Viruses can infect cells when they detect specific attachment sites on the cells’ surface. An international group of researchers, including biochemists from Tübingen, have now discovered the molecular mechanism by which the JCV polyomavirus attaches to these receptors. The researchers deciphered the atomic structure of the virus and for the first time ever were able to prevent the virus from attaching to the host cell and causing infection.
"We know very little about how viruses bind to cells and what they then go on to do," said Prof. Dr. Thilo Stehle, biochemist from the University of Tübingen. In principle it was known that the structure of the viruses and the cell surface properties had to match for the virus to be able to attach to the cell, rather like keys fitting into a lock. The head of the Interfaculty Institute for Biochemistry (IFIB) at the University of Tübingen explains that a single key is often not enough. There are several sites on the virus surface that can bind to sugar or protein molecules (i.e. receptors) on the cell surface. The viruses recognises and attaches to its host cells by way of these receptors, thus initiating infection.
Stehle's team and their international research partners have used a concrete example to describe the attachment mechanism with atomic accuracy. The researchers deciphered the atomic structure of the envelope protein of the JC virus, a polyomavirus that is very common in the human population. In addition, the atomic structure of JVC is the first atomic structure of human polyomaviruses ever published.
The JC virus is very common, infecting 70 to 90 percent of humans. However, the virus does not cause disease symptoms in the majority of people because the human immune system is able to effectively fend off the virus. This means that JCV infections are restricted to the renal tissue where they do not cause any damage. However, if the human immune system does not do its job properly, the JC virus can lead to life-threatening infections, for example in people suffering from AIDS or autoimmune diseases whose immune system needs to be suppressed with drugs. In cases like these, the virus can invade the human brain where it destroys the myelin sheath of the neurons, thereby preventing neuronal signals from being transferred from one neuron to another. JCV-infected people with severe immune deficiency develop progressive multifocal leukoencephalopathy (PML), a rare and usually fatal disease. Due to the clinical importance of JCV, the American NIH (National Institute of Health) has provided funding for the team's research.
The study found that the virus binds to a specific sugar molecule (LSTc, lactoseries tetrasaccharide c) on the host cell. The scientists were able to demonstrate the importance of their finding in the laboratory: when they brought JC viruses into contact with LSTc before the viruses were able to infect cells, the viruses were no longer infective because the attachment sites (receptors) had already bound LSTc. In order to obtain detailed insights into the attachment processes on the molecular level, the researchers from Tübingen took an atomically accurate snapshot of the virus at the moment it bound to LSTc. They let the JCV envelope protein (VP1) crystallise in order to be able to carry out X-ray structure analysis. This highly complex method delivers a very detailed picture of the protein structure. When LSTc was added to the VP1 crystals, the researchers were able to see in real-time how LSTc bound to VP1. The researchers from Tübingen showed that VP1 recognises LSTc from its L-shaped structure. In addition, the researchers identified the site on VP1 that enables the virus to attach to LSTc. They did this by introducing mutations at the specific site. The JC virus lost its ability to infect cells and proliferate. With this finding, the researchers have identified a site on the JC virus that can be modulated in a way that prevents the viruses from attaching to cells, something that is essential for infection and proliferation. “Our results form a powerful basis for developing new antiviral drugs,” said the researchers going on to add “because they make it possible to prevent viral infection at the very first step of infection, namely the recognition and attachment to the host cell.”The results of the study were published in the 21st October 2010 edition of “Cell Host & Microbe” in an article entitled “Structure-function analysis of the human JC polyomavirus establishes the LSTc pentasaccharide as a functional receptor motif”. The research was carried out by IFIB researchers at the University of Tübingen, researchers led by Prof. Walter Atwood at Brown University, USA, and researchers led by Prof. Ten Feizi at Imperial College London, Great Britain.Thilo Stehle has worked for many years on the question as to whether viruses have mechanisms or structures in common that enable them to recognise host cell structures. “We see our results as a basis for future research that will be carried out with other viruses. In addition, our findings also enable us to enter the drug design business.” Although the researchers were able to block viral infection with LSTc, this sugar is unsuitable as a drug as sugars generally only bind weakly. “Our goal is to develop a molecule that binds to the same site on VP1 as LSTc. However, the molecule needs to bind a lot more strongly in order to be able to effectively prevent the attachment of the virus,” said Stehle.It was only possible to obtain these research results in collaboration with top international teams. All the cooperation partners brought their specific skills to the project. The research team from London owns the world’s largest screening platform with which proteins and viruses can be tested for their ability to recognise different carbohydrates. This group identified LSTc as the binding partner of JCV. The research group from Brown University is a world leader in investigating the infectious pathways of human polyomaviruses and showed that LSTc indeed functions as receptor in infectious viruses. In addition, the group also showed that JC viruses lost their infectiousness when they were unable to recognise host cell LSTcs.As part of the project, the Interfaculty Institute for Biochemistry at the University of Tübingen aimed to analyse the atomic structure of the binding of the virus to LSTc. Only a few research groups around the world are able to successfully decipher virus-cell interactions with atomic accuracy, including Stehle’s team who have long-standing experience in this field. “Nevertheless, we had to overcome quite a few problems. One of these is the fact that the ligand only binds to the virus with low affinity, something that made the creation of the complex and the visualisation of the binding rather difficult in technical terms,” said Thilo Stehle. The protein crystals produced in the Tübingen laboratory were relatively small and the laboratory’s X-ray source was not strong enough for the efficient representation of the crystals’ structure. The scattering data were therefore recorded with a high-energy electron synchroton (Swiss Light Source) in Villigen, Switzerland.
Further information: Prof. Dr. Thilo StehleUniversity of TübingenInterfaculty Institute for BiochemistryHoppe-Seyler-Straße 472076 TübingenTel.: +49 (0)7071/29-73043E-mail: thilo.stehle[at]uni-tuebingen.de
Orginal publcation: Structure-function analysis of the human JC polyomavirus establishes the LSTc pentasaccharide as a functional receptor motif, Cell Host & Microbe 2010. An der Arbeit beteiligt sind außer dem IFIB der Universität Tübingen Forscher um Prof. Walter Atwood von der Brown-University, USA, und um Prof. Ten Feizi vom Imperial College London, Großbritannien.