PP2A-Cdc55 phosphatase regulates actomyosin ring contraction and septum formation during cytokinesis

PP2A-B56 regulates Mid1 protein levels for proper cytokinesis in fission yeast

Protein phosphorylation regulates many steps in the cell division process including cytokinesis. In fission yeast cells, the anillin-like protein Mid1 sets the cell division plane and is regulated by phosphorylation. Multiple protein kinases act on Mid1, but no protein phosphatases have been shown to regulate Mid1. Here, we discovered that the conserved protein phosphatase PP2A-B56 is required for proper cytokinesis by promoting Mid1 protein levels. We find that par1∆ cells lacking the primary B56 subunit divide asymmetrically due to the assembly of misplaced cytokinetic rings that slide toward cell tips. These par1∆ mutants have reduced whole-cell levels of Mid1 protein, leading to reduced Mid1 at the cytokinetic ring. Restoring proper Mid1 expression suppresses par1∆ cytokinesis defects. This work identifies a new PP2A-B56 pathway regulating cytokinesis through Mid1, with implications for control of cytokinesis in other organisms.

PP2A-B56 regulates Mid1 protein levels for proper cytokinesis in fission yeast

Protein phosphorylation regulates many steps in the cell division process including cytokinesis. In fission yeast cells, the anillin-like protein Mid1 sets the cell division plane and is regulated by phosphorylation. Multiple protein kinases act on Mid1, but no protein phosphatases have been shown to regulate Mid1. Here, we discovered that the conserved protein phosphatase PP2A-B56 is required for proper cytokinesis by promoting Mid1 protein levels. We find that par1Δ cells lacking the primary B56 subunit divide asymmetrically due to the assembly of misplaced cytokinetic rings that slide towards cell tips. These par1Δ mutants have reduced whole-cell levels of Mid1 protein, leading to reduced Mid1 at the cytokinetic ring. Restoring proper Mid1 expression suppresses par1Δ cytokinesis defects. This work identifies a new PP2A-B56 pathway regulating cytokinesis through Mid1, with implications for control of cytokinesis in other organisms.

A coordinated kinase and phosphatase network regulates Stu2 recruitment to yeast kinetochores

Cells coordinate diverse events at anaphase onset, including separase activation, cohesin cleavage, chromosome separation, and spindle reorganization. Regulation of the XMAP215 family member and microtubule polymerase, Stu2, at the metaphase-anaphase transition determines a specific redistribution from kinetochores to spindle microtubules. We show that cells modulate Stu2 kinetochore-microtubule localization by Polo-like kinase1/Cdc5-mediated phosphorylation of T866, near the Stu2 C-terminus, thereby promoting dissociation from the kinetochore Ndc80 complex. Cdk/Cdc28 likely primes Cdc5:Stu2 interaction. Cdc28 activity is also required for Stu2 nuclear import. PP2ACdc55 actively opposes Cdc5 activity on Stu2T866 during metaphase. This counter-regulation allows for switchlike redistribution of Stu2pT866 at anaphase onset when separase inhibits PP2ACdc55. Blocking Stu2T866 phosphorylation disrupts anaphase spindle progression, and we infer that PP2ACdc55 regulates the mitotic spindle by dephosphorylating Stu2 and other MAPs. These data support a model in which increased phosphorylation at anaphase onset results from phosphatase inhibition and point to a larger regulatory network that facilitates rapid cytoskeletal modulation required for anaphase spindle maintenance.

Central Role of the Actomyosin Ring in Coordinating Cytokinesis Steps in Budding Yeast

Eukaryotic cells must accurately transfer their genetic material and cellular components to their daughter cells. Initially, cells duplicate their chromosomes and subsequently segregate them toward the poles. The actomyosin ring, a crucial molecular machinery normally located in the middle of the cells and underneath the plasma membrane, then physically divides the cytoplasm and all components into two daughter cells, each ready to start a new cell cycle. This process, known as cytokinesis, is conserved throughout evolution. Defects in cytokinesis can lead to the generation of genetically unstable tetraploid cells, potentially initiating uncontrolled proliferation and cancer. This review focuses on the molecular mechanisms by which budding yeast cells build the actomyosin ring and the preceding steps involved in forming a scaffolding structure that supports the challenging structural changes throughout cytokinesis. Additionally, we describe how cells coordinate actomyosin ring contraction, plasma membrane ingression, and extracellular matrix deposition to successfully complete cytokinesis. Furthermore, the review discusses the regulatory roles of Cyclin-Dependent Kinase (Cdk1) and the Mitotic Exit Network (MEN) in ensuring the precise timing and execution of cytokinesis. Understanding these processes in yeast provides insights into the fundamental aspects of cell division and its implications for human health.

