Voelcker Academy

Research Symposium 2009


previous next
Darren Sanchez

Proteolysis and the Cell Cycle: Cyclin-Dependent Kinase Inhinitor Regulation During the Vertebrate Cell Cycle

Darren Sanchez

Mentor(s): Dong Hyun Kim, and Renee Yew

The long-term goal of our studies is to understand how cyclin-dependent kinase (CDK) inhibitors are regulated during the vertebrate cell cycle to accurately trigger the start of S phase. Cell division is one of the most fundamental processes in living organisms and understanding the regulation of cell cycle progression is critical for determining why cancer cells divide unchecked. In cancer cells, CDK inhibitors are not expressed or are expressed at abnormally low levels and while the genes encoding Cip/Kip-type CDK inhibitors are not mutated, CDK inhibitor protein turnover appears to be aberrantly regulated. It is currently unclear how the proteolysis of CDK inhibitor proteins is coordinated at the molecular level with the events of the cell cycle or environmental stress responses. To understand the underlying mechanisms regulating CDK inhibitor function, we use the biochemically tractable Xenopus egg extract system to study the relationship between CDK inhibitor proteolysis and DNA replication and repair. Studying the Xenopus Cip-type CDK inhibitor, p27Xic1 or Xic1 (Xenopus inhibitor of CDK 1), our studies suggest that Cip-type CDK inhibitors are targeted to DNA by Proliferating Cell Nuclear Antigen (PCNA) and are ubiquitinated by the Cul4A-DDB1-Cdt2 ubiquitin ligase during DNA polymerase switching and DNA repair. We hypothesize that PCNA plays a central role in coordinating the proteolysis of Cip-type CDK inhibitors at sites of DNA replication and repair during the normal cell cycle and during a cell cycle checkpoint. During a DNA damage checkpoint, we propose that Cip-type CDK inhibitors are stabilized at sites of initiation to halt DNA replication while those at sites of DNA repair are ubiquitinated and degraded to allow for efficient PCNA-dependent repair.