Retroviruses are cagy buggers. Rather than waste their time and energy manufacturing their own machinery to make more of themselves, they stage a mass takeover of the cells of other organisms, inserting their DNA into that of the host so the host does the job for them. The retroviruses up on easy street and the host ends up with AIDS, cancer or some other retrovirus-caused disease. The more we can learn about how retroviruses do this, the better our ability to disable their ability to disable us. Now, Hideki Aihara, Zhiqi Yin and Ke Shi and colleagues from Cornell University and St. Louis University have made a major stride in that direction. Reporting in the February 18 issue of the journal Nature, the researchers tell how an experimental procedure that involves beaming X-rays at immobilized molecules allowed them to discover how a cancer-causing retrovirus called RSV brings together many copies of a protein (known as integrase) to form tiny molecular claws that insert RSV genetic material into that of a host cell, conscripting it to make more retroviruses.
Because RSV is a close relative of the HIV-1 retrovirus, the work has potential application for improving HIV/AIDS therapies.
“It can certainly help with the development of anti-retrovirals to target the integrase functions,” says Aihara, lead author of the paper and an associate professor in the Department of Biochemistry, Molecular Biology, and Biophysics. “Ultimately, we want to inhibit HIV integration, and for that purpose we need to know HIV intregrase’s complex structure.”
To conduct the study, researchers had to first figure out how to make a stable protein-DNA complex just the right size for analysis and immobilize it in crystalline form — a process that took several years. Next they bombarded it with X-rays and captured data on how the X-rays ping around as they travel through the crystal. Once the X-ray-scattering patterns were available, it took another three years of complex calculations using the Minnesota Supercomputing Institute’s state-of-the-art computing capabilities to derive from them the precise position and configuration of the freeze-framed molecules. In the end, they came up with a big surprise: Whereas other viruses use a complex of four integrase molecules to guide the host and viral DNA into position and connect them, RSV uses eight.
“The structure looked very different from what we anticipated,” Aihara says. “Initially it looked odd, but we started looking into details and it sort of all made sense.”
Aihara now has his eye on doing the same things with HIV integrase, which is more difficult to work with than RSV integrase, but would yield results even more useful for designing anti-HIV therapies. “We would really like to see whether this unexpected assembly is also the case for HIV,” he says. “We think it is, but we [need] evidence.” - Mary Hoff
Caption: By crystallizing molecules and then studying how X-rays bounce through the crystals, researchers discovered that a retrovirus known as RSV uses an eight-protein complex to grab onto a host cell’s DNA and insert viral DNA into it. The maneuver turns the cell into a retrovirus-making machine, sickening or killing the host. Image courtesy of Hideki Aihara.