The attachment of the small protein Ubiquitin is one of the most abundant post-translational modifications in eukaryotes and plays a role in virtually all signal transduction pathways. Ubiquitylation marks proteins for degradation to control the abundance of key regulators in the cell and thus plays a role e.g. in cell cycle transition and the establishment of protein gradients in polarized cells and developmental processes. Aberrant ubiquitylation enzyme function gives rise to cellular dysfunctions that cause numerous human diseases. Although the basic principles of Ubiquitin protein modification have been identified, the exact catalytic mechanism of the ubiquitylation reaction is still unclear.
The ubiquitylation reaction involves in general the sequential action of an activating (E1), a conjugating (E2), and a ligating (E3) enzyme. Apart from certain classes of E3s, ubiquitylation enzymes contain a catalytic cysteine residue that forms a thioester intermediate with the carboxyl terminus of Ubiquitin, before the latter is covalently attached to a substrate lysine residue via an isopeptide bond (Figure 1). We seek to understand:
• How ubiquitylation enzymes function on an atomic level
• How their activities are controlled
• How ubiquitylation regulates cellular behavior
Figure 1. Left panel: Schematic representation of the Ubiquitination reaction involving a HECT-type E3. Question marks highlight the unknown mechanisms underlying Ub transfer and isopeptide bond formation. Right panel: 3D protein structure of a HECT domain (E3 ligase) - UbcH7 (E2 enzyme) complex