What does the virus of a virus look like?

The maturation process of virophages give us new insights.

July 06, 2018
First, a virus infects a host, such as a unicellular organism in the plankton in the ocean, and then its development is blocked because it is itself attacked by a virus? In this unusual tripartite event, the second virus (termed a virophage) helps the host to survive. Researchers at the Max Planck Institute for Medical Research in Heidelberg have now studied the atomic structure and maturation process of the Mavirus virophage, and thus contributed to understanding this system. Their results could, in the future, allow virus particles to be constructed whose properties can be modified, making them useful in biotechnology.

It looks like a football with its surface pattern of pentagons and hexagons, but is very much smaller and with more corners: a virus particle.  This is the part of the virophage, in which all the information and components are present, that are required for its multiplication. Just as the sewn leather sections of a football hold the air, the proteins of the shell of the virophage particle ensure that the genetic information (DNA) of the virus and other components are not released before the virus is inside the host cell. The researchers at the Dept. of Biomolecular Mechanisms at the MPI have now been able to determine the atomic structure of the shell protein molecules and define steps in the maturation of the assembled shell.

“We found that the shell is formed in a two-step process,” explains Diana Born, first author of the paper and PhD student in the group of Dr. Jochen Reinstein. The shell consists mainly of one protein, the major capsid protein. In the first step, hundreds of these molecules, together with other components such as penton, aggregate to form the shell and protect the enclosed virophage DNA. Under the proteins of the shell is an enzyme, a protease, which, in the second step, modifies the individual molecules of the coat protein by removing a short section of each.  One effect of this is to make the protein more stable under acid conditions. “This change in the shell probably enables the virophage to infect a new host cell,” say Diana Born and Jochen Reinstein. They believe that it also loosens the binding of the virophage DNA to the shell, which would be important for the release and subsequent duplication of the hereditary information in the host cell.

“It is particularly interesting that the proteins of the shell assemble correctly, without instructions from other proteins on how large the shell should be,” says Jochen Reinstein. This is very unusual for a particle with a diameter of around 75 nm (1 nm is one billionth of a meter). The researchers want to exploit this mechanism in order to influence the size of the particles via slight changes in the protein. This way, it should be possible in the future to create virus-like particles whose properties and contents can be varied at will and thus allow applications in biotechnology: for example as tiny bioreactors or as protective sheaths for the transport of sensitive biological molecules within cells.

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