Just like a ball, driven into the goal, causing the net to bulge out and wrap itself closely around the leather: This is how it appears when the Simian Virus 40 (SV40) penetrates into a cell. The virus docks onto the cell membrane, which in turn invaginates deeply, wraps itself tightly around the intruder and buds into a vesicle that is finally pinched off inside the cell.
Until recently, scientists were unable to explain how this dramatic rearrangement of the cell membrane took place in order to make it possible for the virus to penetrate, because only few proteins seem to be involved in this process.
A new work, which has just appeared in Nature Cell Biology, now throws new light on the mechanism by which the SV40 outwits its host: it exploits the components of the cell membrane itself, fats.
If a virus wants to reproduce itself, the same question generally arises: How does it get into a cell in order to use the latter’s reproductive mechanism for its own purposes? After all, although viruses carry with them a short piece of genetic information depending upon their type, they need to penetrate into the cell and its nucleus in order to propagate their genome. There, the cells’ replication machinery is reprogrammed to produce new viruses, which finally abandon the cell and infect further cells.
SV40 has developed a unique strategy. Instead of binding to a protein receptor in the plasma membrane and entering vesicles created by an apparatus around the protein molecule clathrin, this polyomavirus attaches itself to lipids. It does however not bind to one receptor, but many lipid molecules, in a similar way to a Velcro fastener. The individual connections are weak, but many connections taken together are strong. As soon as the virus has connected itself to many fats, the plasma membrane of the cell changes dramatically: It undergoes deep invagination and, in the course of time, completely surrounds the virus and finally forms a vesicle, which is pinched off inside the cell.
Interestingly the many connections are not only important for virus binding, but also for this membrane deformation process. The researchers could show that different molecules binding to the same fat cannot deform the membranes. At least five fat connections were required for membrane deformation. The membrane is then organized so closely around the virus that hardly any space remains between its surface and the virus. The virus in this way optimizes the number of connections with the membrane and can exert a strong force on the membrane sufficient to deform it without the help of cellular proteins.
In addition, the correct fats must be present on the surface of the virus. The carbon chains forming part of the fat must be of the correct length. If they are too short, then the membrane does not invaginate. This has been demonstrated by experiments with structurally altered fats.
“It surprised us that there is a relationship between structure and function even with fats,” observed Helge Ewers, ETH group leader, who signs as primary author of the paper. With proteins, such key/lock principles are common. “With this work we have proven that it can also be the same for fats.”
In co-operation with the Curie Institute in France the ETH researchers were able to show that SV40 is not the only pathogen that gains entry to cells via multi-lipid connections. This route is also taken by bacterial toxins, such as, for instance, the cholera toxin or mouse polyomavirus. It thus seems to be a widespread mechanism.
It is not yet possible, however, to use this knowledge therapeutically. Antiviral medicines continue to eliminate the infected cells. Finding active substances that block the viral fat connection is considered by Ewers and Helenius to be a difficult task.
SV40 naturally infects Asian apes, such as macaques and rhesus monkeys. It can also be passed on to humans. In the 1960s it was discovered in cultures of kidney cells from rhesus monkeys. The cells were used for the production of vaccines against polio.
During the inoculation from 1955 to 1963 several million humans were probably infected with SV40. Like other polyomaviruses, SV40 can cause tumors under certain conditions. However, in most cases the infection remains symptom-free. In humans no direct connections between an SV40-infection and the emergence of cancer could be proved. Oncogenes of this virus, however, play a role in the emergence of cancer cells from human cells in cell culture.