Saccharomyces cerevisiae Dma proteins participate in cytokinesis by controlling two different pathways

Phosphorylation of the F-BAR protein Hof1 drives septin ring splitting in budding yeast

A double septin ring accompanies cytokinesis in yeasts and mammalian cells. In budding yeast, reorganisation of the septin collar at the bud neck into a dynamic double ring is essential for actomyosin ring constriction and cytokinesis. Septin reorganisation requires the Mitotic Exit Network (MEN), a kinase cascade essential for cytokinesis. However, the effectors of MEN in this process are unknown. Here we identify the F-BAR protein Hof1 as a critical target of MEN in septin remodelling. Phospho-mimicking HOF1 mutant alleles overcome the inability of MEN mutants to undergo septin reorganisation by decreasing Hof1 binding to septins and facilitating its translocation to the actomyosin ring. Hof1-mediated septin rearrangement requires its F-BAR domain, suggesting that it may involve a local membrane remodelling that leads to septin reorganisation. In vitro Hof1 can induce the formation of intertwined septin bundles, while a phosphomimetic Hof1 protein has impaired septin-bundling activity. Altogether, our data indicate that Hof1 modulates septin architecture in distinct ways depending on its phosphorylation status.

GOLPH3 protein controls organ growth by interacting with TOR signaling proteins in Drosophila

The oncoprotein GOLPH3 (Golgi phosphoprotein 3) is an evolutionarily conserved phosphatidylinositol 4-phosphate effector, mainly localized to the Golgi apparatus, where it supports organelle architecture and vesicular trafficking. Overexpression of human GOLPH3 correlates with poor prognosis in several cancer types and is associated with enhanced signaling downstream of mTOR (mechanistic target of rapamycin). However, the molecular link between GOLPH3 and mTOR remains elusive. Studies in Drosophila melanogaster have shown that Translationally controlled tumor protein (Tctp) and 14-3-3 proteins are required for organ growth by supporting the function of the small GTPase Ras homolog enriched in the brain (Rheb) during mTORC1 (mTOR complex 1) signaling. Here we demonstrate that Drosophila GOLPH3 (dGOLPH3) physically interacts with Tctp and 14-3-3ζ. RNAi-mediated knockdown of dGOLPH3 reduces wing and eye size and enhances the phenotypes of Tctp RNAi. This phenotype is partially rescued by overexpression of Tctp, 14-3-3ζ, or Rheb. We also show that the Golgi localization of Rheb in Drosophila cells depends on dGOLPH3. Consistent with dGOLPH3 involvement in Rheb-mediated mTORC1 activation, depletion of dGOLPH3 also reduces levels of phosphorylated ribosomal S6 kinase, a downstream target of mTORC1. Finally, the autophagy flux and the expression of autophagic transcription factors of the TFEB family, which anti correlates with mTOR signaling, are compromised upon reduction of dGOLPH3. Overall, our data provide the first in vivo demonstration that GOLPH3 regulates organ growth by directly associating with mTOR signaling proteins.

Downregulation of the Tem1 GTPase by Amn1 after cytokinesis involves both nuclear import and SCF-mediated degradation

At mitotic exit the cell cycle engine is reset to allow crucial processes, such as cytokinesis and replication origin licensing, to take place before a new cell cycle begins. In budding yeast, the cell cycle clock is reset by a Hippo-like kinase cascade called the mitotic exit network (MEN), whose activation is triggered at spindle pole bodies (SPBs) by the Tem1 GTPase. Yet, MEN activity must be extinguished once MEN-dependent processes have been accomplished. One factor contributing to switching off the MEN is the Amn1 protein, which binds Tem1 and inhibits it through an unknown mechanism. Here, we show that Amn1 downregulates Tem1 through a dual mode of action. On one side, it evicts Tem1 from SPBs and escorts it into the nucleus. On the other, it promotes Tem1 degradation as part of a Skp, Cullin and F-box-containing (SCF) ubiquitin ligase. Tem1 inhibition by Amn1 takes place after cytokinesis in the bud-derived daughter cell, consistent with its asymmetric appearance in the daughter cell versus the mother cell. This dual mechanism of Tem1 inhibition by Amn1 may contribute to the rapid extinguishing of MEN activity once it has fulfilled its functions.

