Contents
Research Briefs
Virus Anatomists
The P22 virus in full view (top). A cross section (below) reveals the mechanism that packs DNA into the virus. Targeting this mechanism could stop the virus from replicating. (Images by Gabriel Lander, The Scripps Research Institute)
John E. Johnson and his colleagues are peering into the tiny machinery of a virus to understand how it works. They are trying to figure out how a virus packs its DNA, a key process for the replication of some viruses. Johnson, a professor of molecular biology at The Scripps Research Institute, says his detailed studies of viral structure can help locate potential drug targets to keep a virus from replicating.
A focus of Johnson’s research is the bacteriophage P22, which infects the food-borne pathogen Salmonella. The P22 consists of little more than a sphere-like shell, called a capsid, its DNA, and a tail used to attach to Salmonella cells prior to infection.
By using a cryo-electron microscope (cryo-EM), Johnson, graduate student Gabriel Lander, and their colleagues have determined the method by which the P22 switches off the process of packing its DNA into the capsid. “A structure in the interior of the virus acts as a pressure sensor that tells the virus when it’s full of DNA,” says Johnson. When the capsid is full, the molecular configuration changes at the opening of the capsid’s base, which triggers a halt to the loading of DNA into the capsid. Assembly of the tail then begins, completing the virus.
Understanding how the P22 replicates was possible through use of the cryo-EM at the National Resource for Automated Molecular Microscopy (NRAMM) in La Jolla, California. Over the past three years, the NCRR-funded NRAMM has developed innovative tools that automate the process of image collection, speeding results and reducing labor for scientists. “Using cryo-EM typically requires a repetitive task of acquiring and processing thousands of images from different viruses in order to average them into one 3-D image,” says Bridget Carragher, director of the NRAMM. “We have developed technology that performs many of the tasks a microscopist would do, including image selection, to automate this process.”
“If the cryo-EM were not automated, it would have taken many months to obtain all the necessary data. With the automated system it took only a week,” says Lander. “This opens the possibility of doing studies that would previously not be feasible.”
Lander hopes this research will open the door to future clinical applications on similar viruses that affect humans, such as the herpesvirus, which causes oral and genital herpes, chicken pox, and mononucleosis. “If we could manufacture a drug that targets the pressure sensor during assembly, the herpesvirus would not package its DNA properly, rendering the virus unable to infect cells,” he says.
—Al Staropoli
NCRR Resources: The cryo-EM at the National Resource for Automated Molecular Microscopy has helped to conduct more than 50 studies to date and is open for use by external scientists. Researchers can request use of the cryo-EM resource by completing an online application. Applications are accepted year-round and reviewed within one month of receipt.
