Phosphatase PP2A and microtubule-mediated pulling forces disassemble centrosomes during mitotic exit

Abstract

Centrosomes are microtubule-nucleating organelles that facilitate chromosome segregation and cell division in metazoans. Centrosomes comprise centrioles that organize a micron-scale mass of protein called pericentriolar material (PCM) from which microtubules nucleate. During each cell cycle, PCM accumulates around centrioles through phosphorylation-mediated assembly of PCM scaffold proteins. During mitotic exit, PCM swiftly disassembles by an unknown mechanism. Here, we used Caenorhabditis elegans embryos to determine the mechanism and importance of PCM disassembly in dividing cells. We found that the phosphatase PP2A and its regulatory subunit SUR-6 (PP2ASUR-6), together with cortically directed microtubule pulling forces, actively disassemble PCM. In embryos depleted of these activities, ∼25% of PCM persisted from one cell cycle into the next. Purified PP2ASUR-6 could dephosphorylate the major PCM scaffold protein SPD-5 in vitro. Our data suggest that PCM disassembly occurs through a combination of dephosphorylation of PCM components and force-driven fragmentation of the PCM scaffold.

Material aging causes centrosome weakening and disassembly during mitotic exit

“…We conclude that PP2A normally functions to eliminate "self-healing" 397 PCM scaffold interactions, thus making PCM brittle during anaphase and susceptible to 398 microtubule-mediated fracture in telophase. Consistent with this conclusion, LB-100 399 inhibition of PP2A or depletion of its regulatory subunit SUR-6 inhibited SPD-5 scaffold 400 disassembly in telophase ( Figure 6E) (Enos et al, 2018). We speculate that PCM may 401 be less porous in this mutant ductile state compared to the wild-type brittle state, which 402 could delay PCM disassembly further by preventing access of additional disassembly 403 machinery and/or delaying the departure of PLK-1 and SPD-2.…”

“…During 153 spindle assembly, the C. elegans PCM scaffold is subject to microtubule-mediated pulling 154 forces, but it maintains its spherical shape and structural integrity. However, during 155 telophase, those same pulling forces deform and fracture the PCM scaffold ( Figure 156 1A) (Enos et al, 2018;Severson and Bowerman, 2003). We hypothesized that PCM 157 undergoes an intrinsic mechanical transition from a strong, tough state in metaphase to 158 a weak state in telophase.…”

A two-step mechanism for the inactivation of microtubule organizing center function at the centrosome

“…Notably, treatment with either drug led to depolymerization of the microtubules, 254 perhaps due to the hyperactivation of the depolymerizing kinesin KLP-7 during PP2A 255 inactivation (Schlaitz et al, 2007). Consistent with these pharmacological inhibition results, a 256 recent study implicated the PP2A subunit LET-92 in SPD-5 disassembly (Enos et al, 2018). 257…”

“…Given that greater cortical forces exist in the 190 posterior of the one-cell C. elegans embryo, it has been hypothesized that these forces could 191 be responsible for the asynchrony observed in the disassembly of the anterior vs. the posterior 192 centrosome (Grill, Gonczy, Stelzer, & Hyman, 2001). Moreover, a recent study implicated the 193 GPR-1/2/LIN-5/DHC-1 complex in SPD-5 disassembly from the PCM (Enos et al, 2018). 194…”

“…Thus, PCM disassembly appears to be initiated by dephosphorylation by the PP2A subunit 315 LET-92. As LET-92 plays a number of roles at the centrosome and phosphatase activity can 11 directly regulate mitotic kinases (Enos et al, 2018;Kitagawa et al, 2011;Song et al, 2011), 317 further studies will be necessary to determine if its role in PCM dissolution is direct or indirect. 318…”