BioWorld International Correspondent

LONDON - An international team of researchers has shown that it is possible to block assembly of HIV in the test tube with a peptide, thus pinpointing a potential target for antiviral drugs.

The site at which the peptide binds has been mapped to a hydrophobic groove on the capsid protein of HIV, which is highly conserved in HIV-1 and HIV-2, and in the simian immunodeficiency virus.

Hans-Georg Kraeusslich, professor of virology at the University of Heidelberg in Germany, told BioWorld International: "What we have identified is not a drug, but we have identified a region on the viral structure that is a target for antiviral development. We believe that it may be possible to identify other, more stable compounds that bind to the same site as the peptide but are also taken up by cells."

The identification of a new target for antiviral drugs against HIV is important, and he said, "We need not just new drugs that hit the same viral targets, but alternative targets have to be identified if we are to overcome the problems of cross-resistance that are common in the treatment of HIV."

A report of the study appears in the August 2005 edition of Nature Structural and Molecular Biology. Its title is "A peptide inhibitor of HIV-1 assembly in vitro."

Kraeusslich and his collaborators, who are based in France, Germany and the UK, have called the inhibitory molecule the capsid assembly inhibitor (CAI). It binds to the capsid (CA) protein of HIV.

The authors wrote that "CAI integrates the potential of two inhibitors in a single molecule: a drug based on the CAI-CA interaction can be expected to block immature virus assembly in the infected cell and may also target escaping virions by being incorporated into the particle and subsequently blocking the functionally separate formation of the mature capsid."

In an accompanying paper in Nature Structural and Molecular Biology, Felix Rey of the Institute Pasteur in Paris, together with Kraeusslich, reported on the crystal structure of CA in complex with CAI. The title of that paper is "The HIV-1 capsid protein C-terminal domain in complex with a virus assembly inhibitor."

Kraeusslich and his team now are hoping to identify other compounds that will fit the CAI-binding site, both by screening libraries of compounds and by trying to design compounds that will fit the structure of the binding site.

Their work on HIV assembly has been rooted in the belief that the formation of a new virus particle from its components would be a good target for antiviral therapy. But that had never been explored in detail, partly because there was no good assay system to test compounds that interfered with viral assembly.

Kraeusslich and his colleagues overcame that problem by using an in vitro assembly system that contained purified CA protein or other parts of the main structural protein, Gag, and allowed assembly of virus-like particles.

Using phage display, they identified peptides that bound to the CA protein. They then analyzed a subset of those peptides to see if they could inhibit the assembly of the virus-like particles in vitro. That allowed them to identify CAI, which is 12 amino acids long.

When HIV is released from a cell, it initially forms an immature viral particle containing a shell of Gag. After the particle has left the cell, a viral protease cuts the Gag protein into several smaller components. That leads to a change in the structure of the virus, which is needed for it to become infectious.

"In our in vitro assembly system," Kraeusslich explained, "we can make both the immature particles and the mature particles. We have shown that CAI can block both of these stages."

Because peptides are easily broken down in the body, and are not taken up readily by cells, the team was not surprised to see that it did not inhibit the release of HIV from cells in tissue culture.

"In other words, CAI is far from being a drug, but it is nevertheless capable of blocking both assembly stages in an in vitro assembly system, and it does so quite efficiently. No such compound has been known before," Kraeusslich concluded. The team is hoping to use CAI to gain a clearer understanding of the viral assembly process.