Mutation to bird flu protein alters receptor specificity

A new study found that a single mutation to the hemagglutinin protein of an H5N1 virus isolated from a dairy worker in Texas this spring switched the protein’s binding specificity from avian- to human-type receptors (Science 2024, DOI: 10.1126/science.adt0180).

Hemagglutinin—the “H” in “H5N1”—is a protein that helps influenza viruses bind to host cells; the hemagglutinin achieves this by recognizing receptors that line those cells. The receptors are characterized by certain linkages between sialic acid and galactose sugars at the ends of glycan chains connected to proteins on host cell surfaces. Influenza viruses that primarily spread in birds preferentially bind to avian-type receptors, whereas influenza viruses that spread in people preferentially bind to human-type receptors.

The clade 2.3.4.4b H5N1 viruses that have spread through wild birds and poultry over the past several years—and have been detected in over 700 dairy herds in the US—have been found to preferentially bind to avian-type receptors. Scientists agree that in order for H5N1 to spread efficiently between people, it would need to adapt to preferentially bind to human-type receptors, which in humans are expressed in the upper respiratory tract.

James Paulson, a biochemist at Scripps Research, was among several scientists who reported earlier this year that clade 2.3.4.4b H5N1 viruses spreading in cattle had not adapted to preferentially bind to human-type receptors. The new research, led by Paulson and Ian Wilson, a structural biologist at Scripps Research, supports these findings. The study found that the hemagglutinin from an H5N1 virus detected in a dairy worker in Texas exclusively bound to avian-type receptors. (Humans also have avian-type receptors, but they’re not located in parts of the body that promote H5N1 transmission between people.)

Previous studies have found that mutations at certain positions in hemagglutinin proteins could make it easier for influenza viruses to bind to human-type receptors. In the new work, the researchers obtained the genetic sequence of the hemagglutinin from the H5N1 virus found in the Texas worker and then modified the protein by adding a mutation. The mutated hemagglutinin swapped the amino acid glutamine at position 226 for a leucine (Gln²²⁶Leu).

Analyses showed that incorporating the Gln²²⁶Leu mutation completely switched the hemagglutinin’s binding specificity to human-type receptors. That the lone mutation in the hemagglutinin was capable of inducing the switch differed from the results of past studies, which reported that three or more mutations were normally needed to make the change.

The mutated hemagglutinin’s binding affinity to human-type receptors was weak, but Paulson noted that this was also the case when the H1N1 virus that caused the 2009 swine flu pandemic first emerged in humans. Within a couple of years, he says, that H1N1 virus adapted to have dramatically higher binding affinity.

The team also tested other mutations that are believed to affect receptor binding specificity. These mutations on their own did not switch the hemagglutinin’s specificity. But one of them did enhance the binding affinity of the hemagglutinin to human-type receptors when combined with the Gln²²⁶Leu mutation.

“We’re suggesting that you need to continue surveillance of the viruses,” Wilson says. “What’s happening in birds, what’s happening in cows, and what’s happening with people getting infected with these viruses, and look for signs that there are such changes which can lead to changes in receptor specificity.”

The researchers also noted that factors beyond hemagglutinin receptor binding specificity, including the protein’s fusion pH, are important toward dictating transmission of H5N1.

Jenna Guthmiller, an immunologist and virologist at the University of Colorado Anschutz School of Medicine who was not involved in the study, says it serves as “a good proof of principle that these mutations are sufficient for switching” from avian- to human-type receptors.