Ubiquitin-conjugating enzyme Cdc34 and ubiquitin ligase SCF interact through multiple conformations [Protein Synthesis and Degradation]

November 25th, 2014 by Sandoval, D., Hill, S., Ziemba, A., Lewis, S., Kuhlman, B., Kleiger, G.

In the ubiquitin-proteasome system, protein substrates are degraded via covalent modification by a polyubiquitin chain. The polyubiquitin chain must be assembled rapidly in cells, since a chain of at least four ubiquitins is required to signal for degradation, and chain-editing enzymes in the cell may cleave premature polyubiquitin chains before achieving this critical length. The ubiquitin conjugating enzyme Cdc34 and ubiquitin ligase SCF are capable of building polyubiquitin chains onto protein substrates both rapidly and processively; this may be explained at least in part by the atypically fast rate of Cdc34 and SCF association. This rapid association has been attributed to electrostatic interactions between Cdc34s acidic C-terminal tail and a feature on SCF called the basic canyon. However, the structural aspects of the Cdc34-SCF interaction, and how they permit rapid complex formation, remain elusive. Here, we use protein crosslinking to demonstrate that the Cdc34-SCF interaction occurs in multiple conformations, where several residues from Cdc34s acidic tail are capable of contacting a broad region of the SCF basic canyon. Similar patterns of crosslinking are also observed between Cdc34 and the Cul1 paralog Cul2, implicating the same mechanism for the Cdc34-SCF interaction in other members of the cullin-RING ubiquitin ligases. We discuss how these results can explain the rapid association of Cdc34 and SCF.