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Hemagglutinin (influenza) Edit Wikipedia article
|Influenza C hemagglutinin stalk|
x-ray structure of the haemagglutinin-esterase-fusion glycoprotein of influenza c virus
Influenza hemagglutinin (HA) or haemagglutinin (British English) is a glycoprotein found on the surface of influenza viruses. It is responsible for binding the virus to cells with sialic acid on the membranes, such as cells in the upper respiratory tract or erythrocytes. It is also responsible for the fusion of the viral envelope with the endosome membrane, after the pH has been reduced. The name "hemagglutinin" comes from the protein's ability to cause red blood cells (erythrocytes) to clump together ("agglutinate") in vitro.
HA has at least 18 different antigens. These subtypes are named H1 through H18. H16 was discovered in 2004 on influenza A viruses isolated from black-headed gulls from Sweden and Norway. H17 was discovered in 2012 in fruit bats. Most recently, H18 was discovered in a Peruvian bat in 2013. The first three hemagglutinins, H1, H2, and H3, are found in human influenza viruses.
A highly pathogenic avian flu virus of H5N1 type has been found to infect humans at a low rate. It has been reported that single amino acid changes in this avian virus strain's type H5 hemagglutinin have been found in human patients that "can significantly alter receptor specificity of avian H5N1 viruses, providing them with an ability to bind to receptors optimal for human influenza viruses". This finding seems to explain how an H5N1 virus that normally does not infect humans can mutate and become able to efficiently infect human cells. The hemagglutinin of the H5N1 virus has been associated with the high pathogenicity of this flu virus strain, apparently due to its ease of conversion to an active form by proteolysis.
Function and mechanism
HA has two functions. Firstly, it allows the recognition of target vertebrate cells, accomplished through the binding to these cells' sialic acid-containing receptors. Secondly, once bound it facilitates the entry of the viral genome into the target cells by causing the fusion of host endosomal membrane with the viral membrane.
HA binds to the monosaccharide sialic acid which is present on the surface of its target cells, which causes the viral particles to stick to the cell's surface. The cell membrane then engulfs the virus and the portion of the membrane that encloses it pinches off to form a new membrane-bound compartment within the cell called an endosome, which contains the engulfed virus. The cell then attempts to begin digesting the contents of the endosome by acidifying its interior and transforming it into a lysosome. However, as soon as the pH within the endosome drops to about 6.0, the original folded structure of the HA molecule becomes unstable, causing it to partially unfold and release a very hydrophobic portion of its peptide chain that was previously hidden within the protein.
This so-called "fusion peptide" acts like a molecular grappling hook by inserting itself into the endosomal membrane and locking on. Then, when the rest of the HA molecule refolds into a new structure (which is more stable at the lower pH), it "retracts the grappling hook" and pulls the endosomal membrane right up next to the virus particle's own membrane, causing the two to fuse together. Once this has happened, the contents of the virus, including its RNA genome, are free to pour out into the cell's cytoplasm.
HA is a homotrimeric integral membrane glycoprotein. It is shaped like a cylinder, and is approximately 13.5 nanometres long. The three identical monomers that constitute HA are constructed into a central α helix coil; three spherical heads contain the sialic acid binding sites. HA monomers are synthesized as precursors that are then glycosylated and cleaved into two smaller polypeptides: the HA1 and HA2 subunits. Each HA monomer consists of a long, helical chain anchored in the membrane by HA2 and topped by a large HA1 globule.
Since hemagglutinin is the major surface protein of the influenza A virus and is essential to the entry process, it is the primary target of neutralizing antibodies. Neutralizing antibodies against flu have been found to act by two different mechanisms, mirroring the dual functions of hemagglutinin:
Most commonly, antibodies against hemagglutinin act by inhibiting attachment. This is because these antibodies bind near the top of the hemagglutinin "head" (blue region in figure at right) and physically block the interaction with sialic acid receptors on target cells. In contrast, some antibodies have been found to have no effect on attachment. Instead, this latter group of antibodies acts by preventing membrane fusion (only in vitro; the efficacy of these antibodies in vivo is believed to be a result of Antibody-Dependent Cell-Mediated Cytotoxicity and the Complement system). Most of these antibodies, like the human antibodies F10, FI6, CR6261, recognize sites in the stem/stalk region (orange region in figure at right), far away from the receptor binding site.
