Tion via a thiolester linkage to an internal cysteine in E1, which then transfers the ubiquitin to a cysteine residue of an E2 protein. The E2 interacts with an E3 to mediate covalent attachment of ubiquitin onto substrate proteins. Repeated rounds of E2/E3mediated ubiquitin transfer result in polyubiquitylation, allowing substrate proteins to be recognized and destroyed by the proteasome. Vertebr