By Dean Haycock

Special To BioWorld International

James Watson's account of the discovery of the structure of DNA, the double helix, would have lost much of its bestseller appeal if the competing researchers had agreed to cooperate. Fierce competition makes better press copy than accounts of scientists agreeing to publish simultaneously.

A good example of coordinated publication by separate research groups working on the same problem appears in the Sept. 17 issue of Nature. Authors of two papers representing laboratories in the United Kingdom, Spain and the United States agreed to publish their work simultaneously to avoid being forced to publish less complete accounts of their research so as not to be “scooped.“

Teams led by Nikola Pavletich, a investigator at the Howard Hughes Medical Institute at the Memorial Sloan-Kettering Cancer Center, in New York, and by Ernest Laue, a lecturer in the department of biochemistry at the University of Cambridge, in the U.K., both succeeded in showing how important tumor suppressor proteins control the activity of an enzyme that plays a crucial role in controlling the cell cycle. Because this control system is defective in cancer cells, the work helps explain how, on a structural level, mutations in these key molecules lead to disease.

“We knew that we were both working on the same thing,“ Laue told BioWorld International. “It was done entirely in competition, but in the very late stages they contacted us and asked whether we would be willing to publish together. I was very happy to do that. It was very decent of them to contact us in the first place. It gave both of us the opportunity to do a good job with the analysis and the structure determination without having to worry that we were going to be scooped by the other group.“

The work represents a large effort by both groups to solve an important problem in cancer and structural biology.

“I think when two very large efforts are at a comparable stage, science certainly is best served by not rushing things and doing a half-baked job,“ Pavletich said.

Biggest Challenge: Purifying The Kinase

The paper by Pavletich and his colleagues is titled “Structural basis for inhibition of the cyclin-dependent kinase Cdk6 by the tumor suppressor p16INK4a.“ Like the work described by Laue and his collaborators in “Crystal structure of the complex of the cyclin D-dependent kinase Cdk6 bound to the cell-cycle inhibitor p19INK4d,“ the study provides a detailed picture of Cdk6, an enzyme that controls a crucial phase of the cell cycle.

Kinases such as Cdk6 are enzymes that regulate other proteins by adding phosphate groups to them. These cyclin-dependent kinases are themselves turned on and off in a closely regulated manner as cells progress through different stages in their normal cycles. The structures revealing how the enzyme binds tumor suppressors indicate for the first time how these crucial inhibitors work.

For both Pavletich and Laue, the most challenging aspect of this project was purifying the kinase.

“I think that our group and Nikola Pavletich's group are probably the first groups that have succeeded in over-expressing this protein and purifying it from recombinant sources in sufficient amounts to do this sort of structural analysis,“ Laue said.

The challenge was produced in part by the very brief time these enzymes exist in cells, Pavletich noted.

Cdk6 and other enzymes with related functions are closely regulated by cell-cycle inhibitors or tumor suppressors such as p19INK4d and p16INK4a. If these inhibitors are unable to bind properly to Cdk6, the enzyme is unregulated and the cell cycle progresses without control. Certain hereditary cancers such as familial melanoma are caused by loss of a tumor suppressor gene. Alterations in both suppressor genes and growth regulatory genes are present in most cancers.

X-ray analysis of the crystal structure of Cdk6 complexed with p19INK4d or p16INK4a indicates that the tumor suppressors inhibit the binding of a third protein, a Cdk6 activator called cyclin D. The tumor suppressors do this by binding to a site close to where cyclin D normally binds to the enzyme. Not only does the tumor suppressor block access of the activator to Cdk6, it also changes the three-dimensional structure of the enzyme in such a way that ATP, the source of phosphate for the kinase, cannot bind. This ability explains how tumor suppressors can inhibit the enzyme even after it has bound cyclin D.

For groups seeking potential drugs that target the ATP binding site, the picture of the enzyme provided by these structures may aid the design of compounds that are more specific for Cdk6, Pavletich said.

Furthermore, the two groups report that mutations known to occur in cancer cells produce alterations in regions of tumor suppressors that are involved in binding Cdk6. This confirms the link between p16INK4a binding and enzyme inhibition. Laue and his co-authors note that the newly described molecular picture also indicates how binding of p19INK4d would inhibit binding of another Cdk6 inhibitor called p27Kip1. Prevention of p27Kip1 binding to the Cdk6 enzyme might leave more of that inhibitor available to affect other enzymes involved in cell cycle control.

Next: How Cyclin D Turns Enzyme On

“Each of these proteins has many, many interactions with other proteins in the cell. Now we can start thinking seriously about trying to attack some of the other structures as well,“ Laue said.

The Cambridge scientists have had a longstanding collaboration with Du Pont Pharmaceutical Co., in Wilmington, Del. Through this association, they began working with researchers at Mitotix Inc., of Cambridge, Mass. Scientists from both firms as well as from the Centro Nacional de Biotecnología, Campus de Cantoblanco, in Madrid, Spain, contributed to the paper with the Cambridge group.

Pavletich noted that the tumor suppressors represent one way of turning Cdk6 off. His group now wants to study the way cyclin D turns the enzyme on.

“The information we are most excited about is when we can see the molecule in detail,“ Pavletich said. *

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