Contact: Robert Irion (408/459-2495)

SCIENTISTS MAP THE STRUCTURE OF A PROTEIN-RNA COMPLEX IN A RETROVIRUS

* This news release is embargoed until 6 p.m. EST Thursday, November 16, 1995. The study will appear in the November 17 issue of the journal Science.

SANTA CRUZ, CA--Biochemists have published the first detailed view of the molecular tango between two critical units in the life cycle of a retrovirus--a virus that uses RNA, not DNA, to direct its genetic infiltration of a host cell.

The research reveals a precise fit between the viral RNA and a section of a protein that triggers the virus to make copies of itself. The protein segment lies snugly within a curved groove in the RNA, like a sausage resting in its bun. In an infected cell, this protein- RNA complex acts as a "switch" that turns on the assembly line for further virus production.

The scientists derived this structure by studying the bovine immunodeficiency virus, or BIV, which infects cows and is genetically related to HIV. The work may set the stage for a better grasp of a crucial step in the life cycle of HIV, which causes AIDS in humans. However, the researchers caution, there are significant differences in the ways that proteins bind to RNA in the two viruses, so an immediate application to HIV research is unlikely.

Joseph Puglisi, an assistant professor of chemistry and biochemistry at UC Santa Cruz, led the study, which appears in the November 17 issue of the journal Science. Coauthors are Lily Chen, a former postdoctoral researcher at UC San Francisco, now at the University of Vermont; Scott Blanchard, a technician in Puglisi's lab; and Alan Frankel, associate professor of biochemistry and biophysics at UC San Francisco.

"The interactions between RNA and protein are of paramount importance to the life cycles of retroviruses," says Puglisi, a member of the Center for the Molecular Biology of RNA at UCSC. "This is only the fifth or sixth example of a detailed structure of a protein-RNA complex, and it's the first time anyone has solved a protein-RNA structure in a retrovirus."

The complex studied here is a popular one: Tat-TAR, the subject of research in many labs investigating how retroviruses reproduce. The retrovirus, be it BIV or HIV, co-opts the genetic machinery of the host cell to make numerous proteins, including Tat, and to copy strands of the virus's own genetic material, RNA. The specific section of RNA to which the Tat protein sticks is called TAR. Once the Tat-TAR tete-a-tete occurs, the cell spits out raw material for new copies of the virus much more quickly.

In living cells, Tat is too large to study all at once. Thus, the team focused on one small bit of the Tat protein. The researchers used a technique called nuclear magnetic resonance (NMR) spectroscopy to find the structure of the Tat-TAR linkup. NMR probes the relative positions of atoms held in a magnetic field by tickling particular atoms with bursts of radio waves. A computer analyzes the data to predict the most likely positions of atoms with respect to each other. The final solution published in Science is the "best fit" of twenty structures churned out by the computer as good matches to the data. Before now, all other structures of protein-RNA complexes came from crystallography studies, rather than from NMR.

"At the structural level of how the protein sees the RNA, this is very much state-of-the-art," says Frankel. "It's a very important step in understanding how the complex between RNA and protein forms."

The final structure came as some surprise to the team. Biologists know of many examples of proteins attaching to one of the two grooves in DNA, which forms a double-stranded helix. RNA, which is single-stranded, also can twist into a spiral shape. However, the resulting grooves are much narrower--seemingly inhospitable rest stops for a bulky protein. In the TAR section of the viral RNA, a minor distortion widens the groove just enough to accommodate the Tat protein, which folds neatly back on itself to fit within the RNA's clutches.

"The irregularity opens up that narrow groove and basically beckons the protein to come on in," says Puglisi. "Just a small change in the structure lets the protein fit perfectly and make this very intimate contact."

The paper in Science presents both an overall picture of how the protein dovetails into the RNA groove and a detailed account of which parts of the Tat protein interact with which bits of the TAR RNA. The structure is consistent with studies from Frankel's lab at UCSF, where researchers probed the Tat-TAR interaction with biochemical, biophysical, and genetic methods.

Puglisi and Frankel hope to use the structure as a starting point to designing a substance that could cripple this stage in the proliferation of BIV. "These central steps in the life cycles of retroviruses can be their Achilles' heels if we figure out how to block the protein and RNA from interacting," Puglisi says. "Because this structure is so simple, it doesn't take a genius to think up some synthetic molecules that could do that."

But crossing over from BIV to HIV is not a simple proposition, he notes. Most notably, the Tat-TAR system in HIV appears to involve a third molecule, from the host cell, to form the final complex that triggers the genetic machinery to make more RNA.

Editor's notes: You may contact the principal researchers as follows:

Joseph Puglisi: (408) 459-3961 (office), (408) 459-3962 (lab), or puglisi@chemistry.ucsc.edu Alan Frankel: (415) 476-9994 or frankel@cgl.ucsf.edu

A color transparency of the protein-RNA interaction is available.

Press Releases Home | Search Press Releases | Press Release Archive | Services for Journalists

Maintained by:pioweb@cats.ucsc.edu