Cdk1-Clb4 controls the interaction of astral microtubule plus ends with subdomains of the daughter cell cortex

Microtubule specialization by +TIP networks: from mechanisms to functional implications

To fulfill their actual cellular role, individual microtubules become functionally specialized through a broad range of mechanisms. The 'search and capture' model posits that microtubule dynamics and functions are specified by cellular targets that they capture (i.e., a posteriori), independently of the microtubule-organizing center (MTOC) they emerge from. However, work in budding yeast indicates that MTOCs may impart a functional identity to the microtubules they nucleate, a priori. Key effectors in this process are microtubule plus-end tracking proteins (+TIPs), which track microtubule tips to regulate their dynamics and facilitate their targeted interactions. In this review, we discuss potential mechanisms of a priori microtubule specialization, focusing on recent findings indicating that +TIP networks may undergo liquid biomolecular condensation in different cell types.

Activation of the DNA damage checkpoint perturbs asymmetric localization of Kar9 to spindle pole bodies in Saccharomyces cerevisiae

During cell cycle progression in Saccharomyces cerevisiae, spindle pole bodies (SPBs) are duplicated during the G1/S-phase transition. SPBs are crucial for the organization of both the spindle and astral microtubules, and their orientation defines the direction of nuclear division. In this process, an old SPB, which serves as the template SPB during the duplication process, is oriented toward the bud side. The patterning microtubule plus-end tracking protein, Kar9, plays an important role in the orientation of SPBs by asymmetrically localizing to the old SPB. Here, methylglyoxal (MG), a metabolite derived from glycolysis, was found to perturb asymmetric Kar9 localization and influence proper positioning of the old SPB. MG activated the DNA damage checkpoint pathway, and MG-induced perturbation of asymmetric Kar9 localization was abolished by the deletion of MEC1, a sensor for the DNA damage checkpoint pathway. Methyl methanesulfonate, a DNA-alkylating agent, also perturbed asymmetric Kar9 localization. Our results suggest that activation of the DNA damage checkpoint pathway perturbs the asymmetric Kar9 localization required for proper positioning of SPBs.

APC/CCdc20-mediated degradation of Clb4 prompts astral microtubule stabilization at anaphase onset

Key for accurate chromosome partitioning to the offspring is the ability of mitotic spindle microtubules to respond to different molecular signals and remodel their dynamics accordingly. Spindle microtubules are conventionally divided into three classes: kinetochore, interpolar, and astral microtubules (kMTs, iMTs, and aMTs, respectively). Among all, aMT regulation remains elusive. Here, we show that aMT dynamics are tightly regulated. aMTs remain unstable up to metaphase and are stabilized at anaphase onset. This switch in aMT dynamics, important for proper spindle orientation, specifically requires the degradation of the mitotic cyclin Clb4 by the Anaphase Promoting Complex bound to its activator subunit Cdc20 (APC/CCdc20). These data highlight a unique role for mitotic cyclin Clb4 in controlling aMT regulating factors, of which Kip2 is a prime candidate, provide a framework to understand aMT regulation in vertebrates, and uncover mechanistic principles of how the APC/CCdc20 choreographs the timing of late mitotic events by sequentially impacting on the three classes of spindle microtubules.

Structure and regulation of the microtubule plus-end tracking protein Kar9

In many eukaryotes, coordination of chromosome segregation with cell cleavage relies on the patterned interaction of specific microtubules with actin filaments through dedicated microtubule plus-end tracking proteins (+TIPs). However, how these +TIPs are spatially controlled is unclear. The yeast +TIP Kar9 drives one of the spindle aster microtubules along actin cables to align the mitotic spindle with the axis of cell division. Here, we report the crystal structure of Kar9's folded domain, revealing spectrin repeats reminiscent of the +TIPs MACF/ACF7/Shot and PRC1/Ase1. Point mutations abrogating spectrin-repeat-mediated dimerization of Kar9 reduced and randomized Kar9 distribution to microtubule tips, and impaired spindle positioning. Six Cdk1 sites surround the Kar9 dimerization interface. Their phosphomimetic substitution inhibited Kar9 dimerization, displaced Kar9 from microtubules, and affected its interaction with the myosin motor Myo2. Our results provide molecular-level understanding on how diverse cell types may regulate and pattern microtubule-actin interactions to orchestrate their divisions.

Polo-like kinase acts as a molecular timer that safeguards the asymmetric fate of spindle microtubule-organizing centers

The microtubules that form the mitotic spindle originate from microtubule-organizing centers (MTOCs) located at either pole. After duplication, spindle MTOCs can be differentially inherited during asymmetric cell division in organisms ranging from yeast to humans. Problems with establishing predetermined spindle MTOC inheritance patterns during stem cell division have been associated with accelerated cellular aging and the development of both cancer and neurodegenerative disorders. Here, we expand the repertoire of functions Polo-like kinase family members fulfill in regulating pivotal cell cycle processes. We demonstrate that the Plk1 homolog Cdc5 acts as a molecular timer that facilitates the timely and sequential recruitment of two key determinants of spindle MTOCs distribution, that is the γ-tubulin complex receptor Spc72 and the protein Kar9, and establishes the fate of these structures, safeguarding their asymmetric inheritance during Saccharomyces cerevisiae mitosis.

Remote control of microtubule plus-end dynamics and function from the minus-end

In eukaryotes, the organization and function of the microtubule cytoskeleton depend on the allocation of different roles to individual microtubules. For example, many asymmetrically dividing cells differentially specify microtubule behavior at old and new centrosomes. Here we show that yeast spindle pole bodies (SPBs, yeast centrosomes) differentially control the plus-end dynamics and cargoes of their astral microtubules, remotely from the minus-end. The old SPB recruits the kinesin motor protein Kip2, which then translocates to the plus-end of the emanating microtubules, promotes their extension and delivers dynein into the bud. Kip2 recruitment at the SPB depends on Bub2 and Bfa1, and phosphorylation of cytoplasmic Kip2 prevents random lattice binding. Releasing Kip2 of its control by SPBs equalizes its distribution, the length of microtubules and dynein distribution between the mother cell and its bud. These observations reveal that microtubule organizing centers use minus to plus-end directed remote control to individualize microtubule function.