The stem (also called HA2), contains most of the membrane fusion machinery of the hemagglutinin protein, and antibodies targeting this region block key structural changes that drive the membrane fusion process. However, at least one fusion-inhibiting antibody was found to bind closer to the top of hemagglutinin, and is thought to work by cross-linking the heads together, the opening of which is thought to be the first step in the membrane fusion process.
- FI6 antibody
- Antigenic shift
- Sialic acid
- H5N1 genetic structure
- Russell RJ, Kerry PS, Stevens DJ, Steinhauer DA, Martin SR, Gamblin SJ, Skehel JJ (November 2008). "Structure of influenza hemagglutinin in complex with an inhibitor of membrane fusion". Proc. Natl. Acad. Sci. U.S.A. 105 (46): 17736–41. doi:10.1073/pnas.0807142105. PMC 2584702. PMID 19004788.
- Nelson DL, Cox MM (2005). Lehninger's Principles of Biochemistry (4th ed.). New York: WH Freeman.
- Fouchier RA, Munster V, Wallensten A, et al. (March 2005). "Characterization of a Novel Influenza A Virus Hemagglutinin Subtype (H16) Obtained from Black-Headed Gulls". J. Virol. 79 (5): 2814–22. doi:10.1128/JVI.79.5.2814-2822.2005. PMC 548452. PMID 15709000.
- Unique new flu virus found in bats http://www.nhs.uk/news/2012/03march/Pages/cdc-finds-h17-bat-influenza.aspx
- Suxiang Tong; et al. (October 2013). "New World Bats Harbor Diverse Influenza A Viruses". PLoS Pathogens 9 (10): e1003657. doi:10.1371/journal.ppat.1003657. PMC 3794996. PMID 24130481.
- Suzuki Y (March 2005). "Sialobiology of influenza: molecular mechanism of host range variation of influenza viruses". Biol. Pharm. Bull. 28 (3): 399–408. doi:10.1248/bpb.28.399. PMID 15744059.
- Gambaryan A, Tuzikov A, Pazynina G, Bovin N, Balish A, Klimov A (January 2006). "Evolution of the receptor binding phenotype of influenza A (H5) viruses". Virology 344 (2): 432–8. doi:10.1016/j.virol.2005.08.035. PMID 16226289.
- Hatta M, Gao P, Halfmann P, Kawaoka Y (September 2001). "Molecular basis for high virulence of Hong Kong H5N1 influenza A viruses". Science 293 (5536): 1840–2. doi:10.1126/science.1062882. PMID 11546875.
- Senne DA, Panigrahy B, Kawaoka Y, et al. (1996). "Survey of the hemagglutinin (HA) cleavage site sequence of H5 and H7 avian influenza viruses: amino acid sequence at the HA cleavage site as a marker of pathogenicity potential". Avian Dis. 40 (2): 425–37. doi:10.2307/1592241. JSTOR 1592241. PMID 8790895.
- White JM, Hoffman LR, Arevalo JH, et al. (1997). "Attachment and entry of influenza virus into host cells. Pivotal roles of hemagglutinin". In Chiu W, Burnett RM, Garcea RL. Structural Biology of Viruses. Oxford University Press. pp. 80–104.
- Stegmann T, Booy, P.F., Wilschut, J. Dec 1987, "Effects of Low pH on Influenza Virus" The Journal of Biological Chemistry, Vol. 262, No. 36, pp. 17744-17749, 1987
- Goh, Boon Chong; Rynkiewicz, Michael J.; Cafarella, Tanya R.; White, Mitchell R.; Hartshorn, Kevan L.; Allen, Kimberly; Crouch, Erika C.; Calin, Oliviana; Seeberger, Peter H. (2013-11-26). "Molecular Mechanisms of Inhibition of Influenza by Surfactant Protein D Revealed by Large-Scale Molecular Dynamics Simulation". Biochemistry 52 (47): 8527–8538. doi:10.1021/bi4010683. ISSN 0006-2960. PMC 3927399. PMID 24224757.
