Archives

  • 2018-07
  • 2018-10
  • 2018-11
  • 2019-04
  • 2019-05
  • 2019-06
  • 2019-07
  • 2019-08
  • 2019-09
  • 2019-10
  • 2019-11
  • 2019-12
  • 2020-01
  • 2020-02
  • 2020-03
  • 2020-04
  • 2020-05
  • 2020-06
  • 2020-07
  • 2020-08
  • 2020-09
  • 2020-10
  • 2020-11
  • 2020-12
  • 2021-01
  • 2021-02
  • 2021-03
  • 2021-04
  • 2021-05
  • 2021-06
  • 2021-07
  • 2021-08
  • 2021-09
  • 2021-10
  • 2021-11
  • 2021-12
  • 2022-01
  • 2022-02
  • 2022-03
  • 2022-04
  • 2022-05
  • 2022-06
  • 2022-07
  • 2022-08
  • 2022-09
  • 2022-10
  • 2022-11
  • 2022-12
  • 2023-01
  • 2023-02
  • 2023-03
  • 2023-04
  • 2023-05
  • 2023-06
  • 2023-08
  • 2023-09
  • 2023-10
  • 2023-11
  • 2023-12
  • 2024-01
  • 2024-02
  • 2024-03
  • The question arises as to

    2019-07-08

    The question arises as to how the massive conformational changes determining whether APC/CCDC20–MCC is inhibited or able to catalyze MCC ubiquitylation are naturally controlled? This transition likely involves APC/C regions near the subunit APC15, because reducing cellular APC15 levels stabilizes MCC on APC/C 88, 89, 90. Indeed, recombinant APC/CCDC20–MCC complexes lacking APC15 preferentially adopt the closed conformation that inhibits UBE2C-dependent ubiquitylation 48, 51, although APC15 is not required for APC/CCDC20–MCC to swing away from the APC2–APC11 catalytic core 48, 51, nor is there evidence that APC15 ever cycles on and off APC/C. It is possible that all APC/CCDC20–MCC complexes continually cycle between the closed and open states. However, differences between the structural studies indicate a role for phosphoregulation: different ratios of open versus closed configurations of recombinant APC/CCDC20–MCC are observed 48, 51, depending on whether the APC/CCDC20 accumulates phosphorylation during expression in insect Vitamin D3 [53], or whether potential phosphorylation is mimicked by glutamate replacements for 100 possible mitotic phosphorylation sites [54]. The two preparations likely differ in terms of where negative charges are placed, raising the possibility that certain negatively charged phosphorylation sites may modulate the conformation of APC/CCDC20–MCC in vivo to regulate termination of the spindle assembly checkpoint.
    Concluding Remarks Recent studies have provided unprecedented details of APC/C structure and enzymology, which explain how the activity of this massive E3 ligase is controlled, and how ubiquitylation is achieved to temporally regulate cell division. Although one pervasive question has been why the APC/C has such an enormous molecular mass, it seems that the large size enables both nuanced and extreme conformational changes – and their coupling to phosphorylation and the binding of many partner proteins. Step-by-step regulation is achieved through each complex allowing precisely the needed APC/C functions while excluding others (Figure 4). For example, apo, unphosphorylated APC/C excludes CDC20 and UBE2C, while phosphorylated APC/C allows coactivator binding. The many activities of APC/CCDC20 are further tuned, including MCC in a closed and inactive state; an open configuration that excludes APC/C substrates but allows UBE2C-catalyzed ubiquitylation of MCC, and substrate bound in a manner that excludes MCC and allows UBE2C for its direct modification. Ubiquitylated substrates and UBE2S capture yet alternative locations on the cullin–RING core for polyubiquitylation to drive progression through mitosis, while EMI1 subsequently prevents substrate and E2 binding to allow cyclin accumulation and another cell cycle. Despite the wealth of structural data, many open questions remain (see Outstanding Questions), especially relating to how these discrete APC/C complexes transition from one state – or binding partner and activity – to another. It seems likely that the multisite, avid nature of the interactions of APC/C could contribute to this regulation, as all elements within most APC/C partners are required for their high-affinity binding. It seems likely that the dismantling of one interaction, for example through a post-translational modification or the binding of another protein, could precipitate disassembly through a domino-like effect. Future studies will also be required to visualize emerging APC/C regulation, including by phosphorylation, SUMOylation, association with enigmatic binding partners 91, 92, 93, 94, and localization of APC/C to its different regulators and substrates within cells. In addition to APC/C, humans are estimated to express >500 different E3 ligases, roughly half of which are CRLs whose catalytic core becomes mobile upon activation [12]. However, because few activity states have been structurally visualized for most E3 ligases, the studies on APC/C described herein provide paradigmatic molecular principles determining distinct E3 ligase activities across a biological pathway. Although the structural details will differ between ligases, it seems that much like APC/C, E3s are generally restrained in inactive conformations until needed, conformationally flexible when activated, and harnessed into distinct conformations for different functions.