Editing Spindle checkpoint (section)

=== Checkpoint deactivation ===

Several mechanisms exist to deactivate the SAC after correct bi-orientation of [[sister chromatids]]. Upon microtubule-kinetochore attachment, a mechanism of stripping via a [[dynein|dynein-dynein motor complex]] transports spindle checkpoint proteins away from the kinetochores.<ref name = "Nature">{{cite journal | vauthors = Musacchio A, Salmon ED | title = The spindle-assembly checkpoint in space and time | journal = Nature Reviews. Molecular Cell Biology | volume = 8 | issue = 5 | pages = 379–93 | date = May 2007 | pmid = 17426725 | doi = 10.1038/nrm2163 | s2cid = 205494124 }}</ref> The stripped proteins, which include MAD1, MAD2, MPS1, and [[CENPF|CENP-F]], are then redistributed to the [[spindle pole]]s. The stripping process is highly dependent on undamaged microtubule structure as well as dynein motility along microtubules. As well as functioning as a regulator of the C-MAD2 positive feedback loop, p31<sup>comet</sup> also may act as a deactivator of the SAC. Unattached kinetochores temporarily inactivate p31<sup>comet</sup>, but attachment reactivates the protein and inhibits MAD2 activation, possibly by inhibitory phosphorylation. Another possible mechanism of SAC inactivation results from energy-dependent dissociation of the MAD2-CDC20 complex through non-degradative ubiquitylation of CDC20. Conversely, the de-ubiquitylating enzyme [[protectin]] is required to maintain the SAC. Thus, unattached kinetochores maintain the checkpoint by continuously recreating the MAD2-CDC20 subcomplex from its components. The SAC may also be deactivated by APC activation induced [[proteolysis]]. Since the SAC is not reactivated by the loss of sister-chromatid cohesion during anaphase, the proteolysis of cyclin B and inactivation of the CDK1-cyclin-B kinase also inhibits SAC activity. Degradation of MPS1 during anaphase prevents the reactivation of SAC after removal of sister-chromatid cohesion. After checkpoint deactivation and during the normal anaphase of the cell cycle, the anaphase promoting complex is activated through decreasing MCC activity. When this happens the enzyme complex [[Polyubiquitination|polyubiquitinates]] the anaphase inhibitor [[securin]]. The ubiquitination and destruction of securin at the end of metaphase releases the active protease called separase. Separase cleaves the cohesion molecules that hold the sister chromatids together to activate anaphase.<ref name = "Morgan">{{Cite book | edition = 1 | publisher = New Science Press, Ltd | isbn = 978-0-87893-508-6 | last = Morgan | first = David O. | title = The Cell Cycle: Principles of Control (Primers in Biology) | date = 2006-09-06 }}</ref>

==== New model for SAC deactivation in ''S. cerevisiae'': the mechanical switch ====
A new mechanism has been suggested to explain how end-on microtubule attachment at the kinetochore is able to disrupt specific steps in SAC signaling.  In an unattached kinetochore, the first step in the formation of the MCC is phosphorylation of Spc105 by the kinase Mps1.  Phosphorylated Spc105 is then able to recruit the downstream signaling proteins Bub1 and 3; Mad 1,2, and 3; and Cdc20.  Association with Mad1 at unattached kinetochores causes Mad2 to undergo a conformational change that converts it from an open form (O-Mad2) to a closed form (C-Mad2.)  The C-Mad2 bound to Mad1 then dimerizes with a second O-Mad2 and catalyzes its closure around Cdc20.  This C-Mad2 and Cdc20 complex, the MCC, leaves Mad1 and C-Mad2 at the kinetochore to form another MCC.  The MCCs each sequester two Cdc20 molecules to prevent their interaction with the APC/C, thereby maintaining the SAC.<ref name="Morgan" />  Mps1's phosphorylation of Spc105 is both necessary and sufficient to initiate the SAC signaling pathway, but this step can only occur in the absence of microtubule attachment to the kinetochore.  Endogenous Mps1 is shown to associate with the calponin-homology (CH) domain of Ndc80, which is located in the outer kinetochore region that is distant from the chromosome.  Though Mps1 is docked in the outer kinetochore, it is still able to localize within the inner kinetochore and phosphorylate Spc105 because of flexible hinge regions on Ndc80.  However, the mechanical switch model proposes that end-on attachment of a microtubule to the kinetochore deactivates the SAC through two mechanisms. The presence of an attached microtubule increases the distance between the Ndc80 CH domain and  Spc105. Additionally, Dam1/DASH, a large complex consisting of 160 proteins that forms a ring around the attached microtubule, acts as a barrier between the two proteins.  Separation prevents interactions between Mps1 and Spc105 and thus inhibits the SAC signaling pathway.<ref>{{cite journal | vauthors = Aravamudhan P, Goldfarb AA, Joglekar AP | title = The kinetochore encodes a mechanical switch to disrupt spindle assembly checkpoint signalling | journal = Nature Cell Biology | volume = 17 | issue = 7 | pages = 868–79 | date = July 2015 | pmid = 26053220 | pmc = 4630029 | doi = 10.1038/ncb3179 }}</ref>

This model is not applicable to SAC regulation in higher order organisms, including animals.  A main facet of the mechanical switch mechanism is that in ''S. cerevisiae'' the structure of the kinetochore only allows for attachment of one microtubule. Kinetochores in animals, on the other hand, are much more complex meshworks that contain binding sites for a multitude of microtubules.<ref>{{Cite book|title=Molecular Biology of The Cell (6th ed.)|vauthors = Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K,Walter P|publisher=Garland Science, Taylor & Francis Group|year=2015|isbn=978-0-8153-4432-2|location=New York, NY|pages=988}}</ref>  Microtubule attachment at all of the kinetochore binding sites is not necessary for deactivation of the SAC and progression to anaphase.  Therefore, microtubule-attached and microtubule-unattached states coexist in the animal kinetochore while the SAC is inhibited.  This model does not include a barrier that would prevent Mps1 associated with an attached kinetochore from phosphorylating Spc105 in an adjacent unattached kinetochore. Furthermore, the yeast Dam1/DASH complex is not present in animal cells.