Asymmetric distribution of glucose transporter mRNA provides a growth advantage in yeast

Asymmetric localization of mRNA is important for cell fate decisions in eukaryotes and provides the means for localized protein synthesis in a variety of cell types. Here, we show that hexose transporter mRNAs are retained in the mother cell of S. cerevisiae until metaphase-anaphase transition (MAT) and then are released into the bud. The retained mRNA was translationally less active but bound to ribosomes before MAT Importantly, when cells were shifted from starvation to glucose-rich conditions, HXT2 mRNA, but none of the other HXT mRNAs, was enriched in the bud after MAT This enrichment was dependent on the Ras/cAMP/PKA pathway, the APC ortholog Kar9, and nuclear segregation into the bud. Competition experiments between strains that only expressed one hexose transporter at a time revealed that HXT2 only cells grow faster than their counterparts when released from starvation. Therefore, asymmetric distribution of HXT2 mRNA provides a growth advantage for daughters, who are better prepared for nutritional changes in the environment. Our data provide evidence that asymmetric mRNA localization is an important factor in determining cellular fitness.

Coupling spindle position with mitotic exit in budding yeast: The multifaceted role of the small GTPase Tem1

The budding yeast S. cerevisiae divides asymmetrically and is an excellent model system for asymmetric cell division. As for other asymmetrically dividing cells, proper spindle positioning along the mother-daughter polarity axis is crucial for balanced chromosome segregation. Thus, a surveillance mechanism named Spindle Position Checkpoint (SPOC) inhibits mitotic exit and cytokinesis until the mitotic spindle is properly oriented, thereby preventing the generation of cells with aberrant ploidies. The small GTPase Tem1 is required to trigger a Hippo-like protein kinase cascade, named Mitotic Exit Network (MEN), that is essential for mitotic exit and cytokinesis but also contributes to correct spindle alignment in metaphase. Importantly, Tem1 is the target of the SPOC, which relies on the activity of the GTPase-activating complex (GAP) Bub2-Bfa1 to keep Tem1 in the GDP-bound inactive form. Tem1 forms a hetero-trimeric complex with Bub2-Bfa1 at spindle poles (SPBs) that accumulates asymmetrically on the bud-directed spindle pole during mitosis when the spindle is properly positioned. In contrast, the complex remains symmetrically localized on both poles of misaligned spindles. We have recently shown that Tem1 residence at SPBs depends on its nucleotide state and, importantly, asymmetry of the Bub2-Bfa1-Tem1 complex does not promote mitotic exit but rather controls spindle positioning.

Nuclear movement in fungi

Nuclear movement within a cell occurs in a variety of eukaryotic organisms including yeasts and filamentous fungi. Fungal molecular genetic studies identified the minus-end-directed microtubule motor cytoplasmic dynein as a critical protein for nuclear movement or orientation of the mitotic spindle contained in the nucleus. Studies in the budding yeast first indicated that dynein anchored at the cortex via its anchoring protein Num1 exerts pulling force on an astral microtubule to orient the anaphase spindle across the mother-daughter axis before nuclear division. Prior to anaphase, myosin V interacts with the plus end of an astral microtubule via Kar9-Bim1/EB1 and pulls the plus end along the actin cables to move the nucleus/spindle close to the bud neck. In addition, pushing or pulling forces generated from cortex-linked polymerization or depolymerization of microtubules drive nuclear movements in yeasts and possibly also in filamentous fungi. In filamentous fungi, multiple nuclei within a hyphal segment undergo dynein-dependent back-and-forth movements and their positioning is also influenced by cytoplasmic streaming toward the hyphal tip. In addition, nuclear movement occurs at various stages of fungal development and fungal infection of plant tissues. This review discusses our current understanding on the mechanisms of nuclear movement in fungal organisms, the importance of nuclear positioning and the regulatory strategies that ensure the proper positioning of nucleus/spindle.

Intrinsic and Extrinsic Determinants Linking Spindle Pole Fate, Spindle Polarity, and Asymmetric Cell Division in the Budding Yeast S. cerevisiae

The budding yeast S. cerevisiae is a powerful model to understand the multiple layers of control driving an asymmetric cell division. In budding yeast, asymmetric targeting of the spindle poles to the mother and bud cell compartments respectively orients the mitotic spindle along the mother-bud axis. This program exploits an intrinsic functional asymmetry arising from the age distinction between the spindle poles-one inherited from the preceding division and the other newly assembled. Extrinsic mechanisms convert this age distinction into differential fate. Execution of this program couples spindle orientation with the segregation of the older spindle pole to the bud. Remarkably, similar stereotyped patterns of inheritance occur in self-renewing stem cell divisions underscoring the general importance of studying spindle polarity and differential fate in yeast. Here, we review the mechanisms accounting for this pivotal interplay between intrinsic and extrinsic asymmetries that translate spindle pole age into differential fate.

Molecular basis of Kar9-Bim1 complex function during mating and spindle positioning

The Kar9 pathway promotes nuclear fusion during mating and spindle alignment during metaphase in budding yeast. How Kar9 supports the different outcome of these two divergent processes is an open question. Here, we show that three sites in the C-terminal disordered domain of Kar9 mediate tight Kar9 interaction with the C-terminal dimerization domain of Bim1 (EB1 orthologue). Site1 and Site2 contain SxIP motifs; however, Site3 defines a novel type of EB1-binding site. Whereas Site2 and Site3 mediate Kar9 recruitment to microtubule tips, nuclear movement and karyogamy, solely Site2 functions in spindle positioning during metaphase. Site1 in turn plays an inhibitory role during mating. Additionally, the Kar9-Bim1 complex is involved in microtubule-independent activities during mating. Together, our data reveal how multiple and partially redundant EB1-binding sites provide a microtubule-associated protein with the means to modulate its biochemical properties to promote different molecular processes during cell proliferation and differentiation.

