Advanced cryo-EM imaging reveals high-resolution structure of viral RNA replication complex

Advanced cryo-EM imaging reveals high-resolution structure of viral RNA replication complex

For the first time, scientists at the Morgridge Institute for Research have generated near atomic resolution images of a major viral protein complex responsible for replicating the RNA genome of a member of the positive-strand RNA viruses, the large class of viruses that includes coronaviruses and many other pathogens. The results should aid development of new types of antivirals and provide mechanistic insights into the virus life cycle. "The rapidly advancing ability to visualize such crucial structures is game changing," says Paul Ahlquist, director of the John W. and Jeanne M. Rowe Center for Virology Research at the Morgridge Institute and professor of oncology and molecular virology at the University of Wisconsin-Madison. Other authors of the study included Nuruddin Unchwaniwala, Hong Zhan, Janice Pennington, Mark Horswill and Johan den Boon. Using an advanced technique called cryoelectron microscope (cryo-EM) tomography, Ahlquist and his team built upon their previous work, which first revealed the existence of this crown-like viral RNA replication complex. The new research, published July 20 in the Proceedings of the National Academy of Sciences (PNAS), shows the replication crown complex at a dramatically improved resolution of approximately 8.5 angstroms, which corresponds to the spacing of a few atoms. Cryo-EM has recently gone through a quantum leap in its capabilities. In this study our research group combined multiple advances to greatly improve sample preparation, image acquisition and image processing, and to map the position of specific protein domains in the complex." Paul Ahlquist, Director of the John W. and Jeanne M. Rowe Center for Virology Research at the Morgridge Institute The positive-strand RNA viruses addressed in this work are the largest of six genetic classes of viruses and include many important pathogens such as the Zika, dengue and chikungunya viruses, as well as coronaviruses like SARS-CoV-2, cause of the current COVID-19 pandemic. In each positive-strand RNA virus, most of the viral genes are devoted to a single process: replicating the viral RNA genome. "Given this massive investment of resources, viral RNA genome replication is arguably one of the most important processes in infection, and It is already a major target for virus control," Ahlquist says. Within an infected cell, viral RNA replication occurs at modified cellular membranes, often in association with spherules, virus-induced vesicles approximately 50–100 nanometers in size. Ahlquist and his team previously showed that in each such genome replication complex, a copy of the viral RNA genome or chromosome is protected inside the spherule vesicle to function as a replication template. The replication complex repeatedly copies this archival viral RNA chromosome to produce new progeny genomes that are released through a membranous neck on the vesicle into the cytoplasm, where they are incorporated as the payload of new infectious virions. Related Stories



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