Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

Exercise 6

4 Avian influenza viruses readily infect birds, but are transmitted to humans very rarely. Similarly, human influenza viruses spread readily to other humans, but have never been detected in birds. The key to this speci- ficity lies in the viral capsid protein, hemagglutinin, which binds to sialic acid residues on cell-surface glycoproteins, triggering virus entry into the cell (Movie 23.8). Hemag- glutinin on human viruses recognizes sialic acid in a 2-6 linkage with galactose, whereas avian hemagglutinin rec- ognizes sialic acid in a 2-3 linkage with galactose. Humans make carbohydrate chains that have only the 2-6 linkage between sialic acid and galactose; birds make only the 2-3 linkage; but pigs make carbohydrate chains with both linkages. How does this situation make pigs ideal hosts for generating new strains of human influenza viruses?