L10 extra reading Flashcards
factors in spindle assembly
Mierzwa and Gerlich (2014)
A key factor in central spindle assembly is the microtubule bundling protein required for cytokinesis 1 (PRC1), which as a homodimer selectively binds to the interface between antiparallel microtubules once Cdk1- and Plk1-mediated inhibitory phosphorylations have been removed at anaphase onset (Bieling et al., 2010; Cundell et al., 2013; Jiang et al., 1998; Mollinari et al., 2002; Neef et al., 2007; Subramanian et al., 2010; Zhu et al., 2006). Central spindle assembly further depends on the heterotetrameric centralspindlin complex, which comprises two subunits of the kinesin-6 motor protein MKLP1 and the Rho-family GTPase-activating protein CYK-4 (also termed MgcRacGAP), respectively (Mishima et al., 2002; Pavicic-Kaltenbrunner et al., 2007). Centralspindlin is activated at anaphase onset through removal of an inhibitory Cdk1 phosphorylation from its MKLP1 subunit (Mishima et al., 2004) and through Aurora B-mediated phosphorylation of MKLP1 (Guse et al., 2005), which leads to a release from its inhibitor 14-3-3 and the formation of higher-order clusters that stabilize the central spindle (Douglas et al., 2010; Hutterer et al., 2009).
chromosomal passenger complex?
Mierzwa and Gerlich (2014)
A third essential component of the central spindle is the chromosomal passenger complex, comprising the kinase Aurora B, the inner centromere protein (INCENP), borealin, and survivin (Carmena et al., 2012). The chromosomal passenger complex relocates from centromeres to the spindle center at anaphase onset, depending on the removal of a Cdk1 phosphorylation from the INCENP subunit (Hümmer and Mayer, 2009) and the kinesin MKLP2 (Gruneberg et al., 2004). Besides its role in regulating other central spindle components by phosphorylation, the chromosomal passenger complex may also directly contribute to microtubule bundling (Carmena et al., 2012; Glotzer, 2009).
The length of the central spindle is regulated by Aurora B kinase, involving the kinesins KIF4A and KIF2A. In vitro reconstitution experiments showed that KIF4A is transported by kinesins to microtubule plus ends at the microtubule overlap zones, which then stops microtubule growth (Bieling et al., 2010; Subramanian et al., 2010). In cells, Aurora B stabilizes the central spindle by local activation of KIF4A (Nunes Bastos et al., 2013) and by inhibition of the microtubule depolymerase KIF2A (Uehara et al., 2013).
In summary, a decline of Cdk1 activity and relocalization of Aurora B from chromosomes to microtubules promotes microtubule bundling and assembly of the central spindle during early anaphase.
RhoA
Mierzwa and Gerlich (2014)
Once activated at the equatorial cell cortex, RhoA promotes actomyosin ring assembly and contraction through two regulatory pathways. On the one hand, RhoA activates Diaphanous-related formins to stimulate nucleation of unbranched actin filaments (Castrillon and Wasserman, 1994; Severson et al., 2002; Watanabe et al., 2008). On the other hand, RhoA activates myosin II indirectly, through activating the myosin light chain kinase ROCK and through inhibiting a counteracting phosphatase MYPT (Matsumura, 2005). In animal cells, the actomyosin ring initially appears as a broad equatorial band, which then narrows to a ribbon (Hu et al., 2011; Lewellyn et al., 2011; Mabuchi, 1994). Actin and myosin filaments either assemble directly at the cell equator or derive from adjacent cortex regions by lateral cortical flow (Murthy and Wadsworth, 2005; Uehara et al., 2010; Vale et al., 2009; Yumura et al., 2008; Zhou and Wang, 2008). In C. elegans embryos, formin-mediated actin nucleation is required only until early stages of cleavage furrow ingression, whereas myosin II activity is required until complete contraction of the contractile ring (Davies et al., 2014). In addition to actin and myosin filaments, the contractile ring further contains actin crosslinking proteins (Reichl et al., 2008), septin filaments (Joo et al., 2007; Mavrakis et al., 2014; Neufeld and Rubin, 1994), and anillin, a scaffold protein that links actin, myosin, septin, the central spindle, and the plasma membrane (Field and Alberts, 1995; Piekny and Maddox, 2010).
ESCRT
Park et al. (2024)
In the final stage of cell division, cells undergo cytoplasmic separation, known as cytokinesis. In the final stages of cytokinesis, an intercellular bridge is formed between two daughter cells. The midbody, which is the central region of the microtubule-rich intercellular bridge, is severed by membrane separation during cytokinetic abscission (Stoten and Carlton 2018). The ESCRT-III complex plays a pivotal role in modulating cytokinesis (Figure 2D). The ESCRT-III complex and VPS4 are assembled into 17 nm-diameter filaments with constriction zones in intercellular bridges (Carlton and Martin-Serrano 2007; Guizetti et al. 2011). Furthermore, the depletion of ESCRT-III subunits, such as CHMP2A, CHMP1A, or CHMP1B, blocked cytokinetic abscission (Bajorek et al. 2009; Guizetti et al. 2011). During cytokinesis, ESCRT-III proteins are recruited to the midbody through interactions with ESCRT-II components and adaptor proteins, such as VPS36, EAP30, ALIX, TSG101, and CEP55 (Fabbro et al. 2005; Morita et al. 2007; Carlton et al. 2008; Goliand et al. 2014; Christ et al. 2016; Sun et al. 2016). ALIX accumulation in the midbody results in the formation of spiral-like structures at the abscission site that recruit CHMP4B to the midbody (Pust et al. 2023). CHMP4B and CHMP6 initially localize adjacent to the midbody and later constrict themselves to split the plasma membrane (Morita et al. 2007; Guizetti et al. 2011; Mierzwa et al. 2017). Subsequently, CHMP2B, CHMP3, and CHMP4B form a series of cortical rings to construct ESCRT filaments (Elia et al. 2011; Guizetti et al. 2011; Christ et al. 2016; Mierzwa et al. 2017). In addition, CHMP8 plays a crucial role in abscission (Agromayor et al. 2009; Bajorek et al. 2009). The assembly of CHMPs and CHMP-like proteins promotes the recruitment of interactors to construct multiprotein complexes required for efficient cytokinetic abscission (Yang et al. 2008; Lee et al. 2012; Wenzel et al. 2022). MIT domain containing 1 (MITD1) interacts with CHMP1B, CHMP2A, and CHMP8, and this interaction is important for ESCRT filament remodeling (Lee et al. 2012). Another interactor, spastin, which interacts with CHMP1B and CHMP8, facilitates constriction of the abscission site by cleaving microtubules (Yang et al. 2008). Thus, ESCRT-III and its interacting proteins collaborate to induce membrane remodeling events and complete cell division.
