Research ArticleStructural Biology

Dimerization regulates the human APC/C-associated ubiquitin-conjugating enzyme UBE2S

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Science Signaling  20 Oct 2020:
Vol. 13, Issue 654, eaba8208
DOI: 10.1126/scisignal.aba8208

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Protected from destruction by dimerization

The activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase complex, is critical for mitotic progression and exit. The APC/C ubiquitinates substrates in conjunction with the E2 ubiquitin-conjugating enzyme UBE2S. This enzyme can autoubiquitinate itself and promote its own turnover, raising the question of how appropriate amounts of UBE2S are maintained in nonmitotic cells. Liess et al. found that dimerization of UBE2S prevented its autoubiquitination and kept this protein in an inactive state. Cells that expressed wild-type UBE2S maintained UBE2S concentrations high enough to exit from drug-induced mitotic arrest, unlike those expressing the dimerization-defective form of UBE2S. Thus, UBE2S may dimerize to prevent its autoubiquitination and turnover in noncycling cells and ensure its availability for future mitotic cycles.

Abstract

At the heart of protein ubiquitination cascades, ubiquitin-conjugating enzymes (E2s) form reactive ubiquitin-thioester intermediates to enable efficient transfer of ubiquitin to cellular substrates. The precise regulation of E2s is thus crucial for cellular homeostasis, and their deregulation is frequently associated with tumorigenesis. In addition to driving substrate ubiquitination together with ubiquitin ligases (E3s), many E2s can also autoubiquitinate, thereby promoting their own proteasomal turnover. To investigate the mechanisms that balance these disparate activities, we dissected the regulatory dynamics of UBE2S, a human APC/C-associated E2 that ensures the faithful ubiquitination of cell cycle regulators during mitosis. We uncovered a dimeric state of UBE2S that confers autoinhibition by blocking a catalytically critical ubiquitin binding site. Dimerization is stimulated by the lysine-rich carboxyl-terminal extension of UBE2S that is also required for the recruitment of this E2 to the APC/C and is autoubiquitinated as substrate abundance becomes limiting. Consistent with this mechanism, we found that dimerization-deficient UBE2S turned over more rapidly in cells and did not promote mitotic slippage during prolonged drug-induced mitotic arrest. We propose that dimerization attenuates the autoubiquitination-induced turnover of UBE2S when the APC/C is not fully active. More broadly, our data illustrate how the use of mutually exclusive macromolecular interfaces enables modulation of both the activities and the abundance of E2s in cells to facilitate precise ubiquitin signaling.

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