BioWorld International Correspondent
LONDON - Small molecules that could "freeze" two parts of the rabies virus together so that the virus cannot replicate could provide new treatments for rabies - and other viruses that have the same structure.
The latest insights into the 3-dimensional structure of the rabies virus show how keeping a hinged protein clamped shut could prevent the virus from unleashing its genetic material. The discovery also could apply to the measles virus, and the viruses that cause Marburg disease and Ebola disease.
Winfried Weissenhorn, of the European Molecular Biology Laboratory (EMBL) outstation in Grenoble, France, said, "Our results have implications for the design of new drugs against rabies, but they also suggest new therapeutic approaches in a variety of diseases, some of which are even more threatening than rabies."
Rabies is a fatal central nervous system disease, which frequently is transmitted to humans by the bite of infected animals, such as dogs and bats. Although an effective vaccine and postexposure treatment are available, each year 40,000 people die of rabies worldwide, mainly in China, Bangladesh and Pakistan.
The detailed structural images of the rabies virus obtained by Weissenhorn and his colleagues, with collaborators at the Institut de Virologie Moleculaire et Structurale (IVMS) are published in the June 15, 2006, issue of Science. Their paper is titled "Crystal structure of the rabies virus nucleoprotein-RNA complex."
Although rabies, Ebola and measles are diseases with widely differing symptoms, they all are caused by viruses that carry their genetic information in a single strand of RNA, rather than double-stranded DNA.
When the rabies virus enters a human cell, its RNA must reach the nucleus in order to take over the cellular machinery, directing it to produce new copies of the rabies virus. To ensure that the RNA reaches the nucleus undamaged, it is condensed into a helix and covered with a protein called nucleoprotein.
Rob Ruigrok, head of the IVMS, said: "Nucleoprotein is vital for the rabies virus. It is one of the few proteins that the virus brings into the host cell, and it wraps around the RNA like a protection shield. Without this shield, the RNA would be degraded by the enzymes of the human immune system that try to eliminate the invader."
In order to investigate exactly how that protection shield works, Aurelie Albertini, from Ruigrok's team, obtained crystals of nucleoprotein bound to RNA. Amy Wernimont from Weissenhorn's group at EMBL Grenoble then examined the crystals with high-intensity X-ray sources at the European Synchrotron Radiation Facility (ESRF), to produce a high-resolution image of the protein.
Weissenhorn said: "We found that nucleoprotein acts like a clamp. It consists of two domains that - like two jaws - clasp around the RNA strand. Many nucleoproteins bind side-by-side along the length of an RNA molecule and make it inaccessible for degrading enzymes but also for the machinery needed to replicate the virus. That means that the protection shield must be flexible and able to distinguish between different types of enzymes trying to gain access."
Writing in Science, the authors suggested that although the tight wrapping of RNA by nucleoprotein may be to prevent immune recognition, it also may be important for the virus to protect its genome during virus assembly and budding from the host cell's membrane.
Weissenhorn and his colleagues speculated in their paper that the nucleoprotein and the RNA have to break apart in order for replication of the RNA to take place. Their images have identified a flexible region in the nucleoprotein/RNA complex, which may act as a hinge that moves the upper jaw out of the way when the virus needs to replicate its RNA.
Ruigrok said: "This dynamic mechanism makes nucleoproteins an excellent drug target. Small agents that bind to the protein in such a way as to block its flexibility and keep it in the closed state, would prevent replication of the virus and would stop it from spreading."
Weissenhorn, Ruigrok and their colleagues went on to look at electron micrographs of the relevant parts of measles virus and Marburg virus, and concluded that those viruses, too, have a two-domain structure. In their conclusion, they wrote in Science: "This suggests that these enveloped viruses employ an RNA sequestering mechanism similar to that observed for the rabies virus [nucleoprotein]-RNA complex. The [nucleoprotein]-RNA polymer has thus evolved as the ideal template for the polymerase activity, which exposes the genomic RNA only temporarily to the host cell defense systems during replication."