Tetrameric Assembly of Monoubiquitin Accurately Mimics the Lys11 Polyubiquitin Chain Structure.

Specific lysine residues on the ubiquitin surface were selected during the course of evolution to form different polyubiquitin chain structures that signal diverse cellular processes. A vast number of ubiquitin receptors specifically recognize and decode the signals conferred by these polyubiquitin chains. The mechanisms of formation and the structure of Lys11-linked ubiquitin, which signals for cell-cycle and innate immune control, have been elucidated. Here, we present a new crystal structure of monomeric ubiquitin that accurately mimics one of the structures of Lys11-linked ubiquitin. Analysis of the ubiquitin:ubiquitin interface demonstrates structural fitness and specificity. The interaction is exclusively hydrophilic, leaving the Ile44 hydrophobic patch, a major recognition site for ubiquitin receptors, exposed. These noncovalent ubiquitin:ubiquitin interactions are nearly identical to those reported for Lys11-linked ubiquitin and seem to play a significant role in stabilizing the crystal structure without the isopeptide bond. In vitro cross-linking analysis with wild-type ubiquitin or its mutants partially mimics the interactions in the crystal. We suggest that these interactions may play a biological role in transmitting Lys11-linked ubiquitin chain-type cellular signals.