Using ingenious molecular espionage, scientists have found how a single key enzyme, seemingly the Swiss army knife in HIV's toolbox, differentiates and dynamically binds both DNA and RNA as part of the virus' fierce attack on host cells. The work is described this week in the journal Nature.
The enzyme, reverse transcriptase (RT), is already the target of two of the three major classes of existing anti-HIV drugs. The new work, using single-molecule fluorescent imaging to trace RT's activity in real time, not only reveals novel insights into how this critical viral enzyme functions, but also clarifies how some of the anti-HIV pharmaceuticals work.
The research team, at Harvard University and the National Cancer Institute, was led by Xiaowei Zhuang at Harvard and Stuart Le Grice at NCI. Elio A. Abbondanzieri at Harvard and Gregory Bokinsky, formerly at Harvard and now at the Lawrence Berkeley National Laboratory, are lead authors.
"Our experiments allowed us, for the first time, a peek at how individual RT molecules interact with the HIV genome," says Zhuang, professor of chemistry and chemical biology and of physics in Harvard's Faculty of Arts and Sciences, as well as an investigator with the Howard Hughes Medical Institute. "We found that RT binds RNA and DNA primers with opposite orientations and that RT's function is dictated by this binding orientation."
HIV begins its assault by injecting its single-stranded RNA into a host cell. Three subsequent steps are all mediated by RT: The viral RNA is converted into single-stranded DNA, the single-stranded DNA is replicated into double-stranded DNA, and the original viral RNA is degraded. Another enzyme mediates the final step of the genome conversion, where the viral double-stranded DNA is inserted into the host's DNA, allowing it to take advantage of the host's genetic machinery to replicate and propagate itself.