Lords of the SUMO chains

Max Planck scientists identify a new class of SUMO E3 enzymes

Scientists at the Max Planck Institute of Immunobiology and Epigenetics (MPI-IE) in Freiburg identified a novel class of enzymes involved in protein sumoylation. This modification is essential for the regulation of diverse cellular pathways, in particular, for the reaction to diverse stress stimuli. The detailed biochemical characterization of the founding member ZNF451 revealed a yet unknown enzymatic mechanism and provided important insights in how SUMO chains are formed. Two complementary studies on the characterization of this new enzyme have been published in Nature Structural & Molecular Biology

Protein modifications are a fast and efficient way to alter the functional properties of the cellular protein pool in response to environmental stimuli. One well-known modification is sumoylation, where the small protein SUMO (Small Ubiquitin related Modifier) is attached to a target protein. Sumoylation regulates various cellular processes such as transcription, replication, chromosome segregation and DNA repair. In particular, sumoylation is important to respond to diverse stress stimuli. As stress exposure causes the onset of many diseases including cancer the understanding of the basic principles of protein sumoylation and the identification of its main regulators is fundamental. 

The attachment of SUMO to its target protein (substrate) requires three sequential enzymatic steps that involve the activating E1 enzyme, the E2 conjugating enzyme and the E3 ligating enzyme. The E3 enzymes finally select the substrates. Although thousands of proteins are modified with SUMO, only a handful of E3 enzymes are currently known. Researches of the Max Planck Institute of Immunobiology and Epigenetics in Freiburg in collaboration with the University of Eastern Finland in Kuopio, and the Memorial Sloan Kettering Cancer Center in New York were now able to identify and characterize a novel family of these rare SUMO E3 enzymes: the ZNF451 family.

Already in 2008, the ZNF451 protein was identified as transcriptional co-regulator by researchers in the group of Jorma Palvimo at University of Eastern Finland. Palvimo suspected that this protein could act as E3 ligase and thus, contacted Andrea Pichler, group leader at the Max-Planck-Institute Freiburg and expert in the analysis of E3 ligases. Pichler was excited to test the protein for its putative E3 ligase activity: „The experiences my team and I collected studying other E3 ligases provided an encouraging advantage, but it was still a very long way to go.“

“The main challenge was, that ZNF451 in some assays did not behave as E3 enzymes usually do. Classic E3s are known to interact with their E2 enzymes to allow an efficient transfer of SUMO from the E2 to the substrate (Figure 2, left panel). But at first we simply could not detect this interaction.” says Nathalie Eisenhardt co-first author of the study. However, after thorough and sophisticated biochemical analysis combined with intricate cell biology studies the team was finally able to prove the enzymatic activity of the protein. Nathalie Eisenhardt explains: “In case of ZNF451 we learned that an additional SUMO is required for its interaction with the E2 and this finding allowed us to uncover a novel mode of action (Figure 2 middle panel).” And Andrea Pichler adds excitingly: “But there was more. It turned out that this additional SUMO interacting interface can be used to extend a so-called SUMO chain. With this finding we simultaneously provided novel insights into how SUMO chains are formed (Figure 2, right panel). Thus, we dubbed ZNF451 and its three relatives as our ‘Lords of the SUMO chains’.”

SUMO chains are known to have important functions in response to stress (e.g. DNA damage, virus infection, heat shock etc.) suggesting a potential biological function for ZNF451. Indeed, cells that lacked ZNF451 did not show any defects under normal growth conditions. However, when the team stressed these cells, they could not anymore efficiently react and modify the proteins with SUMO. These exciting findings open many avenues for future research. “We are now interested in finding key substrates for the ZNF451 family of E3 ligases. In addition, we would like to focus on the role of the ZNF451 family in stress-induced diseases. In particular, we envision that these enzymes could represent promising drug targets for these diseases and this is what we would like to understand in the upcoming years” says Andrea Pichler.