Molecular architecture and mechanism of the anaphase-promoting complex

MASTL promotes cyclin B1 destruction by enforcing Cdc20-independent binding of cyclin B1 to the APC/C

When cells enter mitosis, the anaphase-promoting complex/cyclosome (APC/C) is activated by phosphorylation and binding of Cdc20. The RXXL destruction box (D-box) of cyclin B1 only binds Cdc20 after release of the spindle checkpoint in metaphase, initiating cyclin B1 ubiquitination upon chromosome bi-orientation. However, we found that cyclin B1, through Cdk1 and Cks, is targeted to the phosphorylated APC/C^Cdc20 at the start of prometaphase, when the spindle checkpoint is still active. Here, we show that MASTL is essential for cyclin B1 recruitment to the mitotic APC/C and that this occurs entirely independently of Cdc20. Importantly, MASTL-directed binding of cyclin B1 to spindle checkpoint-inhibited APC/C^Cdc20 critically supports efficient cyclin B1 destruction after checkpoint release. A high incidence of anaphase bridges observed in response to MASTL RNAi may result from cyclin B1 remaining after securin destruction, which is insufficient to keep MASTL-depleted cells in mitosis but delays the activation of separase.

The ABBA motif binds APC/C activators and is shared by APC/C substrates and regulators

The anaphase-promoting complex or cyclosome (APC/C) is the ubiquitin ligase that regulates mitosis by targeting specific proteins for degradation at specific times under the control of the spindle assembly checkpoint (SAC). How the APC/C recognizes its different substrates is a key problem in the control of cell division. Here, we have identified the ABBA motif in cyclin A, BUBR1, BUB1, and Acm1, and we show that it binds to the APC/C coactivator CDC20. The ABBA motif in cyclin A is required for its proper degradation in prometaphase through competing with BUBR1 for the same site on CDC20. Moreover, the ABBA motifs in BUBR1 and BUB1 are necessary for the SAC to work at full strength and to recruit CDC20 to kinetochores. Thus, we have identified a conserved motif integral to the proper control of mitosis that connects APC/C substrate recognition with the SAC.

The biochemistry of mitosis

In this article, we will discuss the biochemistry of mitosis in eukaryotic cells. We will focus on conserved principles that, importantly, are adapted to the biology of the organism. It is vital to bear in mind that the structural requirements for division in a rapidly dividing syncytial Drosophila embryo, for example, are markedly different from those in a unicellular yeast cell. Nevertheless, division in both systems is driven by conserved modules of antagonistic protein kinases and phosphatases, underpinned by ubiquitin-mediated proteolysis, which create molecular switches to drive each stage of division forward. These conserved control modules combine with the self-organizing properties of the subcellular architecture to meet the specific needs of the cell. Our discussion will draw on discoveries in several model systems that have been important in the long history of research on mitosis, and we will try to point out those principles that appear to apply to all cells, compared with those in which the biochemistry has been specifically adapted in a particular organism.

Regulation of cancer-related pathways by protein NEDDylation and strategies for the use of NEDD8 inhibitors in the clinic

Post-translational modification of proteins with ubiquitin and ubiquitin-like molecules (UBLs) controls a vast if not every biological process in the cell. It is not surprising that deregulation in ubiquitin and UBL signalling has been implicated in the pathogenesis of many diseases and that these pathways are considered as major targets for therapeutic intervention. In this review, we summarise recent advances in our understanding of the role of the UBL neural precursor cell expressed developmentally downregulated-8 (NEDD8) in cancer-related processes and potential strategies for the use of NEDD8 inhibitors as chemotherapeutics.

Regulation of mitotic progression by the spindle assembly checkpoint

Equal segregation of sister chromatids during mitosis requires that pairs of kinetochores establish proper attachment to microtubules emanating from opposite poles of the mitotic spindle. The spindle assembly checkpoint (SAC) protects against errors in segregation by delaying sister separation in response to improper kinetochore–microtubule interactions, and certain checkpoint proteins help to establish proper attachments. Anaphase entry is inhibited by the checkpoint through assembly of the mitotic checkpoint complex (MCC) composed of the 2 checkpoint proteins, Mad2 and BubR1, bound to Cdc20. The outer kinetochore acts as a catalyst for MCC production through the recruitment and proper positioning of checkpoint proteins and recently there has been remarkable progress in understanding how this is achieved. Here, we highlight recent advances in our understanding of kinetochore–checkpoint protein interactions and inhibition of the anaphase promoting complex by the MCC.

