New find exposes apoptosis drug challenge

The three-dimensional structure of the anti-apoptosis protein Bcl-w has unexpectedly revealed a protective 'tail' which sits in a groove preventing cell death from being triggered accidentally, explaining why attempts at designing drugs capable of specifically triggering cell death may be more challenging than first appreciated.

The finding, which has been published in the EMBO journal by scientists from Melbourne's Walter and Eliza Hall Institute, the Ludwig Institute for Cancer Research and the University of Otago in New Zealand, may lead to new cancer therapeutics specifically targeted to the site, as well as better understanding of the events that trigger cell death.

Bcl-w is a member of the Bcl-2 family of proteins, which includes several members with a pro-survival function in cells. A group of small proteins known as the BH3-only proteins, can specifically bind to the pro-survival molecules, causing initiation of apoptotic cell death. But the BH3-only proteins are sequestered until they are activated by cell damage.

"They seem to function as sensors of cellular well-being," said Dr David Huang, one of the WEHI researchers involved in the project.

Previous structural work had shown the presence of the BH3-binding groove, but it was thought that the groove was freely accessible to binding by ligands like the BH3-only proteins. As it turns out, the studies supporting this idea used a truncated version of Bcl-2 proteins, so that the effect of the tail was not noted.

"An important feature in folding of the pro-survival Bcl-2 family of proteins is that the groove is hidden by the tail. The tail binds to the groove but can be displaced by BH3-only proteins," said Dr Catherine Day, whose work at Otago University includes support from the Marsden Fund.

The tail, which is encoded by the C-terminal amino acids of the protein, probably blocks other proteins from binding to the groove and accidentally triggering cell death, she explained.

The discovery is significant for the researchers, who are trying to develop drugs to mimic the action of BH3 and trigger cell death for use as anti-cancer therapeutics. The new information means that drugs would have to closely mimic BH3-only proteins in order to knock the tail out of the way and bind to the groove.

"One reason we embarked upon this was because we were interested in seeing if understanding of the Bcl-2 family could be exploited for therapeutic purposes," said Huang.

The next step, he said, will be to screen a library of compounds using full-length rather than the truncated structures used previously, hopefully finding molecules that more closely resemble BH3-only proteins.

Molecular interations

In addition, the researchers have a series of structural studies in progress at the moment, focusing on the complex formed by BH3-only proteins with Bcl-2 family members. According to WEHI researcher Dr Mark Hinds, solving the three-dimensional structures of a number of family members will be necessary to provide insight into the precise molecular interactions necessary to set off cell death.

"We want to understand every interaction in the complex so we can get a complete picture. It will give us the best chance possible of getting specific and effective agents," he said.

A multi-disciplinary team at the WEHI, including chemists, biologists, structural biologists and clinicians headed by Prof Jerry Adams, has embarked upon this task with funding from the prestigious Leukaemia and Lymphoma Society of America.

"Our interest is scientific, trying to understand the mechanism at the molecular level with the bonus of being able to use it to design therapeutics," said Huang.

Reference: Mark G Hinds, Martin Lackmann, Gretchen L Skea, Penny J Harrison, David CS Huang, and Catherine L Day. 'The structure of Bcl-w reveals a role for the C-terminal residues in modulating biological activity.' EMBO J. 2003 22: 1497-1507