Abstract

Ubiquitination is a reversible post-translational modification that regulates diverse processes in eukaryotic cells, including protein degradation, transcription, intracellular trafficking, and the DNA damage response. The 76-amino acid ubiquitin protein is covalently attached to lysine side chains via its C-terminus, and ubiquitin itself can be ubiquitinated to give rise to a polyubiquitin chain. The precise nature of the ubiquitin modification – monoubiquitin or polyubiquitin, the particular lysine used to build up a polyubiquitin chain – governs the biological consequence of the modification. I will describe structural and biochemical studies of enzymes responsible for both ubiquitination and deubiquitination, as well as of the different nature of different types of polyubiquitin chains. I will present recent results on the four-protein DUB module of the SAGA complex, which deubiquitinates histone H2B and plays a role in transcription activation and elongation. The structure of the DUB complex provides insights into how the different subunits activate the enzymatic activity of the complex and target it to nucleosomal substrates. I will also present studies of the SUMO dependent E3 ligase, RNF4 , and how the way in which it binds to ubiquitin-conjugating E2 enzymes governs the switch between mono and polyubiquitination.