Nek2A destruction marks APC/C activation at the prophase-to-prometaphase transition by spindle-checkpoint restricted Cdc20

Nek2A is a presumed APC/CCdc20 substrate, which, like cyclin A, is degraded in mitosis while the spindle checkpoint is active. Cyclin A prevents spindle checkpoint proteins from binding to Cdc20 and is recruited to the APC/C in prometaphase. We found that Nek2A and cyclin A avoid stabilization by the spindle checkpoint in different ways. First, enhancing mitotic checkpoint complex (MCC) formation by nocodazole treatment inhibited the degradation of geminin and cyclin A while Nek2A disappeared at normal rate. Secondly, depleting Cdc20 effectively stabilized cyclin A but not Nek2A. Nevertheless, Nek2A destruction critically depended on Cdc20 binding to the APC/C. Thirdly, in contrast to cyclin A, Nek2A was recruited to the APC/C before the start of mitosis. Interestingly, the spindle checkpoint very effectively stabilized an APC/C-binding mutant of Nek2A, which required the Nek2A KEN box. Apparently, in cells, the spindle checkpoint primarily prevents Cdc20 from binding destruction motifs. Nek2A disappearance marks the prophase-to-prometaphase transition, when Cdc20, regardless of the spindle checkpoint, activates the APC/C. However, Mad2 depletion accelerated Nek2A destruction, showing that spindle checkpoint release further increases APC/CCdc20 catalytic activity.

Structure of an APC3-APC16 complex: insights into assembly of the anaphase-promoting complex/cyclosome

The anaphase-promoting complex/cyclosome (APC/C) is a massive E3 ligase that controls mitosis by catalyzing ubiquitination of key cell cycle regulatory proteins. The APC/C assembly contains two subcomplexes: the "Platform" centers around a cullin-RING-like E3 ligase catalytic core; the "Arc Lamp" is a hub that mediates transient association with regulators and ubiquitination substrates. The Arc Lamp contains the small subunits APC16, CDC26, and APC13, and tetratricopeptide repeat (TPR) proteins (APC7, APC3, APC6, and APC8) that homodimerize and stack with quasi-2-fold symmetry. Within the APC/C complex, APC3 serves as center for regulation. APC3's TPR motifs recruit substrate-binding coactivators, CDC20 and CDH1, via their C-terminal conserved Ile-Arg (IR) tail sequences. Human APC3 also binds APC16 and APC7 and contains a >200-residue loop that is heavily phosphorylated during mitosis, although the basis for APC3 interactions and whether loop phosphorylation is required for ubiquitination are unclear. Here, we map the basis for human APC3 assembly with APC16 and APC7, report crystal structures of APC3Δloop alone and in complex with the C-terminal domain of APC16, and test roles of APC3's loop and IR tail binding surfaces in APC/C-catalyzed ubiquitination. The structures show how one APC16 binds asymmetrically to the symmetric APC3 dimer and, together with biochemistry and prior data, explain how APC16 recruits APC7 to APC3, show how APC3's C-terminal domain is rearranged in the full APC/C assembly, and visualize residues in the IR tail binding cleft important for coactivator-dependent ubiquitination. Overall, the results provide insights into assembly, regulation, and interactions of TPR proteins and the APC/C.

Insights into the anaphase-promoting complex: a molecular machine that regulates mitosis

The anaphase-promoting complex/cyclosome (APC/C) is a large multimeric complex that functions as a RING domain E3 ubiquitin ligase to regulate ordered transitions through the cell cycle. It does so by controlling the ubiquitin-mediated proteolysis of cell cycle proteins, notably cyclins and securins, whose degradation triggers sister chromatid disjunction and mitotic exit. Regulation of APC/C activity and modulation of its substrate specificity are subject to intricate cell cycle checkpoints and control mechanisms involving the switching of substrate-specifying cofactors, association of regulatory protein complexes and post-translational modifications. This review discusses the recent progress towards understanding the overall architecture of the APC/C, the molecular basis for degron recognition and ubiquitin chain synthesis, and how these activities are regulated.