G1 cyclin driven DNA replication

The mitotic cell cycle is driven by Cyclin-Dependent Kinases (CDK). CDK activation requires the binding of activatory subunits termed cyclins. Different waves of cyclins are expressed during the cell cycle, enabling CDKs to trigger phase specific events. For instance, S phase cyclins promote the initiation of DNA replication but not chromosome segregation. There are at least 2 explanations for how such regulation is achieved. According to one of the visions, cyclins confer intrinsic substrate specificity to the CDK catalytic subunit. Alternatively a quantitative model has been proposed, according to which ever-increasing CDK activity is required to trigger cell cycle events from G1 to M. If a quantitative control prevails, then an early cyclin should trigger later cycle events if accumulated at high enough levels at the right time and place. We show here that a G1 phase cyclin bears the potential to trigger DNA replication and promote S and G2 phase specific transcription.

Control of microtubule organization and dynamics: two ends in the limelight

Microtubules have fundamental roles in many essential biological processes, including cell division and intracellular transport. They assemble and disassemble from their two ends, denoted the plus end and the minus end. Significant advances have been made in our understanding of microtubule plus-end-tracking proteins (+TIPs) such as end-binding protein 1 (EB1), XMAP215, selected kinesins and dynein. By contrast, information on microtubule minus-end-targeting proteins (-TIPs), such as the calmodulin-regulated spectrin-associated proteins (CAMSAPs) and Patronin, has only recently started to emerge. Here, we review our current knowledge of factors, including microtubule-targeting agents, that associate with microtubule ends to control the dynamics and function of microtubules during the cell cycle and development.

Spindle pole body history intrinsically links pole identity with asymmetric fate in budding yeast

BACKGROUND

Budding yeast is a unique model for exploring differential fate in a cell dividing asymmetrically. In yeast, spindle orientation begins with the old spindle pole body (SPB) (from the preceding cell cycle) contacting the bud by its existing astral microtubules (aMTs) while the new pole delays astral microtubule organization. This appears to prime the inheritance of the old pole by the bud. The basis for this asymmetry and the discrimination of the poles by virtue of their history remain a mystery.

RESULTS

Here, we report that asymmetric aMT organization stems from an outstanding structural asymmetry linked to the SPB cycle. We show that the γ-tubulin nucleation complex (γTC) favors the old spindle pole, an asymmetry inherent to the outer plaque (the cytoplasmic face of the SPB). Indeed, Spc72 (the receptor for the γTC) is acquired by the new SPB outer plaque partway through spindle assembly. The significance of this asymmetry was explored in cells expressing an Spc72(1-276)-Cnm67 fusion that forced symmetric nucleation at the SPB outer plaques. This manipulation triggered simultaneous aMT organization by both spindle poles from the outset and led to symmetric contacts between poles and the bud, effectively disrupting the program for spindle polarity. Temporally symmetric aMT organization perturbed Kar9 polarization by randomizing the choice of the pole to be guided toward the bud. Accordingly, the pattern of SPB inheritance was also randomized.

CONCLUSIONS

Spc72 differential recruitment imparting asymmetric aMT organization represents the most upstream determinant linking SPB historical identity and fate.

The yeast homologue of the microtubule-associated protein Lis1 interacts with the sumoylation machinery and a SUMO-targeted ubiquitin ligase

The two yeast members of the CLIP-170/Bik1p and Lis1/Pac1p families of microtubule-associated proteins are shown to interact with the sumoylation machinery and the STUbL complex Ris1p–Nis1p. Pac1p can be modified by both SUMO and ubiquitin. The She1 regulator of dynactin is identified as a novel inhibitor of Pac1p modification.

Microtubules and microtubule-associated proteins are fundamental for multiple cellular processes, including mitosis and intracellular motility, but the factors that control microtubule-associated proteins (MAPs) are poorly understood. Here we show that two MAPs—the CLIP-170 homologue Bik1p and the Lis1 homologue Pac1p—interact with several proteins in the sumoylation pathway. Bik1p and Pac1p interact with Smt3p, the yeast SUMO; Ubc9p, an E2; and Nfi1p, an E3. Bik1p interacts directly with SUMO in vitro, and overexpression of Smt3p and Bik1p results in its in vivo sumoylation. Modified Pac1p is observed when the SUMO protease Ulp1p is inactivated. Both ubiquitin and Smt3p copurify with Pac1p. In contrast to ubiquitination, sumoylation does not directly tag the substrate for degradation. However, SUMO-targeted ubiquitin ligases (STUbLs) can recognize a sumoylated substrate and promote its degradation via ubiquitination and the proteasome. Both Pac1p and Bik1p interact with the STUbL Nis1p-Ris1p and the protease Wss1p. Strains deleted for RIS1 or WSS1 accumulate Pac1p conjugates. This suggests a novel model in which the abundance of these MAPs may be regulated via STUbLs. Pac1p modification is also altered by Kar9p and the dynein regulator She1p. This work has implications for the regulation of dynein's interaction with various cargoes, including its off-loading to the cortex.

Purification, crystallization and preliminary crystallographic analysis of Kar9p from Saccharomyces cerevisiae

Kar9p is required for correct positioning of the mitotic spindle in Saccharomyces cerevisiae. The in vivo function of Kar9p is well understood, but no structural information is available. Additionally, molecular details of how Kar9p interacts with other proteins are scarce. Full-length Kar9p was expressed in Escherichia coli, purified and crystallized. Diffraction data were collected and processed at 7 Å resolution. One crystal showed diffraction to 3 Å resolution. The crystals that diffracted to 7 Å resolution belonged to space group P3, with unit-cell parameters a = b = 195.02, c = 257.15 Å, α = β = 90, γ = 120°. The crystal that diffracted to 3 Å resolution belonged to space group P222, with unit-cell parameters a = 46.37, b = 74.64, c = 133.63 Å, α = β = γ = 90°.

