Single Amino Acid Switch Determines Coronavirus Spillover from Bats to Humans
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Researchers found that a single amino acid difference in the Orf9b (search) protein determines whether a coronavirus infects bat cells or human cells.
The SARS-CoV-2 (search) version of Orf9b (search) disables human immune alarms, while the RaTG13 (search) version activates bat immune defenses to suppress viral spread.
The study, published in Cell Host & Microbe, used the first lab-grown lung cell line from the greater horseshoe bat to map protein-level interactions.
A single amino acid change in a viral protein can mean the difference between a coronavirus that stays confined to bats and one capable of causing a human pandemic, according to new research published May 13 in Cell Host & Microbe.
The study, led by researchers at the UCSF Quantitative Biosciences Institute (search) (QBI), the Icahn School of Medicine at Mount Sinai, Institut Pasteur, and Fred Hutchinson Cancer Center, compared SARS-CoV-2 (search) with RaTG13 (search)—a closely related coronavirus that exclusively infects bat cells. By examining how each virus interacts with immune proteins in bat and human lung cells, the team identified a remarkably small genetic determinant of species tropism.
"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," said Nevan J. Krogan, PhD, director of the UCSF Quantitative Biosciences Institute (search) and senior author of the study. "By mapping these interactions at the protein level—across two viruses and two species—we can read the molecular signatures that predict spillover risk. It's the kind of early warning system the world needs."
A Single Amino Acid Alters Immune Evasion
Both SARS-CoV-2 (search) and RaTG13 (search) carry a gene known as Orf9b (search), and their sequences differ by just one amino acid out of roughly 100. Despite this minimal variation, the functional consequences are profound.
In human cells, the SARS-CoV-2 (search) version of Orf9b (search) disables an immune alarm system, allowing the virus to evade host defenses and multiply unchecked. In contrast, the Orf9b gene carried by RaTG13 (search) activates an immune protein in bat cells that helps suppress viral replication, effectively containing the infection.
This divergent behavior explains how a benign bat virus can, through minor genetic alteration, adapt to human hosts and cause severe disease.
First Lab-Grown Bat Lung Cell Line
A critical innovation enabling the study was the use of the first laboratory-grown lung cell line derived from the greater horseshoe bat—a species known to be a natural host of SARS-related coronaviruses, including SARS-CoV. This cell line allowed the researchers to conduct controlled comparisons of viral protein interactions across species in a physiologically relevant setting.
The animal reservoir of SARS-CoV-2 (search) has not yet been conclusively identified, but SARS-related coronaviruses have been detected in several species, including pangolins and horseshoe bats. Horseshoe bats are recognized as natural hosts of both SARS-CoV and SARS-related coronaviruses.
Toward a Molecular Early Warning System
The findings suggest that systematic mapping of viral protein interactions across potential host species could serve as a surveillance tool for pandemic preparedness. By identifying the molecular signatures that confer human infectivity, researchers may be able to flag high-risk animal viruses before they spill over into human populations.
The study underscores how viruses with very small genomes can achieve dramatically different infection outcomes through minimal genetic changes, reinforcing the importance of protein-level surveillance in zoonotic disease prevention.