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- Tuesday, 07 February 2017 07:52
A new insight into how viruses replicated based on X-ray crystallography work by a team at Thomas Jefferson University could ultimately lead to new antiviral drugs to treat pathogenic DNA viruses.
When a virus infects a host it takes genetic control of the host cells in order to generate myriad self-assembling copies of its viral particles loaded with the viral DNA ready to be sneezed, coughed, vomited or otherwise transported into the environment to infect the next hosts. Understandin how the DNA is loaded into those viral capsids is not entirely clear, particularly for human herpesvirus, one of the most common infectious viruses as well as for the kind of DNA viruses that infect bacteria.
Gino Cingolani of the Department of Biochemistry and Molecular Biology at TJU and colleagues at the Sidney Kimmel Cancer Center have now pieced together the three-dimensional atomic structure of a doughnut-shaped protein, which they explain, acts like a portal through which DNA can enter, and indeed exit, viral capsid. They have thus revealed that this protein begins to transform its structure as soon as it comes into contact with DNA. The team recently published details of the findings in the journal Nature Communications.
"Researchers thought that the portal protein acts as an inert passageway for DNA," explains Cingolani."We have shown that the portal is much more like a sensor that essentially helps measure out an appropriate length of DNA for each capsid particle, ensuring faithful production of new viral particles." This discovery resolves a mystery in viral molecular biology and offers up a new drug target for treating infection with herpesviruses. These viruses are responsible for a wide range of diseases including chicken pox, mononucleosis (often called glandular fever and caused by the herpesvirus known as Epstein-Barr virus), lymphomas and Kaposi's sarcoma.
The new discovery is the culmination of 18 years of research into the characterization of the structure of the portal protein using X-ray crystallography from P22, a virus that infects bacteria, its DNA portal is almost identical to the equivalent protein in human-infecting herpesviruses. In 2011, the team first described this portal protein as having a toroidal, or doughnut shape, although as if it were sitting on a pedestal and having twelvefold rotational symmetry. What intrigued them most was that paradoxically the protein itself did not have much affinity for DNA, which the researchers had assumed would be an essential characteristic of a protein that is required to interact with DNA.
"We figured the portal protein we had studied for over a decade must be an end-stage, or mature version, of a more plastic and dynamic molecular machine," explains Cingolani. "And that it must also adopt other conformations earlier in viral assembly that have the capacity to bind both DNA and other motor proteins, or terminases."
It is well known that many proteins are dynamic macromolecules that can change structure and so function depending on environment and biochemical activity. Indeed , many proteins go through multiple conformations before settling on their final, mature, active state. The intermediate stages are commonly unstable and exist only for a tiny fraction of second, although they may nevertheless have a biochemical role to play in these forms. In the new work, one such intermediate is characterised, this immature conformation of portal protein reveals itself to have a surprisingly asymmetric structure and can bind strongly to both the motor and the DNA itself.
"We think that DNA binds to the immature portal protein and wraps around it like a python, as it enters the viral capsid with the help of the motor protein. This DNA stranglehold causes the portal protein to begin to transform into its final symmetric state that because of its weak binding will ultimately release both the DNA and the motor, cutting off the DNA-loading at an appropriate length," explains Cingolani. "It's a completely novel mechanism for sensing DNA. It's a conformational change from asymmetric to symmetric that's completely unexpected, yet makes perfect sense."
These portal proteins are unique to the viruses that make them. As such they are near-perfect targets for antiviral agents that could be designed to block their activity without interfering with human proteins; a problem seen with many other drugs that attempt to target specific proteins. Some herpesviruses infect us but then lie dormant in human cells sometimes for decades until an exogenous factor, or stress, for instance, reawakens them. This seems to be the case with the painful and debilitating disease shingles, which is essentially a reawakening of a childhood chicken pox infection into a form that can damage nerves. Developing a therapy that could interfere with viral production at different levels could prove a useful therapeutic strategy.
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