounds the sialic acid domain with three primary regions of the hemagglutinin structure. This region is comprised of a loop-helix-loop structure, which surrounds the sialic acid. The 130-loop, 190-helix and 220-loop structures form the triangular opening into the beta-sheet depression. These three structures from the primary binding to the sialic acid and specific regions in their domains (Gamblin et al., 2004). The exact binding geometry differs between species specific hemagglutinin and cell surface proteins, however there are primary cites on the loop-helix-loop complexes that form specific integrations allowing for the binding of the hemagglutinin and the subsequent cell infection. In human-avian receptor complexes the Glu-190 residue on the 190-helix forms a hydrogen bond to the 9-hydrxyl group. Thr-136 as well as amino-acids at residues 135 and 137 on the 130-loop form hydrogen bonds to the sialic acid's carboxylate. Also, Lys-222 and Gln-226 of the 220-loop form bonds with 8-hydroxyl group of the sialic acid (Gamblin et al., 2004) (Figure 1). Together this binding forms around the sialic acid domain of the cell surface glycoprotein or glycolipid in the HA depression, connecting each monomer to the sialic acid on the cell, initiating viral infection .



(Gamblin et al., 2004)

IV. Species Specific Binding

The HAs of viruses recognize different linkage if the HA strain is avian or human. Avian strains have receptor–binding specificity for sialic acid receptors in alpha-2,3 linkage while human strains have specificity for alpha-2,6 linkage. Avian H5 HA hydrogen bonds through Gln-226 to the glycosidic oxygen that is exposed in a trans coformation in the alpha ;2,3 sialic acid-to-galactose link. Alpha-2,6-Linked sialosides bind in a cis conformation, exposing the glycosidic oxygen to solution and nonpolar atoms of the receptor to Leu-226, a human-specific residue. Although each strain contains the both types of alpha linkages, the Gln-226/Gly-228 of the avian strain prefer the alpha-2,6 linkage while the Leu-226/Ser-228 of the human strain prefers the alpha-2,3 linkage. The two can geometrically distinguished because the human strain is much wider between the 226 and 228 positions while the avian strain is much more closed.

V. References

- Ya Ha, David J. Stevens, John J. Skehel, and Don C. Wiley. X-ray structures of H5 avian and H9 swine influenza virus hemagglutinins bound to avian and human receptor analogs. PNAS, Sep 2001; 98: 11181.

- S. J. Gamblin, L. F. Haire, R. J. Russell, D. J. Stevens, B. Xiao, Y. Ha, N. Vasisht, D. A. Steinhauer, R. S. Daniels, A. Elliot, D. C. Wiley, and J. J. Skehel
The Structure and Receptor Binding Properties of the 1918 Influenza Hemagglutinin. Science, Mar 2004; 303: 1838 - 1842.

- Sauter, N. K., Hanson, J. E., Glick, G. D., Brown, J. H., Crowther, R. L., Park, S. J., Skehel, J. J., Wiley, D. C.: Binding of influenza virus hemagglutinin to analogs of its cell-surface receptor, sialic acid: analysis by proton nuclear magnetic resonance spectroscopy and X-ray crystallography. Biochemistry 31 pp. 9609 (1992)

- Subbarao, K.; Katz, J. Avian influenza viruses infecting humans. Cellular and Molecular Life Sciences. Vol: 57, Issue: 12, November, 2000, pp. 1770 - 1784

- Suarez, David L. Evolution of avian influenza viruses Veterinary Microbiology, Vol: 74, Issue: 1-2, May 22, 2000 pp. 15-27

- Horimoto, Taisuke; Kawaoka, Yoshihiro. Pandemic Threat Posed by Avian Influenza A Viruses. Clinical Microbiolical Review., Jan 2001; Vol. 14: pg. 129 - 149.