Scientists searching for molecular clues to cancer have produced
a detailed picture showing how a key protein blocks a central
promoter of cell growth involved in virtually all human cancers.
The discovery sets the stage for developing drugs to mimic the
protein, called p27, with the hope of halting the uncontrolled
cell division that ultimately leads to the formation of tumors.
The three-dimensional image, produced in atom-by-atom detail using
a combination of x-rays and crystals, was published in the July
25th issue of Nature by a team of researchers at Memorial Sloan-Kettering
Cancer Center in New York City led by Nikola P. Pavletich, phd.
The research is supported by the NIH and the Howard Hughes Medical
Cell division in many tissues is regulated by the p27 protein,
which binds to and inhibits enzymes called cyclin-dependent kinases
(CDKs). The crystal structure reveals in intimate detail how p27
paralyzes CDKs, thereby arresting the cell cycle.
Cyclin-dependent kinases are so crucial to cell division that
cells--even those that are cancerous--will not multiply without
them. Thus, the role of CDKs in tumor growth is a main focus of
cancer research, says Joan Massagué, phd, Program Chairman
for Cell Biology at Memorial Sloan-Kettering, and one of the paper's
"Now that we understand exactly how p27 attaches to CDKs
to halt the cell cycle, it may be possible to design or search
for innovative cancer drugs that work in the same way," says
Dr. Massagué, whose laboratory was the first to purify
and clone p27 in 1994.
Research Has Broad Implications
The researchers' high-tech picture also is likely to have broad
implications for other inhibitory proteins in the p27 family,
such as p21 and p57. "We expect these proteins, which are
structurally very similar to p27, to bind to CDKs in almost the
same way as p27," says Dr. Massagué. "This work
is important because it establishes a general principle by which
an entire family of inhibitors will block cell growth."
Under normal, healthy conditions, these inhibitors keep cell growth
in check as part of a universal pathway that, when altered by
genetic mutations, leads to cancer. "What is remarkable is
that this pathway, at one step or another, is crippled in many
cancers," Dr. Massagué says.
Researchers have known for years that p27 exerts its inhibitory
effect by binding to CDKs. Cyclin-dependent kinases are activated
when they bind with cyclins. When a cell is ready to divide, cyclins
latch onto CDKs, and the resulting complex activates other enzymes
to copy cells' DNA.
Now, by using a sophisticated technique called x-ray crystallography,
Drs. Pavletich and Massagué and their colleagues have shown
exactly how p27 attaches to the cyclin-CDK complex. The researchers
bombarded crystals containing millions of copies of the p27-cyclin-CDK
complex with intense x-rays directed from many different angles.
They then used an electronic detector to gather information from
x-rays that ricocheted off the crystals. A powerful computer analyzed
the data, and the researchers used the resulting information to
piece together the molecular structure.
The crystal structure reveals that p27 interacts with both members
of the cyclin-kinase complex, in this case cyclin A and CDK2 (see
Figure 1). Specifically, p27 inhibits cell division by inserting
itself into a pocket in the cyclin-CDK complex that normally binds
the energy molecule adenosine triphosphate (ATP). Without the
phosphate from ATP, CDKs can't activate the enzymes that copy
In recent years, scientists at Memorial Sloan-Kettering have focused
their efforts on elucidating the biologic and chemical controls
that govern the reproduction of individual cells. In 1994, in
Science, Dr. Pavletich reported using x-ray crystallography to
identify the atomic structure of p53, a protein produced by a
gene involved in triggering about half of all cancers. The p53
protein halts cell division by activating the p21 cell cycle-inhibitor
Then in 1995, Drs. Pavletich and Massagué and their colleagues
also used x-ray crystallography to identify one type of cyclin-CDK
complex, which is one of the targets of the p2l and p27 inhibitors.
This report was published in Nature.
The team now plans to use x-ray crystallography to reveal how
another class of cell cycle-inhibitors, among them p15 and p16,
binds to cyclin-CDK complexes to halt cell division.