A novel coordinated function of Myosin II with GOLPH3 controls centralspindlin localization during cytokinesis in Drosophila

In animal cell cytokinesis, interaction of non-muscle myosin II (NMII) with F-actin provides the dominant force for pinching the mother cell into two daughters. Here we demonstrate that celibe (cbe) is a missense allele of zipper, which encodes the Drosophila Myosin heavy chain. Mutation of cbe impairs binding of Zipper protein to the regulatory light chain Spaghetti squash (Sqh). In dividing spermatocytes from cbe males, Sqh fails to concentrate at the equatorial cortex, resulting in thin actomyosin rings that are unable to constrict. We show that cbe mutation impairs localization of the phosphatidylinositol 4-phosphate [PI(4)P]-binding protein Golgi phosphoprotein 3 (GOLPH3, also known as Sauron) and maintenance of centralspindlin at the cell equator of telophase cells. Our results further demonstrate that GOLPH3 protein associates with Sqh and directly binds the centralspindlin subunit Pavarotti. We propose that during cytokinesis, the reciprocal dependence between Myosin and PI(4)P-GOLPH3 regulates centralspindlin stabilization at the invaginating plasma membrane and contractile ring assembly.

Cytokinesis in Eukaryotic Cells: The Furrow Complexity at a Glance

The duplication cycle is the fascinating process that, starting from a cell, results in the formation of two daughter cells and it is essential for life. Cytokinesis is the final step of the cell cycle, it is a very complex phase, and is a concert of forces, remodeling, trafficking, and cell signaling. All of the steps of cell division must be properly coordinated with each other to faithfully segregate the genetic material and this task is fundamental for generating viable cells. Given the importance of this process, molecular pathways and proteins that are involved in cytokinesis are conserved from yeast to humans. In this review, we describe symmetric and asymmetric cell division in animal cell and in a model organism, budding yeast. In addition, we illustrate the surveillance mechanisms that ensure a proper cell division and discuss the connections with normal cell proliferation and organs development and with the occurrence of human diseases.

Recruitment of the mitotic exit network to yeast centrosomes couples septin displacement to actomyosin constriction

In many eukaryotic organisms cytokinesis is driven by a contractile actomyosin ring (CAR) that guides membrane invagination. What triggers CAR constriction at a precise time of the cell cycle is a fundamental question. In budding yeast CAR is assembled via a septin scaffold at the division site. A Hippo-like kinase cascade, the Mitotic Exit Network (MEN), promotes mitotic exit and cytokinesis, but whether and how these two processes are independently controlled by MEN is poorly understood. Here we show that a critical function of MEN is to promote displacement of the septin ring from the division site, which in turn is essential for CAR constriction. This is independent of MEN control over mitotic exit and involves recruitment of MEN components to the spindle pole body (SPB). Ubiquitination of the SPB scaffold Nud1 inhibits MEN signaling at the end of mitosis and prevents septin ring splitting, thus silencing the cytokinetic machinery.

The Mitotic Exit Network (MEN) promotes mitotic exit and cytokinesis but if and how MEN independently controls these two processes is unclear. Here, the authors report that MEN displaces septins from the cell division site to promote actomyosin ring constriction, independently of MEN control of mitotic exit.

Analysis of the Functionality of the Mitotic Checkpoints

During cell division the main goal of the cell is to produce two daughter cells with the same genome as the mother, i.e., maintain its genetic stability. Since this issue is essential to preserve the cell ability to proliferate properly, all eukaryotic cells have developed several pathways, called mitotic checkpoints, that regulate mitotic entry, progression, and exit in response to different cellular signals. Given the evolutive conservation of mechanisms and proteins involved in the cell cycle control from yeast to humans, the budding yeast S. cerevisiae has been very helpful to gain insight in these complex regulations. Here, we describe how the checkpoint can be activated and which cellular phenotypes can be used as markers of checkpoint activation.

