RaTG13 is a bat coronavirus that has gained scientific attention due to its genetic relationship to other coronaviruses, particularly those that have caused human outbreaks. Understanding RaTG13 provides valuable insights into the broader coronavirus family and their potential to cross species barriers.
Discovery and Classification of RaTG13
RaTG13 is classified as a bat coronavirus, specifically a sarbecovirus, a subgenus of betacoronaviruses. It was discovered in 2013 from fecal droppings of horseshoe bats, Rhinolophus affinis, in a mining cave in Tongguan, Mojiang county, Yunnan, China. Initially identified as “Ra4991,” it was later renamed RaTG13, incorporating its collection location (Tongguan) and year (2013).
The study of RaTG13 began as part of broader surveillance efforts to identify SARS-like viruses in bat populations. Researchers collected samples from bats in the Mojiang mining cave between 2012 and 2015. This research was prompted by fatal pneumonia cases among miners who had been cleaning bat guano in the same cave in 2012. From these samples, RaTG13 was identified as a novel coronavirus sequence from a 2013 sample.
RaTG13 and its Connection to SARS-CoV-2
RaTG13 garnered significant attention because of its genomic similarity to SARS-CoV-2, the virus responsible for COVID-19. Initial analyses in February 2020 revealed that RaTG13 shares approximately 96.1% nucleotide identity with SARS-CoV-2 across its entire genome. This high degree of similarity made RaTG13 a focal point in the early investigations into SARS-CoV-2’s origins, suggesting a potential natural evolutionary link.
Despite this genomic closeness, notable differences exist, particularly in the spike protein’s receptor-binding domain (RBD). The spike protein, specifically its RBD, allows the virus to attach to and enter host cells by binding to the ACE2 receptor. While RaTG13’s spike protein shows high conservation, its RBD region has significant nucleotide substitutions and shares only one of six key residues involved in binding to human ACE2. This difference means RaTG13 has a much lower binding affinity for human ACE2 compared to SARS-CoV-2, indicating it is not as efficient at infecting human cells.
Another distinction is the absence of a furin cleavage site in RaTG13’s spike protein, a feature in SARS-CoV-2 thought to enhance its infectivity. This difference, along with RBD variations, suggests that while RaTG13 is closely related, it is not the direct ancestor of SARS-CoV-2. Instead, it represents a parallel lineage within the sarbecovirus subgenus, indicating a common evolutionary origin but not a direct ancestral link to the human pandemic virus.
Understanding Bat Coronaviruses and Zoonotic Spillover
Bats are natural reservoirs for a wide array of viruses, including a remarkable diversity of coronaviruses. Over 1,400 bat species globally host an exceptional range of viral lineages, with more than 4,800 coronavirus sequences detected across six continents. This makes bats a crucial subject for understanding viral ecology and evolution.
Zoonotic spillover describes how a pathogen, such as a virus, jumps from an animal to a human host. This can occur through various pathways, including direct contact with infected animals, their bodily fluids, or via an intermediate animal host. Human activities like encroachment into wildlife habitats, wildlife farming, and trade can increase opportunities for such cross-species transmission events.
Studying bat coronaviruses is important for pandemic preparedness. Understanding the diversity of viruses circulating in bats helps scientists anticipate potential threats and monitor for new pathogens that could jump to humans. The identification of RaTG13, a SARS-like coronavirus in bats, underscores the value of ongoing wildlife surveillance. This research helps identify potential pathogens, understand their host range, and develop strategies to minimize zoonotic risk factors.