PROJECT SUMMARY Human cell division is regulated at the G1/S transition before DNA is replicated. The primary drivers of cells through the G1/S transition are the cyclin-dependent kinases Cdk4 and Cdk6 in complex with D-type cyclins. The goal of this project is to determine the mechanisms that Cyclin D-Cdk4/6 complexes use to drive cell division. This is important for cancer because Cdk4/6 activity is elevated in many cancers including breast cancers, brain cancers, acute lymphoblastic leukemia, and neuroblastoma tumors. Inhibiting Cyclin D-Cdk4/6 is therefore a promising avenue for therapies targeting these cancers. Here, we propose to determine how Cyclin D-Cdk4/6 drives the G1/S transition. First, we will do this by identifying all the targets of Cyclin D-Cdk4/6 kinases in cells using a chemical genetic approach. Second, we will determine the molecular mechanism Cyclin D-Cdk4/6 uses to dock and phosphorylate its primary target, the retinoblastoma protein Rb. In preliminary data, we discovered that Cyclin D binds an alpha helix at the C-terminus of Rb. We now aim to determine the location on Cyclin D that docks Rb’s C-terminal helix using a combination of in vitro biochemistry and structural biology approaches. We propose to use knowledge of this docking interaction to develop a tool compound that disrupts the interaction and arrests the cell cycle. This will allow testing of the importance of the interactions in cells and provides a proof-of-principle that disrupting this interaction could be used in targeted cancer therapeutics. We will pursue two directions to identify a tool compound. First, we will develop a peptide inhibitor based on Rb’s C-terminal helix. Second, we will screen a small molecule library using a FRET assay. Taken together, the proposed experiments will provide an in-depth analysis of the Cyclin D-Rb interaction and will determine how it drives cell division. The broader impact of this study of the Cyclin D molecular docking mechanism and identification of Cyclin D-Cdk4/6 substrates may be the identification of new small molecules that improve cancer therapy.