Researchers have pinpointed a startlingly small genetic change that may explain why some animal viruses jump to humans and trigger catastrophic outbreaks while others remain confined to their original hosts.
A multinational team examined the differences between SARS-CoV-2, the virus behind COVID-19, and RaTG13, a closely related coronavirus found in bats. Despite their kinship, RaTG13 has never been known to infect humans. The comparison revealed that a single amino acid swap in one viral protein dramatically alters how each virus interacts with immune systems across species.
The work was published in Cell Host & Microbe and involved researchers from UCSF, Mount Sinai, Institut Pasteur, and Fred Hutchinson Cancer Center. A key breakthrough came from developing the first laboratory-grown lung cell line derived from the greater horseshoe bat, allowing direct comparisons of viral behavior in both human and bat tissue.
The critical protein, called OrfB9, showed the striking pattern. Among roughly 100 amino acids in the protein, SARS-CoV-2 and RaTG13 versions differ by exactly one. That single difference produced wildly divergent immune responses.
In human lung cells, the SARS-CoV-2 version silenced a crucial immune alarm system, enabling the virus to replicate unchecked. In bat cells, the RaTG13 version triggered immune activation that suppressed the infection. The finding suggests that microscopic genetic changes can determine whether a virus stays trapped in animals or gains a foothold in humans.
Nevan J. Krogan, director of the UCSF Quantitative Biosciences Institute and senior author, emphasized the implications. "The difference between a virus that stays in bats and one that spills over into humans and causes catastrophic disease can come down to remarkably small genetic changes," he said. "By mapping these interactions at the protein level, we can read the molecular signatures that predict spillover risk. It's the kind of early warning system the world needs."
The research offers a roadmap for identifying viruses most likely to jump species before they trigger outbreaks. By understanding which specific protein interactions signal spillover potential, scientists may gain crucial lead time to detect and contain emerging threats before they become global emergencies. The work underscores how pandemic preparedness ultimately depends on reading the molecular language of zoonotic viruses at the smallest scales.
Author Jessica Williams: "One amino acid separating a bat virus from a human plague is a reminder that pandemic prevention isn't about finding smoking guns, it's about understanding molecular mechanics at the edge of species barriers."
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