Proteolysis by the ubiquitin-proteasome-system serves essential functions in the eukaryotic cell division cycle. A multi-subunit ubiquitin ligase termed anaphase promoting complex (APC) controls chromosome separation and exit from mitosis. The APC cooperates with two related WD proteins, called Cdc20 and Hct1 in budding yeast, to ubiquitinate an anaphase inhibitor, various B-type cyclins and other protein substrates during mitosis and G1. The objective of this proposal is to define the molecular steps by which the APC recruits its substrates. We most recently observed that Hct1 can bind a specific subset of APC substrates suggesting that WD proteins may function as substrate receptors. To follow this idea we will study the binding pattern of Cdc20. We will moreover determine interaction domains in substrates, WD proteins and the APC, and further investigate the cell cycle regulation of Cdc20 and Hct1. By these and further studies we wish to build a detailed picture of the substrate recognition process.
