Decoding the SUMO Signal

Cells in our body are controlled by switches that turn the activity of key proteins on or off. This can be done by tagging them with ubiquitin, another small protein. As the cell contains many thousands of proteins selecting the correct protein for tagging is critical. This is achieved by enzymes that pick out the target protein and carryout a chemical reaction linking it to ubiquitin. However we did not know how this reaction worked. By obtaining a detailed snapshot of the enzyme in a primed state about to carry out the tagging reaction we have helped to solve this problem.

We are focused on understanding the role and mechanism of action of ubiquitin and ubiquitin-like proteins in important biological processes and have helped establish conjugation with the Small Ubiquitin-like Modifier (SUMO) as an important regulatory mechanism in eukaryotes. In most cases ubiquitin and ubiquitin-like proteins are transferred to their target proteins from a thioester complex with a conjugating enzyme (E2), catalysed by an E3 ligase. There are thought to be more than 600 specificity-determining ubiquitin E3 ligases that influence almost all aspects of biological activity and most of these are members of the RING family of ubiquitin E3 ligases. Both ubiquitin and the Small Ubiquitin-like Modifier (SUMO) are integrated into the cellular responses to genotoxic and proteotoxic stress. The two pathways of ubiquitin and SUMO modification are linked by the RING domain containing protein Rnf4. We recognised that Rnf4 also contained multiple SUMO interaction motifs (SIMs) and demonstrated that it could function as a ubiquitin E3 ligase with a unique specificity for polySUMO chains.  Arsenic trioxide therapy for Acute Promyelocytic Leukaemia (APL) is mediated by SUMO-dependent degradation of Promyelocytic Leukaemia (PML)-Retinoic Acid Receptor α (RARα) oncogenic fusion protein. We demonstrated  that ubiquitin mediated proteolysis of PML-RARα is mediated by the RNF4 E3 ubiquitin ligase. This allows terminal differentiation of tumour cells and cures disease. Our objective is to establish the molecular basis for the remarkable specificity of arsenic in APL and to determine if it can be employed therapeutically in other situations. We are also investigating the role of RNF4 in the response of human cells to DNA damage; determining the mechanism of RING mediated ubiquitin modification; and carrying out a system wide analysis of SUMO modification in response to stress.