Dynamic Multisite Substrate Recognition by the SCFCdc4 Ubiquitin Ligase

Speaker: Veronika Csizmok, Postdoc, The Hospital For Sick Children, Ontario, Canada
Host: Karen Skriver, Biomolecular Sciences

Abstract
The ubiquitin ligase SCF^Cdc4 mediates the phosphorylation-dependent elimination of numerous substrates through recognition of one or more Cdc4 phosphodegrons (CPDs). One of the best characterized substrates of Cdc4 is the disordered cyclin dependent kinase inhibitor Sic1, which contains multiple CPDs and forms a dynamic complex with Cdc4 leading to switch-like response, referred to as “ultrasensitivity”, for degradation of Sic1.

We investigated the dynamics of substrate-Cdc4 interactions by NMR analysis of assigned methyl groups in the isolated WD40 domain of Cdc4. Our results revealed that extended multisite phosphodegrons of Sic1 engage the WD40 domain through two distinct sites that antagonize each other's interactions, facilitating exchange between multiple degrons and enabling favorable long-range contributions to binding. This novel mechanism help explain the longstanding conundrum of how multiple low affinity sites can generate a rapidly exchangeable yet high affinity interaction.

We also investigated the binding of disordered oncoprotein c-Jun to Cdc4, and our data demonstrate that a multisite, dynamic interaction occurs between the Cdc4 and c-Jun, similar to that previously observed for Sic1. Moreover, we found that the prolyl cis-trans isomerase Pin1 isomerases a Ser-Pro bond at the N-terminal of c-Jun that affects the binding to Cdc4 and thus regulates the ubiquitination and proteasomal degradation of c-Jun. Together, these results provide a detailed picture of interactions of the cancer-associated Cdc4 and facilitate understanding of the oncogenic process mediated by Jun.