Cullin neddylation may allosterically tune polyubiquitin chain length and topology

Conjugation of Nedd8 (neddylation) to Cullins (Cul) in Cul-RING E3 ligases (CRLs) stimulates ubiquitination and polyubiquitination of protein substrates. CRL is made up of two Cul-flanked arms: one consists of the substrate-binding and adaptor proteins and the other consists of E2 and Ring-box protein (Rbx). Polyubiquitin chain length and topology determine the substrate fate. Here, we ask how polyubiquitin chains are accommodated in the limited space available between the two arms and what determines the polyubiquitin linkage topology. We focus on Cul5 and Rbx1 in three states: before Cul5 neddylation (closed state), after neddylation (open state), and after deneddylation, exploiting molecular dynamics simulations and the Gaussian Network Model. We observe that regulation of substrate ubiquitination and polyubiquitination takes place through Rbx1 rotations, which are controlled by Nedd8-Rbx1 allosteric communication. Allosteric propagation proceeds from Nedd8 via Cul5 dynamic hinges and hydrogen bonds between the C-terminal domain of Cul5 (Cul5^CTD) and Rbx1 (Cul5^CTD residues R538/R569 and Rbx1 residue E67, or Cul5^CTD E474/E478/N491 and Rbx1 K105). Importantly, at each ubiquitination step (homogeneous or heterogeneous, linear or branched), the polyubiquitin linkages fit into the distances between the two arms, and these match the inherent CRL conformational tendencies. Hinge sites may constitute drug targets.