The MEN mediates the effects of the spindle assembly checkpoint on Kar9-dependent spindle pole body inheritance in budding yeast

Many asymmetrically dividing cells segregate the poles of the mitotic spindle non-randomly between their two daughters. In budding yeast, the protein Kar9 localizes almost exclusively to the astral microtubules emanating from the old spindle pole body (SPB) and promotes its movement toward the bud. Thereby, Kar9 orients the spindle relative to the division axis. Here, we show that beyond perturbing Kar9 distribution, activation of the spindle assembly checkpoint (SAC) randomizes SPB inheritance. Inactivation of the B-type cyclin Clb5 led to a SAC-dependent defect in Kar9 orientation and SPB segregation. Furthermore, unlike the Clb4-dependent pathway, the Clb5- and SAC-dependent pathways functioned genetically upstream of the mitotic exit network (MEN) in SPB specification and Kar9-dependent SPB inheritance. Together, our study indicates that Clb5 functions in spindle assembly and that the SAC controls the specification and inheritance of yeast SPBs through inhibition of the MEN.

Astral microtubule asymmetry provides directional cues for spindle positioning in budding yeast

Cortical force generators play a central role in the orientation and positioning of the mitotic spindle. In higher eukaryotes, asymmetrically localized cortical polarity determinants recruit or activate such force generators, which, through interactions with astral microtubules, position the mitotic spindle at the future site of cytokinesis. Recent studies in budding yeast have shown that, rather than the cell cortex, the astral microtubules themselves may provide polarity cues that are needed for asymmetric pulling on the mitotic spindle. Such asymmetry has been shown to be required for proper spindle positioning, and consequently faithful and accurate chromosome segregation. In this review, we highlight results that have shed light on spindle orientation in this classical model of asymmetric cell division, and review findings that may shed light on similar processes in higher eukaryotes.

The mother-bud neck as a signaling platform for the coordination between spindle position and cytokinesis in budding yeast

During asymmetric cell division, spindle positioning is critical for ensuring the unequal inheritance of polarity factors. In budding yeast, the mother-bud neck determines the cleavage plane and a correct nuclear division between mother and daughter cell requires orientation of the mitotic spindle along the mother-bud axis. A surveillance device called the spindle position/orientation checkpoint (SPOC) oversees this process and delays mitotic exit and cytokinesis until the spindle is properly oriented along the division axis, thus ensuring genome stability. Cytoskeletal proteins called septins form a ring at the bud neck that is essential for cytokinesis. Furthermore, septins and septin-associated proteins are implicated in spindle positioning and SPOC. In this review, we discuss the emerging connections between septins and the SPOC and the role of the mother-bud neck as a signaling platform to couple proper chromosome segregation to cytokinesis.

The cortical protein Lte1 promotes mitotic exit by inhibiting the spindle position checkpoint kinase Kin4

Lte1 directly inhibits Kin4 activity and restricts its binding to the mother spindle pole body, which allows proper mitotic exit of cells with a correctly aligned spindle.

The spindle position checkpoint (SPOC) is an essential surveillance mechanism that allows mitotic exit only when the spindle is correctly oriented along the cell axis. Key SPOC components are the kinase Kin4 and the Bub2–Bfa1 GAP complex that inhibit the mitotic exit–promoting GTPase Tem1. During an unperturbed cell cycle, Kin4 associates with the mother spindle pole body (mSPB), whereas Bub2–Bfa1 is at the daughter SPB (dSPB). When the spindle is mispositioned, Bub2–Bfa1 and Kin4 bind to both SPBs, which enables Kin4 to phosphorylate Bfa1 and thereby block mitotic exit. Here, we show that the daughter cell protein Lte1 physically interacts with Kin4 and inhibits Kin4 kinase activity. Specifically, Lte1 binds to catalytically active Kin4 and promotes Kin4 hyperphosphorylation, which restricts Kin4 binding to the mSPB. This Lte1-mediated exclusion of Kin4 from the dSPB is essential for proper mitotic exit of cells with a correctly aligned spindle. Therefore, Lte1 promotes mitotic exit by inhibiting Kin4 activity at the dSPB.

Global analysis of Cdc14 phosphatase reveals diverse roles in mitotic processes

Cdc14 phosphatase regulates multiple events during anaphase and is essential for mitotic exit in budding yeast. Cdc14 is regulated in both a spatial and temporal manner. It is sequestered in the nucleolus for most of the cell cycle by the nucleolar protein Net1 and is released into the nucleus and cytoplasm during anaphase. To identify novel binding partners of Cdc14, we used affinity purification of Cdc14 and mass spectrometric analysis of interacting proteins from strains in which Cdc14 localization or catalytic activity was altered. To alter Cdc14 localization, we used a strain deleted for NET1, which causes full release of Cdc14 from the nucleolus. To alter Cdc14 activity, we generated mutations in the active site of Cdc14 (C283S or D253A), which allow binding of substrates, but not dephosphorylation, by Cdc14. Using this strategy, we identified new interactors of Cdc14, including multiple proteins involved in mitotic events. A subset of these proteins displayed increased affinity for catalytically inactive mutants of Cdc14 compared with the wild-type version, suggesting they are likely substrates of Cdc14. We have also shown that several of the novel Cdc14-interacting proteins, including Kar9 (a protein that orients the mitotic spindle) and Bni1 and Bnr1 (formins that nucleate actin cables and may be important for actomyosin ring contraction) are specifically dephosphorylated by Cdc14 in vitro and in vivo. Our findings suggest the dephosphorylation of the formins may be important for their observed localization change during exit from mitosis and indicate that Cdc14 targets proteins involved in wide-ranging mitotic events.

Mutation of Ser172 in yeast β tubulin induces defects in microtubule dynamics and cell division

Ser172 of β tubulin is an important residue that is mutated in a human brain disease and phosphorylated by the cyclin-dependent kinase Cdk1 in mammalian cells. To examine the role of this residue, we used the yeast S. cerevisiae as a model and produced two different mutations (S172A and S172E) of the conserved Ser172 in the yeast β tubulin Tub2p. The two mutants showed impaired cell growth on benomyl-containing medium and at cold temperatures, altered microtubule (MT) dynamics, and altered nucleus positioning and segregation. When cytoplasmic MT effectors Dyn1p or Kar9p were deleted in S172A and S172E mutants, cells were viable but presented increased ploidy. Furthermore, the two β tubulin mutations exhibited synthetic lethal interactions with Bik1p, Bim1p or Kar3p, which are effectors of cytoplasmic and spindle MTs. In the absence of Mad2p-dependent spindle checkpoint, both mutations are deleterious. These findings show the importance of Ser172 for the correct function of both cytoplasmic and spindle MTs and for normal cell division.