- Dilillo DJ, Tan GS, Palese P, Ravetch JV (February 2014). "Broadly neutralizing hemagglutinin stalk-specific antibodies require FcγR interactions for protection against influenza virus in vivo". Nature Medicine 20 (2): 143–51. doi:10.1038/nm.3443. PMC 3966466. PMID 24412922.
- Sui J, Hwang WC, Perez S, Wei G, Aird D, Chen LM, Santelli E, Stec B, Cadwell G, Ali M, Wan H, Murakami A, Yammanuru A, Han T, Cox NJ, Bankston LA, Donis RO, Liddington RC, Marasco WA (March 2009). "Structural and functional bases for broad-spectrum neutralization of avian and human influenza A viruses". Nat. Struct. Mol. Biol. 16 (3): 265–73. doi:10.1038/nsmb.1566. PMC 2692245. PMID 19234466.
- Corti D, Voss J, Gamblin SJ, Codoni G, Macagno A, Jarrossay D, Vachieri SG, Pinna D, Minola A, Vanzetta F, Silacci C, Fernandez-Rodriguez BM, Agatic G, Bianchi S, Giacchetto-Sasselli I, Calder L, Sallusto F, Collins P, Haire LF, Temperton N, Langedijk JP, Skehel JJ, Lanzavecchia A (August 2011). "A neutralizing antibody selected from plasma cells that binds to group 1 and group 2 influenza A hemagglutinins". Science 333 (6044): 850–6. doi:10.1126/science.1205669. PMID 21798894.
- Throsby M, van den Brink E, Jongeneelen M, Poon LL, Alard P, Cornelissen L, Bakker A, Cox F, van Deventer E, Guan Y, Cinatl J, ter Meulen J, Lasters I, Carsetti R, Peiris M, de Kruif J, Goudsmit J (2008). "Heterosubtypic neutralizing monoclonal antibodies cross-protective against H5N1 and H1N1 recovered from human IgM+ memory B cells". PLoS ONE 3 (12): e3942. doi:10.1371/journal.pone.0003942. PMC 2596486. PMID 19079604.
- Ekiert DC, Bhabha G, Elsliger MA, Friesen RH, Jongeneelen M, Throsby M, Goudsmit J, Wilson IA (April 2009). "Antibody recognition of a highly conserved influenza virus epitope". Science 324 (5924): 246–51. doi:10.1126/science.1171491. PMC 2758658. PMID 19251591.
- Barbey-Martin C, Gigant B, Bizebard T, Calder LJ, Wharton SA, Skehel JJ, Knossow M (March 2002). "An antibody that prevents the hemagglutinin low pH fusogenic transition". Virology 294 (1): 70–4. doi:10.1006/viro.2001.1320. PMID 11886266.
- Jmol tutorial of influenza hemagglutinin structure and activity.
- PDB Molecule of the Month Hemagglutinin (April 2006)
- Influenza Research Database Database of influenza protein sequences and structures
- 3D macromolecular structures of influenza hemagglutinin from the EM Data Bank(EMDB)
This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.
Haemagglutinin Provide feedback
Haemagglutinin from influenza virus causes membrane fusion of the viral membrane with the host membrane. Fusion occurs after the host cell internalises the virus by endocytosis. The drop of pH causes release of a hydrophobic fusion peptide and a large conformational change leading to membrane fusion.
Internal database links
|SCOOP:||RmuC MtrG HSBP1 FliD_C Matrilin_ccoil|
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR001364
Haemagglutinin (HA) is one of two main surface fusion glycoproteins embedded in the envelope of influenza viruses, the other being neuraminidase (NA). There are sixteen known HA subtypes (H1-H16) and nine NA subtypes (N1-N9), which together are used to classify influenza viruses (e.g. H5N1). The antigenic variations in HA and NA enable the virus to evade host antibodies made to previous influenza strains, accounting for recurrent influenza epidemics [PUBMED:16178512]. The HA glycoprotein is present in the viral membrane as a single polypeptide (HA0), which must be cleaved by the host's trypsin-like proteases to produce two peptides (HA1 and HA2) in order for the virus to be infectious. Once HA0 is cleaved, the newly exposed N-terminal of the HA2 peptide then acts to fuse the viral envelope to the cellular membrane of the host cell, which allows the viral negative-stranded RNA to infect the host cell. The type of host protease can influence the infectivity and pathogenicity of the virus.
