Newly revealed viral structure suggests a continuum in the evolution of viruses

An international team of scientists led by researchers at The Wistar Institute has combined two different imaging techniques to uncover the molecular-level framework of a common bacteriophage, a virus that infects bacteria. The results, reported in the October issue of Nature Structural Biology, suggest that viruses developed a continuum of progressively more complex architectural strategies to cope with their increasing size as they evolved. An image from the study is featured on the journal’s cover.

The new findings may open a novel approach to developing therapies for certain difficult-to-treat infections. The bacteriophage studied, called PRD1, infects antibiotic-resistant strains of E. coli bacteria, including strains responsible for tens of thousands of cases of food poisoning in the United States each year. The intimate knowledge of PRD1’s structure provided by the current study might help scientists develop a treatment for E. coli infections involving PRD1.

The structural details show that the bacteriophage has similarities to viruses smaller than itself, simple plant and animal viruses whose outer coats are formed from proteins held together by linked “arms.” In addition, however, it also uses small “glue” proteins to cement larger proteins together. This feature makes it more like the human adenoviruses, larger and more complex viruses that infect the respiratory tract and cause other diseases. Taken together, these features place the bacteriophage at an intermediate point on the viral evolutionary tree and help illuminate the overall evolutionary path taken by families of viruses.

The new images show not only the outer coat of the bacteriophage, but also reveal details of its inner membrane, a poorly-understood fatty double layer beneath the coat that forms a protective barrier around the genetic material, or DNA.

“We have been intrigued by the parallels between PRD1 and adenovirus since we discovered striking similarities in their overall structure in earlier studies,” says structural biologist Roger M. Burnett, Ph.D., a professor at The Wistar Institute and senior author on the Nature Structural Biology study. “Our results reveal that PRD1 also has similarities to simpler viruses and reinforce the idea that there is a continuum of viral architectures running through viruses that infect such different hosts as bacteria, plants, and animals, including humans. An appreciation of these parallels is important, as findings in one viral system may provide valuable insights into another. We have also learned more about membranes, which are very hard to study with conventional techniques, and see now how they can be involved in packaging viral DNA.”

The two imaging techniques used by the researchers to dissect the structure of PRD1 are electron microscopy and X-ray crystallography. Computer modeling was used to combine images of an entire virus particle provided by the relatively low-resolution technique of electron microscopy with the high-resolution molecular structure of the coat protein obtained through X-ray crystallography. The resultant “quasi-atomic” structure of the proteins forming the outer envelope of the virus was then stripped away by a kind of graphical “surgery” to reveal details of the other molecules forming the viral interior.