Spindle position is coordinated with cell-cycle progression through establishment of mitotic exit-activating and -inhibitory zones

How spatial information is translated into a chemical signal is a fundamental problem in all organisms. The spindle position checkpoint is a prime example of this problem. This checkpoint senses spindle position and, in budding yeast, inhibits the mitotic exit network (MEN), a signaling pathway that promotes exit from mitosis. We find that spindle position is sensed by a system composed of MEN-inhibitory and -activating zones and a sensor that moves between them. The MEN inhibitory zone is located in the mother cell, the MEN-activating zone in the bud, and the spindle pole body (SPB), where the components of the MEN reside, functions as the sensor. Only when an SPB escapes the MEN inhibitor Kin4 in the mother cell and moves into the bud where the MEN activator Lte1 resides can exit from mitosis occur. In this manner, spatial information is sensed and translated into a chemical signal.

Candida albicans cyclin Clb4 carries S-phase cyclin activity

Cyclin-dependent kinases (CDKs) are key regulators of eukaryotic cell cycle progression. The cyclin subunit activates the CDK and also imparts to the complex, at least in some cases, substrate specificity. Saccharomyces cerevisiae, an organism in which the roles of individual cyclins are best studied, contains nine cyclins (three G(1) cyclins and six B-type cyclins) capable of activating the main cell cycle CDK, Cdc28. Analysis of the genome of the pathogenic yeast Candida albicans revealed only two sequences corresponding to B-type cyclins, C. albicans Clb2 (CaClb2) and CaClb4. Notably, no homolog of the S. cerevisiae S-phase-specific cyclins, Clb5/Clb6, could be detected. Here, we performed an in vitro analysis of the activity of CaClb2 and CaClb4 and of three G(1) cyclins, as well as an analysis of the phenotype of S. cerevisiae cells expressing CaClb2 or CaClb4 instead of Clb5. Remarkably, replacement of CLB5 by CaCLB4 caused rapid diploidization of S. cerevisiae. In addition, both in vivo and in vitro analyses indicate that, in spite of the higher sequence similarity of CaClb2 to Clb5/Clb6, CaClb4 is the functional homolog of Clb5/Clb6. The activity of a CaClb2/CaClb4 cyclin hybrid suggests that the cyclin box domain of CaClb4 carries the functional specificity of the protein. These results have implications for our understanding of the evolution of specificity of the cell cycle cyclins.

Actin-mediated delivery of astral microtubules instructs Kar9p asymmetric loading to the bud-ward spindle pole

We show that Kar9p polarity is instructed by a feedback loop that requires astral microtubules, actin cables, and Myo2p-based transport to enforce Kar9p loading to the bud-ward pole. This novel mechanism also provides the basis for a model unifying Kar9p polarity and the stereotyped pattern of spindle pole inheritance known to occur in yeast.

In Saccharomyces cerevisiae, Kar9p, one player in spindle alignment, guides the bud-ward spindle pole by linking astral microtubule plus ends to Myo2p-based transport along actin cables generated by the formins Bni1p and Bnr1p and the polarity determinant Bud6p. Initially, Kar9p labels both poles but progressively singles out the bud-ward pole. Here, we show that this polarization requires cell polarity determinants, actin cables, and microtubules. Indeed, in a bud6Δ bni1Δ mutant or upon direct depolymerization of actin cables Kar9p symmetry increased. Furthermore, symmetry was selectively induced by myo2 alleles, preventing Kar9p binding to the Myo2p cargo domain. Kar9p polarity was rebuilt after transient disruption of microtubules, dependent on cell polarity and actin cables. Symmetry breaking also occurred after transient depolymerization of actin cables, with Kar9p increasing at the spindle pole engaging in repeated cycles of Kar9p-mediated transport. Kar9p returning to the spindle pole on shrinking astral microtubules may contribute toward this bias. Thus, Myo2p transport along actin cables may support a feedback loop by which delivery of astral microtubule plus ends sustains Kar9p polarized recruitment to the bud-ward spindle pole. Our findings also explain the link between Kar9p polarity and the choice setting aside the old spindle pole for daughter-bound fate.

Mutual regulation of cyclin-dependent kinase and the mitotic exit network

The mitotic exit network (MEN) is a spindle pole body (SPB)-associated, GTPase-driven signaling cascade that controls mitotic exit. The inhibitory Bfa1-Bub2 GTPase-activating protein (GAP) only associates with the daughter SPB (dSPB), raising the question as to how the MEN is regulated on the mother SPB (mSPB). Here, we show mutual regulation of cyclin-dependent kinase 1 (Cdk1) and the MEN. In early anaphase Cdk1 becomes recruited to the mSPB depending on the activity of the MEN kinase Cdc15. Conversely, Cdk1 negatively regulates binding of Cdc15 to the mSPB. In addition, Cdk1 phosphorylates the Mob1 protein to inhibit the activity of Dbf2-Mob1 kinase that regulates Cdc14 phosphatase. Our data revise the understanding of the spatial regulation of the MEN. Although MEN activity in the daughter cells is controlled by Bfa1-Bub2, Cdk1 inhibits MEN activity at the mSPB. Consistent with this model, only triple mutants that lack BUB2 and the Cdk1 phosphorylation sites in Mob1 and Cdc15 show mitotic exit defects.

Coordinating mitosis with cell polarity: Molecular motors at the cell cortex

In many cell divisions, the position of the spindle apparatus is coordinated with polarity signals at the cell cortex so that copies of the genome are delivered to regions of the cell that are designated for differential inheritance by the two progeny. To coordinate spindle position with cell polarity, the spindle interfaces with elements on the cortex, where molecular motors often produce the forces that power displacement. Here we describe the molecular pathways by which cortical motors translocate the spindle in budding yeast, where the mechanisms are understood relatively well, and we compare these pathways to spindle positioning processes in metazoan systems, where the molecular details are less well understood.

