Purdue: Researchers have discovered the structure of a
key protein on the surface of an unusually large virus called the
mimivirus, aiding efforts to determine its hosts and unknown functions. The mimivirus was initially thought to be a
bacterium because it is much larger than most viruses. It was isolated
by French scientists in 1992 but wasn't confirmed to be a virus until
2003. In the laboratory, the virus has been studied
while infecting amoebas, but its natural hosts in the wild and many
details about the virus remain unknown, said Pr Michael Rossmann. He led a team of researchers who discovered the
structure of a key, enzyme-like protein called R135, which is contained
in fibers on the outer surface of the virus. The structure of R135 is
similar to an enzyme called aryl alcohol oxidase, which is found in a
fungus and is involved in biodegrading lignin in plant cell walls.
"This could tell us something about the
mimivirus's natural hosts," Rossmann said. "We think there must be
another host, something different than amoebas and that this enzyme
helps the virus get into this host. Perhaps R135 participates in the
degradation of lignin so that the virus can enter a host such as
lignin-containing algae."
Also suggesting the possibility of alternative
hosts is the recent discovery that mimivirus has been found to be
abundant in oysters. Antibodies against mimivirus have been found in
humans, and the virus has been discovered inside specialized cells in
humans called macrophages, but whether it actually infects people is not
known.
"I wouldn't say it infects humans, but it can
propagate in humans because macrophages take up this virus and it can
propagate in these," said Thomas Klose, a assistant research scientist
at Purdue working with Rossmann. "Overall, there are many unknowns."
Research findings will appear in the June 2 issue of the journal Structure.
Mimiviruses are among the largest known viruses.
The virus approaches the size of bacteria and is about half of a micron
in diameter, more than 10 times larger than the virus that causes the
common cold and large enough to be seen with a light microscope. Other
viruses are too small to be seen with conventional light microscopes.
"It's one of the biggest viruses and has about a
thousand genes, many of which have unknown functions, and it seems to
be like a melting pot of enzymes," Klose said.
The R135 protein likely plays a key role in the virus's relationship with a smaller, "satellite virus" aptly called Sputnik.
Sputnik is referred to as a satellite virus
because it can only be propagated in combination with another virus, in
this case the mimivirus, which infects amoebas in laboratory research.
While inside the single-cell amoeba, the DNA for both Sputnik and
mimivirus are replicated, allowing the viruses to multiply.
"It is very likely that this R135 is the protein that Sputnik uses to co-infect with mimivirus," Rossmann said.
The Structure paper was authored by Klose;
Dominik A. Herbst, a former intern and now a graduate student at the
University of Basel in Switzerland; Purdue graduate students Hanyu Zhu
and Joann P. Max in the Department of Chemistry; Hilkka I. Kenttämaa, a
professor in the Department of Chemistry; and Rossmann.
The researchers used a technique called X-ray crystallography to determine the protein's structure.
The research was funded by the National Institutes of Health.