Molecule of the Month: H5 Hemagglutinin

Since it first emerged in 1996, H5N1 avian influenza (also known as "bird flu") has resulted in the death and culling of millions of birds across the globe, causing significant economic losses in the poultry industry and raising alarms about at-risk wild bird populations. Recent outbreaks have resulted in the illness of not just birds, but have also caused sickness in wild and domestic mammals.

Hemagglutinin is a spike-shaped protein that, along with neuraminidase, covers the surface of influenza viruses. There are currently 18 known subtypes of hemagglutinin (H1 to H18), and 11 known subtypes of neuraminidase (N1 to N11). Different combinations of these proteins determine the viral subtype, such as H1N1 or H3N2. While humans are most often infected with viruses harboring H1, H2, or H3 hemagglutinins, birds can be infected by viruses expressing nearly any hemagglutinin subtype (with the exception of H17 and H18, which were isolated from bats). Only two hemagglutinin subtypes, H5 and H7, cause severe disease in birds, however.

Influenza is able to enter and infect cells through the action of hemagglutinin, which recognizes and attaches to specific molecules on the cell surface. Most hemagglutinins have been found to target sialic acids, a family of nine-carbon sugar molecules that are commonly found at the tips of glycans, or sugar chains, that are attached to proteins and lipids on the cell surface. Sialic acid can be linked to glycans in different ways. The two most common linkage types are called α-2,3 and α-2,6 linkages. Avian influenza viruses, such as H5N1 or H7N9, are adapted to bind to α-2,3-linked sialic acid receptors, which are predominantly found in the respiratory and gastrointestinal tracts of birds. Avian H5 is shown in purple on the right (PDB ID 1JSN). In humans, the majority of cells in the upper respiratory tract (nose, throat, and trachea) have α-2,6-linked sialic acid receptors. Human influenza viruses, such as H1N1 (H1 is shown in blue, PDB ID 1RVT), are adapted to bind to these receptors, making them more easily transmissible among people.

Despite its preference for birds, H5N1 is known to infect humans and other mammals. These infections typically occur from direct contact with infected birds in crowded environments, such as in poultry farms or live bird markets. While human-to-human transmission of H5N1 is very rare and inefficient, numerous cases of transmission between mammals have recently been reported, including an outbreak of H5N1 in multiple dairy cattle farms across the United States. Research has shown that mammary glands of cows are rich in α-2,3-linked sialic acid receptors, allowing H5N1 to replicate and be released in milk, and potentially to be transmitted to other cows through contaminated milking equipment.

How do sialic acids get added to cell surface molecules? This happens through the action sialyltransferases, enzymes that transfer sialic acid from donor molecules, such as CMP-sialic acid, to acceptor molecules, such as glycoproteins and glycolipids. Sialyltransferases, which are found on the membrane of the Golgi, are categorized into several subfamilies based on the type of glycosidic bond they form: α-2,3-sialyltransferases (such as ST3Gal1, PDB ID 2WNB, shown in light green) add sialic acid to galactose residues via an α-2,3 linkage, while α-2,6-sialyltransferases (such as ST6Gal1, PDB ID 6QVT, shown in dark green) attach sialic acid to galactose via an α-2,6 linkage. The abundance of cell surface sialylation, which is involved in diverse processes including cell recognition, signaling, and differentiation, makes it an ideal target for viruses.