Structure, dynamics and optical properties of fluorescent proteins: perspectives for marker development

GFP-like proteins, originally cloned from marine animals, are genetically encoded fluorescence markers that have become indispensable tools for the life sciences. The search for GFP-like proteins with novel and improved properties is still ongoing, however, driven by the persistent need for advanced and specialized fluorescence labels for cellular imaging. Overall, the structures of these proteins are similar, but considerable variations have been found in the covalent structures and stereochemistry of the fluorophore, which govern essential optical properties such as the absorption/emission wavelengths. Moreover, as the fluorophore-enclosing cavity forms its solvation shell, it can also have a significant effect on the absorption/emission wavelengths and the brightness of the fluorophore. Most exciting are recent developments of photoactivatable fluorescence markers which change their color and/or intensity upon irradiation with light of specific wavelengths. A detailed understanding of the structure and dynamics of GFP-like proteins greatly aids in the rational engineering of advanced fluorescence marker proteins. Herein, we review our present knowledge of the structural diversity of GFP-like proteins and discuss how structure and dynamics govern their optical properties, with an emphasis on red fluorescent proteins.

Spindle orientation during asymmetric cell division

Development of a multicellular organism from a fertilized egg depends on a precise balance between symmetric cell divisions to expand the pool of similar cells, and asymmetric cell divisions to create cell-type diversity. Spindle orientation can influence the generation of symmetric or asymmetric cell fates depending on how it is coupled to cell-intrinsic polarity cues, or how it is positioned relative to cell-extrinsic cues such as niche-derived signals. In this review, we describe the mechanism of spindle orientation in budding yeast, Drosophila melanogaster, Caenorhabditis elegans and mammalian neural progenitors, with the goal of highlighting conserved mechanisms and indicating open questions for the future.

Cdc14 inhibition by the spindle assembly checkpoint prevents unscheduled centrosome separation in budding yeast

The spindle assembly checkpoint (SAC) is an evolutionarily conserved surveillance mechanism that delays anaphase onset and mitotic exit in response to the lack of kinetochore attachment. The target of the SAC is the E3 ubiquitin ligase anaphase-promoting complex (APC) bound to its Cdc20 activator. The Cdc20/APC complex is in turn required for sister chromatid separation and mitotic exit through ubiquitin-mediated proteolysis of securin, thus relieving inhibition of separase that unties sister chromatids. Separase is also involved in the Cdc-fourteen early anaphase release (FEAR) pathway of nucleolar release and activation of the Cdc14 phosphatase, which regulates several microtubule-linked processes at the metaphase/anaphase transition and also drives mitotic exit. Here, we report that the SAC prevents separation of microtubule-organizing centers (spindle pole bodies [SPBs]) when spindle assembly is defective. Under these circumstances, failure of SAC activation causes unscheduled SPB separation, which requires Cdc20/APC, the FEAR pathway, cytoplasmic dynein, and the actin cytoskeleton. We propose that, besides inhibiting sister chromatid separation, the SAC preserves the accurate transmission of chromosomes also by preventing SPBs to migrate far apart until the conditions to assemble a bipolar spindle are satisfied.

Spindle alignment regulates the dynamic association of checkpoint proteins with yeast spindle pole bodies

In many polarized cells, the accuracy of chromosome segregation depends on the correct positioning of the mitotic spindle. In budding yeast, the spindle positioning checkpoint (SPOC) delays mitotic exit when the anaphase spindle fails to extend toward the mother-daughter axis. However it remains to be established how spindle orientation is translated to SPOC components at the yeast spindle pole bodies (SPB). Here, we used photobleaching techniques to show that the dynamics with which Bub2-Bfa1 turned over at SPBs significantly increased upon SPOC activation. A version of Bfa1 that was stably associated with SPBs rendered the cells SPOC deficient without affecting other Bub2-Bfa1 functions, demonstrating the functional importance of regulating the dynamics of Bfa1 SPB association. In addition, we established that the SPOC kinase Kin4 is the major regulator of Bfa1 residence time at SPBs. We suggest that upon SPOC activation Bfa1-Bub2 spreads throughout the cytoplasm, thereby inhibiting mitotic exit.

Cell polarity determinants establish asymmetry in MEN signaling

Components of the mitotic exit network (MEN), a signaling pathway that triggers exit from mitosis, localize to the spindle pole body (SPB) that migrates into the daughter cell during anaphase but are largely absent from the SPB that remains in the mother cell. Through the analysis of one of the determinants of this asymmetry, Bfa1, we find that the machinery responsible for establishing cell polarity and cytoplasmic microtubules collaborate to establish MEN asymmetry. In cells defective in the Cdc42 signaling pathway or the formin Bni1, Bfa1 localizes to both SPBs. The quantitative analysis of Bfa1 localization further shows that Bfa1 can associate with both SPBs in a transient and highly dynamic fashion, but the protein is stabilized on the SPB that migrates into the daughter cell during anaphase through microtubule-bud cortex interactions. Our results indicate that mother-daughter cell asymmetry determinants establish MEN signaling asymmetry through microtubule-bud cortex interactions.

Role of spindle asymmetry in cellular dynamics

The mitotic spindle is mostly perceived as a symmetric structure. However, in many cell divisions, the two poles of the spindle organize asters with different dynamics, associate with different biomolecules or subcellular domains, and perform different functions. In this chapter, we describe some of the most prominent examples of spindle asymmetry. These are encountered during cell-cycle progression in budding and fission yeast and during asymmetric cell divisions of stem cells and embryos. We analyze the molecular mechanisms that lead to generation of spindle asymmetry and discuss the importance of spindle-pole differentiation for the correct outcome of cell division.

