Processive ubiquitin chain formation by the anaphase-promoting complex

Abstract

Progression through mitosis requires the sequential ubiquitination of cell cycle regulators by the anaphase-promoting complex, resulting in their proteasomal degradation. Although several mechanisms contribute to APC/C regulation during mitosis, the APC/C is able to discriminate between its many substrates by exploiting differences in the processivity of ubiquitin chain assembly. Here, we discuss how the APC/C achieves processive ubiquitin chain formation to trigger the sequential degradation of cell cycle regulators during mitosis.

Figure 1. Sequential degradation of APC/C substrates during mitosis

The APC/C degrades its many substrates only after they have fulfilled their mitotic functions, thereby creating a sequence of degradation events. Processive APC/C-substrates are degraded earlier in mitosis than distributive substrates, and distributive substrates can re-accumulate more easily during late G1. In early mitosis, APC/C^Cdc20 is inhibited by the spindle assembly checkpoint, but few substrates, such as cyclin A and Nek2 can be degraded under these conditions. Later in mitosis, the APC/C switches to a substrate adaptor with broader specificity, Cdh1. During G1, APC/C^Cdh1 activity decreases through the degradation of the E2s Ube2C and Ube2S. P: prophase, PM: prometaphase, M: metaphase, A: anaphase, T: telophase.

Figure 2. Mechanism of processive ubiquitin chain formation by the APC/C

A. Yeast APC/C catalyzes the attachment of K48-linked ubiquitin chains to its substrates. The E2 Ubc4 first modifies substrates with ubiquitin molecules to initiate chain formation. The E2 Ubc1 then extends K48-linked ubiquitin chains with high processivity. Ubc1 requires a UBA-domain for processive chain formation. As both Ubc4 and Ubc1 have to interact with the RING domain in APC11 for activation, they need to act sequentially. B. Metazoan APC/C forms K11-linked ubiquitin chains on its substrates. Chain initiation is catalyzed by the E2 Ube2C, which interacts with the APC/C through the RING-domain in APC11. A second E2, Ube2S then elongates K11-linked ubiquitin chains with high processivity. Binding of Ube2S to the APC/C is most likely independently of the RING finger, and requires a positively charged C-terminal appendix. C. Mechanism of linkage-specific chain elongation by Ube2S. Activated ubiquitin (the “donor”) is transferred from the E1 to the catalytic site Cys residue of Ube2S. The donor ubiquitin engages in a non-covalent interaction with Ube2S, which is required for processive chain elongation. The “acceptor” ubiquitin, whose Lys11 will be modified, is then recognized by a different surface on Ube2S. Binding of the correct acceptor ubiquitin surface leads to formation of a catalytically competent active site, which is composed of residue of both Ube2S and the acceptor ubiquitin (E34). Thus, K11-linkage specific chain formation by Ube2S depends on substrate-assisted catalysis.