Ubiquitin chain elongation requires E3-dependent tracking of the emerging conjugate

Protein modification with ubiquitin chains is an essential signaling event catalyzed by E3 ubiquitin ligases. Most human E3s contain a signature RING domain that recruits a ubiquitin-charged E2 and a separate domain for substrate recognition. How RING-E3s can build polymeric ubiquitin chains while binding substrates and E2s at defined interfaces remains poorly understood. Here, we show that the RING-E3 APC/C catalyzes chain elongation by strongly increasing the affinity of its E2 for the distal acceptor ubiquitin in a growing conjugate. This function of the APC/C requires its coactivator as well as conserved residues of the E2 and ubiquitin. APC/C's ability to track the tip of an emerging conjugate is required for APC/C-substrate degradation and accurate cell division. Our results suggest that RING-E3s tether the distal ubiquitin of a growing chain in proximity to the active site of their E2s, allowing them to assemble polymeric conjugates without altering their binding to substrate or E2.

Mechanism of polyubiquitination by human anaphase-promoting complex: RING repurposing for ubiquitin chain assembly

Polyubiquitination by E2 and E3 enzymes is a predominant mechanism regulating protein function. Some RING E3s, including anaphase-promoting complex/cyclosome (APC), catalyze polyubiquitination by sequential reactions with two different E2s. An initiating E2 ligates ubiquitin to an E3-bound substrate. Another E2 grows a polyubiquitin chain on the ubiquitin-primed substrate through poorly defined mechanisms. Here we show that human APC's RING domain is repurposed for dual functions in polyubiquitination. The canonical RING surface activates an initiating E2-ubiquitin intermediate for substrate modification. However, APC engages and activates its specialized ubiquitin chain-elongating E2 UBE2S in ways that differ from current paradigms. During chain assembly, a distinct APC11 RING surface helps deliver a substrate-linked ubiquitin to accept another ubiquitin from UBE2S. Our data define mechanisms of APC/UBE2S-mediated polyubiquitination, reveal diverse functions of RING E3s and E2s, and provide a framework for understanding distinctive RING E3 features specifying ubiquitin chain elongation.

Activation of the APC/C ubiquitin ligase by enhanced E2 efficiency

The anaphase-promoting complex/cyclosome (APC/C) is a protein-ubiquitin ligase (E3) that initiates the final events of mitosis by catalyzing the ubiquitination and proteasomal destruction of securin, cyclins, and other substrates [1, 2]. Like other members of the RING family of E3s [3, 4], the APC/C catalyzes direct ubiquitin transfer from an E2-ubiquitin conjugate (E2-Ub) to lysine residues on the protein substrate. The APC/C is activated at specific cell-cycle stages by association with an activator subunit, Cdc20 or Cdh1, which provides binding sites for specific substrate sequence motifs, or degrons. Activator might also stimulate catalytic activity [5, 6], but the underlying mechanisms are not known. Here, we dissected activator function using an artificial fusion substrate in which the N-terminal region of securin was linked to an APC/C core subunit. This fusion substrate bound tightly to the APC/C and was ubiquitinated at a low rate in the absence of activator. Ubiquitination of this substrate was stimulated by activator, due primarily to a dramatic stimulation of E2 sensitivity (Km) and catalytic rate (kcat), which together resulted in a 670-fold stimulation of kcat/Km. Thus, activator is not simply a substrate adaptor, but also enhances catalysis by promoting a more efficient interaction with the E2-Ub. Interestingly, full E2 stimulation required activator interaction with degron motifs on the substrate. We conclude that formation of a complete APC/C-activator-substrate complex leads to a major enhancement of E2 efficiency, providing an unusual substrate-assisted catalytic mechanism that limits efficient ubiquitin transfer to specific substrates.