The spindle position checkpoint: how to deal with spindle misalignment during asymmetric cell division in budding yeast

During asymmetric cell division, spindle positioning is critical to ensure the unequal segregation of polarity factors and generate daughter cells with different sizes or fates. In budding yeast the boundary between mother and daughter cell resides at the bud neck, where cytokinesis takes place at the end of the cell cycle. Since budding and bud neck formation occur much earlier than bipolar spindle formation, spindle positioning is a finely regulated process. A surveillance device called the SPOC (spindle position checkpoint) oversees this process and delays mitotic exit and cytokinesis until the spindle is properly oriented along the division axis, thus ensuring genome stability.

Regulation of mitotic spindle asymmetry by SUMO and the spindle-assembly checkpoint in yeast

During mitosis, the kinetochore microtubules capture and segregate chromosomes, and the astral microtubules position the spindle within the cell. Although the spindle is symmetric, proper positioning of the spindle in asymmetrically dividing cells generally correlates with the formation of morphologically and structurally distinct asters [1]. In budding yeast, the spindle-orientation proteins Kar9 and dynein decorate only one aster of the metaphase spindle and direct it toward the bud [2, 3]. The mechanisms controlling the distribution of Kar9 and dynein remain unclear. Here, we show that SUMO regulates astral-microtubule function in at least two ways. First, Kar9 was sumoylated in vivo. Sumoylation and Cdk1-dependent phosphorylation of Kar9 independently promoted Kar9 asymmetry on the spindle. Second, proper regulation of kinetochore function by SUMO was also required for Kar9 asymmetry. Indeed, activation of the spindle-assembly checkpoint (SAC) due to SUMO and kinetochore defects promoted symmetric redistribution of Kar9 in a Mad2-dependent manner. The control of Kar9 distribution by the SAC was independent of Kar9 sumoylation and phosphorylation. Together, our data reveal that three independent mechanisms contribute to Kar9 asymmetry: Cdk1-dependent phosphorylation, sumoylation, and SAC signaling. Hence, the two seemingly independent spindle domains, kinetochores and astral microtubules, function in a tightly coordinated fashion.

The spindle positioning protein Kar9p interacts with the sumoylation machinery in Saccharomyces cerevisiae

Accurate positioning of the mitotic spindle is important for the genetic material to be distributed evenly in dividing cells, but little is known about the mechanisms that regulate this process. Here we report that two microtubule-associated proteins important for spindle positioning interact with several proteins in the sumoylation pathway. By two-hybrid analysis, Kar9p and Bim1p interact with the yeast SUMO Smt3p, the E2 enzyme Ubc9p, an E3 Nfi1p, as well as Wss1p, a weak suppressor of a temperature-sensitive smt3 allele. The physical interaction between Kar9p and Ubc9p was confirmed by in vitro binding assays. A single-amino-acid substitution in Kar9p, L304P disrupted its two-hybrid interaction with proteins in the sumoylation pathway, but retained its interactions with the spindle positioning proteins Bim1p, Stu2p, Bik1p, and Myo2p. The kar9-L304P mutant showed defects in positioning the mitotic spindle, with the spindle located more distally than normal. Whereas wild-type Kar9p-3GFP normally localizes to only the bud-directed spindle pole body (SPB), Kar9p-L304P-3GFP was mislocalized to both SPBs. Using a reconstitution assay, Kar9p was sumoylated in vitro. We propose a model in which sumoylation regulates spindle positioning by restricting Kar9p to one SPB. These findings raise the possibility that sumoylation could regulate other microtubule-dependent processes.

Intrinsic and cyclin-dependent kinase-dependent control of spindle pole body duplication in budding yeast

Centrosome duplication must be tightly controlled so that duplication occurs only once each cell cycle. Accumulation of multiple centrosomes can result in the assembly of a multipolar spindle and lead to chromosome mis-segregation and genomic instability. In metazoans, a centrosome-intrinsic mechanism prevents reduplication until centriole disengagement. Mitotic cyclin/cyclin-dependent kinases (CDKs) prevent reduplication of the budding yeast centrosome, called a spindle pole body (SPB), in late S-phase and G2/M, but the mechanism remains unclear. How SPB reduplication is prevented early in the cell cycle is also not understood. Here we show that, similar to metazoans, an SPB-intrinsic mechanism prevents reduplication early in the cell cycle. We also show that mitotic cyclins can inhibit SPB duplication when expressed before satellite assembly in early G1, but not later in G1, after the satellite had assembled. Moreover, electron microscopy revealed that SPBs do not assemble a satellite in cells expressing Clb2 in early G1. Finally, we demonstrate that Clb2 must localize to the cytoplasm in order to inhibit SPB duplication, suggesting the possibility for direct CDK inhibition of satellite components. These two mechanisms, intrinsic and extrinsic control by CDK, evoke two-step system that prevents SPB reduplication throughout the cell cycle.

Tracking the ends: a dynamic protein network controls the fate of microtubule tips

Microtubule plus-end tracking proteins (+TIPs) are a diverse group of evolutionarily conserved cellular factors that accumulate at the ends of growing microtubules. They form dynamic networks through the interaction of a limited set of protein modules, repeat sequences and linear motifs that bind to each other with moderate affinities. +TIPs regulate different aspects of cell architecture by controlling microtubule dynamics, microtubule interactions with cellular structures and signalling factors, and the forces that are exerted on microtubule networks.

Optimized and far-red-emitting variants of fluorescent protein eqFP611

Fluorescent proteins (FPs) emitting in the far-red region of the spectrum are highly advantageous for whole-body imaging applications because scattering and absorption of long-wavelength light is markedly reduced in tissue. We characterized variants of the red fluorescent protein eqFP611 with bright fluorescence emission shifted up to 639 nm. The additional red shift is caused by a trans-cis isomerization of the chromophore. The equilibrium between the trans and cis conformations is strongly influenced by amino acid residues 143 and 158. Pseudo monomeric tags were obtained by further genetic engineering. For the red chromophores of eqFP611 variants, molar extinction coefficients of up to approximately 150,000 were determined by an approach that is not affected by the presence of molecules with nonfunctional red chromophores. The bright fluorescence makes the red-shifted eqFP611 variants promising lead structures for the development of near-infrared fluorescent markers. The red fluorescent proteins performed well in cell biological applications, including two-photon imaging.

