Molecular espionage shows a single HIV enzyme's many tasks

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.

Using their molecular probe to spy on this process, Abbondanzieri and colleagues traced RT's multitasking skill to its dynamic active sites, which allow it to bind and process RNA as well as single- or double-stranded DNA.

"Remarkably, RT can spontaneously flip between these two opposite orientations on DNA and RNA to facilitate two distinct catalytic activities," says Abbondanzieri, a postdoctoral researcher in Harvard's Department of Chemistry and Chemical Biology. "These flipping motions, which have never before been seen in a protein-nucleic acid complex, can be likened to a nanoscale version of a gymnastics routine on a pommel horse."

The 180-degree flipping of RT is regulated by nonnucleoside RT inhibitors (NNRTIs), a major class of anti-HIV drugs. Abbondanzieri and coworkers observed NNRTIs inhibiting HIV activity by accelerating RT's flipping between its two active sites, hindering the enzyme's ability to convert single-stranded DNA to double-stranded DNA.

The other co-authors of the paper are Jennifer X. Zhang at Harvard and Jason W. Rausch at the NCI's HIV Drug Resistance Program. This work was funded by the Howard Hughes Medical Institute, the National Institute of General Medical Sciences, the Packard Foundation, the National Cancer Institute, and the Jane Coffin Childs Memorial Fund.