Cyclin-dependent kinases & their regulation

Cyclin-dependent kinases are enzymes that are essential regulators of cell division. They are present in all eukaryotic cells and are the proteins that control the ordered process called the cell cycle. This process takes a single cell, duplicates its DNA, and creates two identical daughter cells from the parent cell. The importance of the discovery of cyclin-dependent kinases and the idea of the cell cycle as controlled by these proteins was recognized in 2001 with the award of the Nobel Prize in Medicine to the cell cycle pioneers Drs. Leeland Hartwell, Tim Hunt and Paul Nurse. The Nobel e-museum has several resources to explain the significance of their findings to look at these click here. There is also an interactive tutorial on the cell cycle on the Nobel site, to explore this click here.

My research focuses on one cyclin-dependent kinase: cyclin-dependent kinase 2 (CDK2). Like all kinases, CDK2 catalyzes a phosphoryl transfer reaction from ATP to specific amino acid residues on protein targets within the cell. CDK2 is one of several CDKs involved in starting the cell cycle. The job of CDK2 during cell division is to initiate the cycle and to maintain the commitment of the cell to completion of DNA replication. The CDK contains the active site of the protein where the chemistry of phosphoryl transfer occurs, but phosphoryl transfer is not performed efficiently without two modifications to the CDK protein. First, CDK2 must bind a cyclin partner. These proteins are essential to CDK function and are present in the cell only when the activity of their particular CDK partner is required. Second, the CDK has to be phosphorylated on a Threonine residue near the active site. Another protein kinase in the cell performs this modification.

Research Interests:

1. The chemistry of CDK2 phosphoryl transfer. CDKs are targets for chemotherapeutic drugs in the treatment of cancer. Cancer is the result of aberrant cell division, producing cells that do not control how fast or how carefully new cells are produced from their parents. Design of effective therapeutics is aided by a detailed understanding of exactly how reactions occur inside enzyme active sites. I am interested in determining the role of Mg2+ in CDK2 activity because this ion is very important in determining the architecture of the active site.

2. CDK2 regulation by its cyclin partners. CDK2 can pair with two different cyclin partners Cyclin A and Cyclin E at different times during the cell cycle. I am interested in the differences in these two protein complexes. Does CDK2/cyclin E or CDK2/cyclin A have higher activity? What portions of these proteins affect the activity of CDK2? Are they the same or different? How does the cyclin limit the number of targets phosphorylated by the CDK2 kinase?

3. The differences between CDKs in mammals and those in human parasites. Both humans and protozoan parasites are eukaryotes, however they diverged long ago and belong to different phylogenetic kingdoms. The reproduction of a single-celled malarial parasite and that of a human or a human cell are very different processes, however they both use similar CDK machinery. The genome sequence of the malarial parasite Plasmodia falciparum has recently been finished and with it came the discovery of several homologs of the cyclin-dependent kinases and cyclins. I am working with several other groups to elucidate the differences between the mammalian CDKs and those of the parasite. There are two reasons for this study. First, we are currently trying to study these enzymes with an eye toward finding inhibitors which will stop the cell cycle of the parasite without stopping the cell cycle of the human host. This will hopefully lead to the development of malarial treatments. Second, evolutionarily, protists and animals diverged from each other early in the development of eukaryotes. It is interesting to compare enzymes that are so vital to eukarkotic survival (all eukaryotes need CDKs to drive cell multiplication) between such divergent species. These comparative studies may shed light on the evolution of cell cycle control or the manipulation of cell cycle control to facilitate the different life cycles of these diverse organisms. Our contribution to this goal is a comparative study of the kinetic mechanisms of mammalian CDKs and those from P. falciparum.