COG7 deficiency in Drosophila generates multifaceted developmental, behavioral and protein glycosylation phenotypes

Congenital disorders of glycosylation (CDG) comprise a family of human multisystemic diseases caused by recessive mutations in genes required for protein N-glycosylation. More than 100 distinct forms of CDGs have been identified and most of them cause severe neurological impairment. The Conserved Oligomeric Golgi (COG) complex mediates tethering of vesicles carrying glycosylation enzymes across the Golgi cisternae. Mutations affecting human COG1, COG2 and COG4-COG8 cause monogenic forms of inherited, autosomal recessive CDGs. We have generated a Drosophila COG7-CDG model that closely parallels the pathological characteristics of COG7-CDG patients, including pronounced neuromotor defects associated with altered N-glycome profiles. Consistent with these alterations, larval neuromuscular junctions of Cog7 mutants exhibit a significant reduction in bouton numbers. We demonstrate that the COG complex cooperates with Rab1 and Golgi phosphoprotein 3 to regulate Golgi trafficking and that overexpression of Rab1 can rescue the cytokinesis and locomotor defects associated with loss of Cog7. Our results suggest that the Drosophila COG7-CDG model can be used to test novel potential therapeutic strategies by modulating trafficking pathways.

Rab1 interacts with GOLPH3 and controls Golgi structure and contractile ring constriction during cytokinesis in Drosophila melanogaster

Cytokinesis requires a tight coordination between actomyosin ring constriction and new membrane addition along the ingressing cleavage furrow. However, the molecular mechanisms underlying vesicle trafficking to the equatorial site and how this process is coupled with the dynamics of the contractile apparatus are poorly defined. Here we provide evidence for the requirement of Rab1 during cleavage furrow ingression in cytokinesis. We demonstrate that the gene omelette (omt) encodes the Drosophila orthologue of human Rab1 and is required for successful cytokinesis in both mitotic and meiotic dividing cells of Drosophila melanogaster We show that Rab1 protein colocalizes with the conserved oligomeric Golgi (COG) complex Cog7 subunit and the phosphatidylinositol 4-phosphate effector GOLPH3 at the Golgi stacks. Analysis by transmission electron microscopy and 3D-SIM super-resolution microscopy reveals loss of normal Golgi architecture in omt mutant spermatocytes indicating a role for Rab1 in Golgi formation. In dividing cells, Rab1 enables stabilization and contraction of actomyosin rings. We further demonstrate that GTP-bound Rab1 directly interacts with GOLPH3 and controls its localization at the Golgi and at the cleavage site. We propose that Rab1, by associating with GOLPH3, controls membrane trafficking and contractile ring constriction during cytokinesis.

The transcriptional activator Pog1 controls cell cycle, and its phosphorylated form is downregulated by the ubiquitin ligase Dma2 in Saccharomyces cerevisiae

The POG1 gene in Saccharomyces cerevisiae is suggested to encode the transcriptional activator that promotes growth in the presence of a mating pheromone. We previously showed that the overexpression of POG1 conferred tolerance to high concentrations of LiCl and sugar on laboratory and baker's yeast strains, respectively. Here, the overexpression of POG1 was shown to induce cell cycle delay at the G1 phase and morphological abnormality. In addition, by yeast two-hybrid screening, the really interesting new gene (RING)-type ubiquitin ligase Dma2, which is involved in cell cycle regulation, was identified as the protein interacting with Pog1. The gene mutation and deletion analysis revealed that the interaction between Pog1 and Dma2 requires the phosphorylation of Thr253 in Pog1 and the forkhead-associated domain in Dma2. The phosphorylation status of Pog1 changed along with progression of the cell cycle. Interestingly, our results showed that Pog1 might be ubiquitinated by Dma2, but a dephosphorylation-mimic mutation in POG1 increased the cellular Pog1 level possibly due to the failure of ubiquitination. Furthermore, growth of the dma1/2-disrupted strain was greatly inhibited by the overexpression of POG1. These results suggest that Pog1 controls the cell cycle and its phosphorylated form is downregulated by Dma2.

Vhs2 is a novel regulator of septin dynamics in budding yeast

In budding yeast, septins are assembled into structures that undergo dramatic changes during the cell cycle. The molecular mechanisms that drive these remodelings are not fully uncovered. In this study, we describe a characterization of Vhs2, a nonessential protein that revealed to be a new player in septin dynamics. In particular, we report that Vhs2 is important to maintain the stability of the double septin ring structure until telophase. In addition, we show that Vhs2 undergoes multiple phosphorylations during the cell cycle, being phosphorylated during S phase until nuclear division and dephosphorylated just before cell division. Importantly we report that cyclin-dependent protein kinase Cdk1 and protein phosphatase Cdc14 control these Vhs2 post-translational modifications. These results reveal that Vhs2 is a novel Cdc14 substrate that is involved in the control of septin organization.