Scientists Determine 3-D Crystal Structure Of Cancer-Causing Protein

February 14, 1997

Work Links to Discovery Made at Rockefeller 86 Years Ago

The three-dimensional picture of a cancer-causing protein illuminates how a mutated gene transforms cells into cancer, report scientists from the Howard Hughes Medical Institute at The Rockefeller University in the Feb. 13 Nature. The determination of this structure clarifies earlier models that sought to explain how the gene, called src, works and offers new information for designing drug therapies to fight cancers.

"These findings help to explain how tinkering with just one protein produces changes in cell behavior," said senior author John Kuriyan, Ph.D., Rockefeller professor and HHMI investigator. "This gives us a better understanding of the breakdown of controls that lead to cancer." Kuriyan heads one of the university's laboratories of Molecular Biophysics.

Cells rely on proteins to regulate their differentiation and growth. Mutations that alter the normal function of control proteins result in unchecked growth and differentiation--cancer. In a process called phosphorylation--often the first step of signal transmission inside a cell--an enzyme called a kinase attaches a highly charged phosphate group to a tyrosine, one of the 20 amino acids that make proteins, acting as a molecular switch that turns proteins off and on.

In this study, Kuriyan and co-workers Frank Sicheri, Ph.D., and Ismail Moarefi, Ph.D., determined the three-dimensional structure of Hck, one member of the Src family of very closely related tyrosine kinases, named for src, the first oncogene that was found to be the mutated form of a cellular protein. The Src family plays an important role in regulating the body's immune system and other cellular behavior. Src is found in nearly all cells of the body, but Hck primarily arises in cells related to the immune system, such as macrophages and B cells. In a related paper published in this issue of Nature, a team of scientists led by Stephen Harrison, Ph.D., an HHMI investigator at Children's Hospital in Boston, determined the crystal structure of Src.

In the cellular signaling event known as signal transduction, proteins cluster together on the surface of the cell in response to an external signal. This clustering is mediated by modular units that carry the phosphorylated tyrosines, or phosphotyrosines. One of these modular units, Src homology (SH) domain 2, is a signaling molecule that enables interactions and contains docking units. Normally, Hck, like the other members of the Src family, is "off," folded into a position much like a snake biting its own tail. In this closed position, SH2 binds to the phosphotyrosine in Hck's tail, but if this phosphotyrosine is deleted, the structure opens up, enabling Src to seek out other proteins to phosphorylate.

Scientists have long known that, in the viral form of the Src protein, the phosphotyrosine is missing, resulting in the transformation of normal cells into cancerous ones. The work by the Hughes-Rockefeller team confirms earlier models that showed the change in the protein's shape that enables it to transform cells, but shows, unexpectedly, that it is the SH3 domain that is the prime controller of the enzyme's activity. "The role of the SH2 domain appears to be to set up the binding site on the enzyme's catalytic machinery for proper binding of the SH3 domain," said Kuriyan.

In a companion paper in the same issue of Nature, Kuriyan, Sicheri and Moarefi collaborated with researchers Michelle LaFevre-Bernt, B.S., and W. Todd Miller, Ph.D., from the State University of New York at Stony Brook to study the interaction of Hck with an HIV-1 protein called Nef. The results of this studied confirmed the finding that SH3 plays an important role in the activation of Hck. "We know that Nef binds tightly to the SH3 domain of Hck," said Kuriyan. "Nef turns out to be a potent activator of Hck activity."

This work was funded by the Howard Hughes Medical Institute. In addition, Miller received support from the federal government's National Institutes of Health. Sicheri is a fellow of the Human Frontiers Science Program.

On a historical note, in work that would win him a Nobel Prize in 1966, Rockefeller's Peyton Rous, M.D., Sc.D., discovered in 1911 that a virus causes cancer in chickens. This virus, which became known as the Rous sarcoma virus, was later found to contain a gene with the capacity to transform healthy cells into cancer cells. The gene, called src, was identified as a mutated version of a gene that is normally present in cells. Rockefeller's Hidesaburo Hanafusa, Ph.D., Leon Hess Professor, was among the first to provide direct evidence for the existence of Src in normal cells. Rockefeller faculty have made significant scientific achievements, including the discovery that DNA is the carrier of genetic information and the launching of the scientific field of modern cell biology. The university has ties to 19 Nobel laureates, including the university president, Torsten Wiesel, M.D., who received the prize in 1981. Recently, the university created four centers to foster collaborations among scientists from complementary fields to pursue investigations of Alzheimer's Disease, human genetics, neurosciences and the links between physics and biology.

Rockefeller University

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