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Other Members of the Cell Cycle Family

The Cks proteins were originally discovered in yeast because of their interaction with another protein family that Reed has studied for over 15 years called the cyclin-dependent kinases. These are one of the crucial regulatory enzymes controlling cellular division and the cell cycle, a biological process that Reed has been studying for more than 30 years—since his days in graduate school.

The cyclin dependent kinases are binary proteins belonging to a large family of eukaryotic kinases—enzymes that exploit an abundant molecule in the cell known as ATP to attach phosphate groups to other proteins in the cell. In mammals, there are several different "cyclins" and a few different kinases that can come together at various points in the cell cycle to carry out specific phosphorylations.

In general, phosphorylation acts as a cellular "signal" that can do everything from turning the proteins on or off, controlling their transport, or even regulating survival of the cell.

"This is one of the primary modes of biological regulation," says Reed. And in the case of the specific action of cyclin-dependent kinases, he adds, "These phosphorylation events drive both mitosis and meiosis."

Cyclin dependent kinase-mediated phosphorylation is that event that gets the cell's transcription machinery running, duplicating the cell's DNA at just the right moment late in the cell cycle before a cell divides during mitosis. Naturally, these proteins must themselves be highly regulated, accumulating rapidly when they are needed to accomplish a specific task and disappearing when they are not. One of the primary modes of regulation is the degradation of the cyclin subunits when they are not needed.

Reed discovered one of these cyclin subunits, cyclin E, in the early 1990s and has since spent a considerable amount of time studying it. The protein activates by binding to the cyclin dependent kinase 2 protein and together the complex is involved in the initiation of DNA replication.

One facet of Reed's work is concentrated on the cyclin dependent kinases, their regulation, and their interactions with other proteins in the cell—besides those that they phosphorylate. The Cks proteins are a facet of this work. In meiosis, the cell cycle is controlled by the Cdk–Cks complex, the structure of which Reed and TSRI Professor John Tainer solved a few years ago. Reed's research suggests that the Cks proteins seem to serve as "adaptors" that allow the cyclin dependent kinases to interact more efficiently with their molecular targets.

Another facet of his work is how the cyclin dependent kinases and their regulation are relevant to human cancer. He has a multifaceted project that aims to understand protein turnover, the role of Cyclin E in carcinogenesis, and how the deregulation of the protein causes cell proliferation and cancer.

"Cancer is a disease of cell proliferation," he says. "And one of the reasons we study [the normal machinery of] cell proliferation is to understand how it works so that we can figure out what goes wrong."

CDK and Cancer

In some malignant cancer cells, the levels of cyclin E do not drop. When cyclin E is overexpressed, cells become genomically and genetically unstable—gaining and losing chromosomes. "This chromosome instability," says Reed, "is one of the hallmarks of cancer."

Reflecting the complexity of the cell, the loss of control is not necessarily related to problems with the cyclin E itself but rather problems with one protein that is supposed to control it.

Last year Reed published a study looking at another cellular protein called "hCdc4" which is a specificity factor for the cells' ubiquitin ligase machinery, which degrades proteins. The cell uses hCdc4 to target cyclin E to help the cell turn it over.

Reed discovered evidence suggesting that hCdc4 is a tumor suppressor protein—its presence works to counter the proliferation of cancer cells. He found that in certain types of cancer, particularly endometrial cancer, the hCDC4 gene is often mutated. Reed's group is now analyzing breast cancer samples for similar hCdc4 mutations. These mutations create forms of the hCdc4 protein that fail to target cyclin E, and this leads to the accumulation of cyclin E in the cell. The accumulation of cyclin E leads to the chromosomal instability, which is known to contribute to cancer.

Reed is trying to work out how and why cyclin E is turned over in order to address some of the problems that it causes with regard to cancer. Similarly, he is trying to figure out the role of the Cks proteins in all of this. Gene expression analysis studies of several different tumor cells have shown that Cks2 (and to a lesser extent Cks1) is one of the most frequently overexpressed proteins in cancer cells.

"Overexpression of this protein may be deleterious to cell integrity and [may be] part of the process of malignant transformation," says Reed, adding that he would like to know why, and upon finding out why would like to use that knowledge to formulate a strategy to stop it.


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CKS2-/- mice have a meiotic defect that blocks spermatogenesis. These are cross sections of semeniferous tubules of (A) mutant and (B) wild-type mice. Meiosis occurs at the outer edge of the tubule and spermatocytes develop into sperm as they move inward toward the lumen. Note elongated sperm with condensed nuclei near the lumen of the wild-type tubule. In contrast, the center of the mutant tubule is populated by dead and dying undivided spermatocyes with large nuclei.