The haemagglutinin glycoprotein is a trimer containing three structurally distinct regions: a globular head consisting of anti-parallel beta-sheets that form a beta-sandwich with a jelly-roll fold (contains the receptor binding site and the HA1/HA2 cleavage site); a triple-stranded, coiled-coil, alpha-helical stalk; and a globular foot composed of anti-parallel beta-sheets [PUBMED:16543414, PUBMED:15475582]. Each monomer consists of an intact HA0 polypeptide with the HA1 and HA2 regions linked by disulphide bonds. The N terminus of HA1 provides the central strand in the 5-stranded globular foot, while the rest of the HA1 chain makes its way to the 8-stranded globular head. HA2 provides two alpha helices, which form part of the triple-stranded coiled-coil that stabilises the trimer, its C terminus providing the remaining strands of the 5-stranded globular foot.
This entry represents the entire haemagglutinin protein (HA0) consisting of both the HA1 and HA2 regions, as found in influenza A and B viruses.
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Cellular component||viral envelope (GO:0019031)|
|Molecular function||host cell surface receptor binding (GO:0046789)|
|Biological process||fusion of virus membrane with host plasma membrane (GO:0019064)|
Below is a listing of the unique domain organisations or architectures in which this domain is found. More...
The graphic that is shown by default represents the longest sequence with a given architecture. Each row contains the following information:
- the number of sequences which exhibit this architecture
a textual description of the architecture, e.g. Gla, EGF x 2, Trypsin.
This example describes an architecture with one
Gladomain, followed by two consecutive
EGFdomains, and finally a single
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We store a range of different sequence alignments for families. As well as the seed alignment from which the family is built, we provide the full alignment, generated by searching the sequence database (reference proteomes) using the family HMM. We also generate alignments using four representative proteomes (RP) sets, the UniProtKB sequence database, the NCBI sequence database, and our metagenomics sequence database. More...
There are various ways to view or download the sequence alignments that we store. We provide several sequence viewers and a plain-text Stockholm-format file for download.
We make a range of alignments for each Pfam-A family:
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- Representative Proteomes (RPs) at 15%, 35%, 55% and 75% co-membership thresholds
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You can see the alignments as HTML or in three different sequence viewers:
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You can download (or view in your browser) a text representation of a Pfam alignment in various formats:
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We make a range of alignments for each Pfam-A family. You can see a description of each above. You can view these alignments in various ways but please note that some types of alignment are never generated while others may not be available for all families, most commonly because the alignments are too large to handle.
1Cannot generate PP/Heatmap alignments for seeds; no PP data available
Key: available, not generated, — not available.
Format an alignment
We make all of our alignments available in Stockholm format. You can download them here as raw, plain text files or as gzip-compressed files.
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This page displays the phylogenetic tree for this family's seed alignment. We use FastTree to calculate neighbour join trees with a local bootstrap based on 100 resamples (shown next to the tree nodes). FastTree calculates approximately-maximum-likelihood phylogenetic trees from our seed alignment.
Note: You can also download the data file for the tree.
Curation and family details
This section shows the detailed information about the Pfam family. You can see the definitions of many of the terms in this section in the glossary and a fuller explanation of the scoring system that we use in the scores section of the help pages.
|Seed source:||Pfam-B_26 (release 1.0)|
|Number in seed:||3|
|Number in full:||3|
|Average length of the domain:||554.30 aa|
|Average identity of full alignment:||42 %|
|Average coverage of the sequence by the domain:||96.85 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 11927849 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||15|
|Download:||download the raw HMM for this family|
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This visualisation provides a simple graphical representation of the distribution of this family across species. You can find the original interactive tree in the More....
This chart is a modified "sunburst" visualisation of the species tree for this family. It shows each node in the tree as a separate arc, arranged radially with the superkingdoms at the centre and the species arrayed around the outermost ring.
How the sunburst is generated
The tree is built by considering the taxonomic lineage of each sequence that has a match to this family. For each node in the resulting tree, we draw an arc in the sunburst. The radius of the arc, its distance from the root node at the centre of the sunburst, shows the taxonomic level ("superkingdom", "kingdom", etc). The length of the arc represents either the number of sequences represented at a given level, or the number of species that are found beneath the node in the tree. The weighting scheme can be changed using the sunburst controls.
