The Mechanism of Linkage-Specific Ubiquitin Chain Elongation by a Single-Subunit E2

SummaryUbiquitin chains of different topologies trigger distinct functional consequences, including protein degradation and reorganization of complexes. The assembly of most ubiquitin chains is promoted by E2s, yet how these enzymes achieve linkage specificity is poorly understood. We have discovered that the K11-specific Ube2S orients the donor ubiquitin through an essential noncovalent interaction that occurs in addition to the thioester bond at the E2 active site. The E2-donor ubiquitin complex transiently recognizes the acceptor ubiquitin, primarily through electrostatic interactions. The recognition of the acceptor ubiquitin surface around Lys11, but not around other lysines, generates a catalytically competent active site, which is composed of residues of both Ube2S and ubiquitin. Our studies suggest that monomeric E2s promote linkage-specific ubiquitin chain formation through substrate-assisted catalysis.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (109 K)Download as PowerPoint slideHighlights► In addition to the thioester bond, Ube2S binds the donor ubiquitin noncovalently ► Ube2S transiently binds the acceptor, primarily through electrostatic interactions ► The catalytic center is composed of residues of Ube2S and ubiquitin ► Linkage formation by Ube2S is promoted by substrate-assisted catalysis