Cytokinetic abscission in Toxoplasma gondii is governed by protein phosphatase 2A and the daughter cell scaffold complex

Cytokinetic abscission marks the final stage of cell division, during which the daughter cells physically separate through the generation of new barriers, such as the plasma membrane or cell wall. While the contractile ring plays a central role during cytokinesis in bacteria, fungi and animal cells, the process diverges in Apicomplexa. In Toxoplasma gondii, two daughter cells are formed within the mother cell by endodyogeny. The mechanism by which the progeny cells acquire their plasma membrane during the disassembly of the mother cell, allowing daughter cells to emerge, remains unknown. Here we identify and characterize five T. gondii proteins, including three protein phosphatase 2A subunits, which exhibit a distinct and dynamic localization pattern during parasite division. Individual downregulation of these proteins prevents the accumulation of plasma membrane at the division plane, preventing the completion of cellular abscission. Remarkably, the absence of cytokinetic abscission does not hinder the completion of subsequent division cycles. The resulting progeny are able to egress from the infected cells but fail to glide and invade, except in cases of conjoined twin parasites.

Downregulation of protein phosphatase 2Aα in asthmatic airway smooth muscle

Airway smooth muscle cell (ASM) is renowned for its involvement in airway hyperresponsiveness through impaired ASM relaxation and bronchoconstriction in asthma, which poses a significant challenge in the field. Recent studies have explored different targets in ASM to alleviate airway hyperresponsiveness, however, a sizeable portion of patients with asthma still experience poor control. In our study, we explored protein phosphatase 2 A (PP2A) in ASM as it has been reported to regulate cellular contractility by controlling intracellular calcium ([Ca2+]i), ion channels, and respective regulatory proteins. We obtained human ASM cells and lung tissues from healthy and patients with asthma and evaluated PP2A expression using RNA-Seq data, immunofluorescence, and immunoblotting. We further investigated the functional importance of PP2A by determining its role in bronchoconstriction using mouse bronchus and human ASM cell [Ca2+]i regulation. We found robust expression of PP2A isoforms in human ASM cells with PP2Aα being highly expressed. Interestingly, PP2Aα was significantly downregulated in asthmatic tissue and human ASM cells exposed to proinflammatory cytokines. Functionally, FTY720 (PP2A agonist) inhibited acetylcholine- or methacholine-induced bronchial contraction in mouse bronchus and further potentiated isoproterenol-induced bronchial relaxation. Mechanistically, FTY720 inhibited histamine-evoked [Ca2+]i response and myosin light chain (MLC) phosphorylation in the presence of interleukin-13 (IL-13) in human ASM cells. To conclude, we for the first time established PP2A signaling in ASM, which can be further explored to develop novel therapeutics to alleviate airway hyperresponsiveness in asthma.NEW & NOTEWORTHY This novel study deciphered the expression and function of protein phosphatase 2Aα (PP2Aα) in airway smooth muscle (ASM) during asthma and/or inflammation. We showed robust expression of PP2Aα in human ASM while its downregulation in asthmatic ASM. Similarly, we demonstrated reduced PP2Aα expression in ASM exposed to proinflammatory cytokines. PP2Aα activation inhibited bronchoconstriction of isolated mouse bronchi. In addition, we unveiled that PP2Aα activation inhibits the intracellular calcium release and myosin light chain phosphorylation in human ASM.

Chitin Synthesis in Yeast: A Matter of Trafficking

Chitin synthesis has attracted scientific interest for decades as an essential part of fungal biology and for its potential as a target for antifungal therapies. While this interest remains, three decades ago, pioneering molecular studies on chitin synthesis regulation identified the major chitin synthase in yeast, Chs3, as an authentic paradigm in the field of the intracellular trafficking of integral membrane proteins. Over the years, researchers have shown how the intracellular trafficking of Chs3 recapitulates all the steps in the intracellular trafficking of integral membrane proteins, from their synthesis in the endoplasmic reticulum to their degradation in the vacuole. This trafficking includes specific mechanisms for sorting in the trans-Golgi network, regulated endocytosis, and endosomal recycling at different levels. This review summarizes the work carried out on chitin synthesis regulation, mostly focusing on Chs3 as a molecular model to study the mechanisms involved in the control of the intracellular trafficking of proteins.