In order to reduce the complexity of the representation, we reduce the number of taxonomic levels that we show. We consider only the following eight major taxonomic levels:
Colouring and labels
Segments of the tree are coloured approximately according to their superkingdom. For example, archeal branches are coloured with shades of orange, eukaryotes in shades of purple, etc. The colour assignments are shown under the sunburst controls. Where space allows, the name of the taxonomic level will be written on the arc itself.
As you move your mouse across the sunburst, the current node will be highlighted. In the top section of the controls panel we show a summary of the lineage of the currently highlighed node. If you pause over an arc, a tooltip will be shown, giving the name of the taxonomic level in the title and a summary of the number of sequences and species below that node in the tree.
Anomalies in the taxonomy tree
There are some situations that the sunburst tree cannot easily handle and for which we have work-arounds in place.
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Some species in the taxonomic tree may not have one or more of the main eight levels that we display. For example, Bos taurus is not assigned an order in the NCBI taxonomic tree. In such cases we mark the omitted level with, for example, "No order", in both the tooltip and the lineage summary.
Unmapped species names
The tree is built by looking at each sequence in the full alignment for the family. We take the name of the species given by UniProt and try to map that to the full taxonomic tree from NCBI. In some cases, the name chosen by UniProt does not map to any node in the NCBI tree, perhaps because the chosen name is listed as a synonym or a misspelling in the NCBI taxonomy.
So that these nodes are not simply omitted from the sunburst tree, we group them together in a separate branch (or segment of the sunburst tree). Since we cannot determine the lineage for these unmapped species, we show all levels between the superkingdom and the species as "uncategorised".
Since we reduce the species tree to only the eight main taxonomic levels, sequences that are mapped to the sub-species level in the tree would not normally be shown. Rather than leave out these species, we map them instead to their parent species. So, for example, for sequences belonging to one of the Vibrio cholerae sub-species in the NCBI taxonomy, we show them instead as belonging to the species Vibrio cholerae.
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For large species trees, you may see blank regions in the outer layers of the sunburst. These occur when there are large numbers of arcs to be drawn in a small space. If an arc is less than approximately one pixel wide, it will not be drawn and the space will be left blank. You may still be able to get some information about the species in that region by moving your mouse across the area, but since each arc will be very small, it will be difficult to accurately locate a particular species.
The tree shows the occurrence of this domain across different species. More...
We show the species tree in one of two ways. For smaller trees we try to show an interactive representation, which allows you to select specific nodes in the tree and view them as an alignment or as a set of Pfam domain graphics.
Unfortunately we have found that there are problems viewing the interactive tree when the it becomes larger than a certain limit. Furthermore, we have found that Internet Explorer can become unresponsive when viewing some trees, regardless of their size. We therefore show a text representation of the species tree when the size is above a certain limit or if you are using Internet Explorer to view the site.
If you are using IE you can still load the interactive tree by clicking the "Generate interactive tree" button, but please be aware of the potential problems that the interactive species tree can cause.
For all of the domain matches in a full alignment, we count the number that are found on all sequences in the alignment. This total is shown in the purple box.
We also count the number of unique sequences on which each domain is found, which is shown in green. Note that a domain may appear multiple times on the same sequence, leading to the difference between these two numbers.
Finally, we group sequences from the same organism according to the NCBI code that is assigned by UniProt, allowing us to count the number of distinct sequences on which the domain is found. This value is shown in the pink boxes.
We use the NCBI species tree to group organisms according to their taxonomy and this forms the structure of the displayed tree. Note that in some cases the trees are too large (have too many nodes) to allow us to build an interactive tree, but in most cases you can still view the tree in a plain text, non-interactive representation. Those species which are represented in the seed alignment for this domain are highlighted.
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There are 12 interactions for this family. More...
We determine these interactions using iPfam, which considers the interactions between residues in three-dimensional protein structures and maps those interactions back to Pfam families. You can find more information about the iPfam algorithm in the journal article that accompanies the website.
For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Hemagglutinin domain has been found. There are 1551 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.
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