Structure of the ubiquitin-activating enzyme loaded with two ubiquitin molecules

The structure of a ternary complex consisting of the ubiquitin-activating enzyme Uba1 and two differently bound ubiquitin molecules has been determined by X-ray crystallography. For the first time, the formation of the ubiquitin-acyladenylate could be structurally visualized together with the attachment of a second ubiquitin covalently linked to the active-site cysteine of Uba1. Model-building studies provide insights into the recruitment of ubiquitin-conjugating enzymes.