The Kaposi’s sarcoma-associated herpes virus (KSHV) is a tumour virus that is involved in the formation of various types of cancer. In infected cells of the body, the viral DNA is largely present in latent form. This means that the virus does not actively proliferate, but is copied passively just once during each cell division. The latent viral DNA is not incorporated directly into the cellular genome, but is present as a separate ring-shaped molecule in the nucleus of the cell. In order not to get lost from the nucleus during the division of its host cell, this molecule attaches itself to the host chromosomes by means of an “anchor”. Scientists of the Helmholtz Centre for Infection Research (HZI) in Braunschweig recently deciphered the detailed structure of the anchor, which is a crucial contribution to understanding its function. The researchers published their results in PNAS.
KSHV causes various types of cancer diseases such as, for example, Kaposi’s sarcoma, from which the name is derived and which is characterised by uncontrolled proliferation of blood vessel cells in the skin. Under normal conditions, the virus is present in the body in latent form, which means it is kind of asleep. Only when the immune system of the patient is compromised, like in an AIDS patient or after organ transplantation, the virus becomes active and harmful to humans.
In order to survive in the body cells which constantly undergo cell division, the virus must attach itself to the genetic material of the human cells. This process of anchoring of the viral DNA is crucial for the survival of KSHV.
The virus therefore developed an anchoring protein for this purpose: the so-called “latency-associated nuclear antigen” (LANA). “Without this protein, the virus would not survive cell division,” says Dr Christiane Ritter, head of the NMR Spectroscopy unit at the HZI. “Therefore, detailed knowledge of the anchoring mechanism is of great interest for the development of potential medications.”
To selectively block the binding of LANA to the viral DNA by means of an agent might be a key for successful therapy of diseases caused by this virus. Dr Thorsten Lührs, former head of the Emmy Nöther Transmission Barriers group and his colleagues recently deciphered the three-dimensional structure of the DNA-binding module of LANA while it was binding directly to a short section of viral DNA.
In order to be able to visualise the interface between LANA and the viral DNA at near atomic resolution, the researchers crystallised a complex consisting of the DNA-binding module of LANA and the viral DNA. Then they elucidated the crystal structure at the “PETRA III” particle accelerator of the DESY (Deutsches Elektronen-Synchrotron) in Hamburg. This method reveals the molecular details of the components of the crystal – i.e. the LANA-DNA complex.
“Using this methodology, we succeeded not only to elucidate the structure, but also discovered a previously unknown third binding site for the anchor on the viral DNA, whose existence was proven in experiments by our collaborators at the Hannover Medical School,” says Jan Hellert, who is the principal author of the study and PhD student at the HZI.
The combination of the two results allows the researchers to draw a model that explains how a complex of three LANA units engages the viral DNA. This allowed them to decipher the function of the complex. “We did not only contribute a significant step to the development of new medications, but also made a contribution to the understanding of the interactions between viruses and humans during the latency phase of a tumour virus at the molecular level,” says Ritter.
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