Microtubule plus end: a hub of cellular activities

Microtubules (MTs) are highly dynamic polymers, which control many aspects of cellular architecture. Growing MT plus ends accumulate a specific set of evolutionary conserved factors, the so-called MT plus-end-tracking proteins (+TIPs). +TIPs regulate MT dynamics and the reciprocal interactions of MTs with the cell cortex, mitotic kinetochores or different cellular organelles. Most +TIPs can directly bind to MTs, but the molecular mechanisms of their specific targeting to the growing plus ends remain poorly understood. Recent studies suggest that the members of one particular +TIP family, EB1 and its homologues, are present in all eucaryotic kingdoms, interact directly with the majority of other known plus-end-associated proteins and may be responsible for their specific accumulation at the MT tips.

Multiple levels of cyclin specificity in cell-cycle control

Cyclins regulate the cell cycle by binding to and activating cyclin-dependent kinases (Cdks). Phosphorylation of specific targets by cyclin-Cdk complexes sets in motion different processes that drive the cell cycle in a timely manner. In budding yeast, a single Cdk is activated by multiple cyclins. The ability of these cyclins to target specific proteins and to initiate different cell-cycle events might, in some cases, reflect the timing of the expression of the cyclins; in others, it might reflect intrinsic properties of the cyclins that render them better suited to target particular proteins.

The cyclin-dependent kinase Cdc28p regulates multiple aspects of Kar9p function in yeast

During mitosis in the yeast Saccharomyces cerevisiae, Kar9p directs one spindle pole body (SPB) toward the incipient daughter cell by linking the associated set of cytoplasmic microtubules (cMTs) to the polarized actin network on the bud cortex. The asymmetric localization of Kar9p to one SPB and attached cMTs is dependent on its interactions with microtubule-associated proteins and is regulated by the yeast Cdk1 Cdc28p. Two phosphorylation sites in Kar9p were previously identified. Here, we propose that the two sites are likely to govern Kar9p function through separate mechanisms, each involving a distinct cyclin. In the first mechanism, phosphorylation at serine 496 recruits Kar9p to one SPB. A phosphomimetic mutation at serine 496 bypasses the requirement of BIK1 and CLB5 in generating Kar9p asymmetry. In the second mechanism, Clb4p may target serine 197 of Kar9p for phosphorylation. This modification is required for Kar9p to direct cMTs to the bud. Two-hybrid analysis suggests that this phosphorylation may attenuate the interaction between Kar9p and the XMAP215-homologue Stu2p. We propose that phosphorylation at serine 197 regulates the release of Kar9p from Stu2p at the SPB, either to clear it from the mother-SPB or to allow it to travel to the plus end.

Gamma-tubulin is required for proper recruitment and assembly of Kar9-Bim1 complexes in budding yeast

Microtubule plus-end-interacting proteins (+TIPs) promote the dynamic interactions between the plus ends (+ends) of astral microtubules and cortical actin that are required for preanaphase spindle positioning. Paradoxically, +TIPs such as the EB1 orthologue Bim1 and Kar9 also associate with spindle pole bodies (SPBs), the centrosome equivalent in budding yeast. Here, we show that deletion of four C-terminal residues of the budding yeast gamma-tubulin Tub4 (tub4-delta dsyl) perturbs Bim1 and Kar9 localization to SPBs and Kar9-dependent spindle positioning. Surprisingly, we find Kar9 localizes to microtubule +ends in tub4-delta dsyl cells, but these microtubules fail to position the spindle when targeted to the bud. Using cofluorescence and coaffinity purification, we show Kar9 complexes in tub4-delta dsyl cells contain reduced levels of Bim1. Astral microtubule dynamics is suppressed in tub4-delta dsyl cells, but it are restored by deletion of Kar9. Moreover, Myo2- and F-actin-dependent dwelling of Kar9 in the bud is observed in tub4-delta dsyl cells, suggesting defective Kar9 complexes tether microtubule +ends to the cortex. Overproduction of Bim1, but not Kar9, restores Kar9-dependent spindle positioning in the tub4-delta dsyl mutant, reduces cortical dwelling, and promotes Bim1-Kar9 interactions. We propose that SPBs, via the tail of Tub4, promote the assembly of functional +TIP complexes before their deployment to microtubule +ends.

Asymmetric recruitment of dynein to spindle poles and microtubules promotes proper spindle orientation in yeast

The orientation of the mitotic spindle plays a key role in determining whether a polarized cell will divide symmetrically or asymmetrically. In most cell types, cytoplasmic dynein plays a critical role in spindle orientation. However, how dynein directs opposite spindle poles toward distinct and predetermined cell ends is poorly understood. Here, we show that dynein distributes preferentially to the spindle pole bodies (SPB) and astral microtubules (MTs) proximal to the bud in metaphase yeast cells. Dynein asymmetry depended on the bud neck kinases Elm1, Hsl1, and Gin4, on the spindle pole components Cnm67 and Cdk1, and on the B-type cyclins Clb1 and Clb2. Furthermore, phenotypic and genetic studies both indicated that dynein is unable to orient the spindle when it localizes to both poles and associated microtubules. Together, our data indicate that proper orientation of the spindle requires dynein to act on a single spindle pole.

Motor proteins at the microtubule plus-end

The plus-end of the microtubule has a central role in the interactions that occur between the microtubule and actin cytoskeletons. The recent identification of a family of proteins that congregate at the plus-end is enabling an increased mechanistic understanding of how this cross talk is accomplished. These proteins, termed plus-end tracking proteins because they appear to associate with the plus-end as it grows, have already been shown to regulate microtubule dynamics and to facilitate the formation of connections between the plus-end and the actin-rich cortex. Several motor proteins, including an actin-based motor, microtubule-based motors that move towards either end of the microtubule and microtubule motors that depolymerize microtubule ends, can now be added to the list of plus-end tracking proteins. Here, we discuss how the presence of these motors at the plus-end seems to drive several fundamental cellular processes involving force generation at the interface between microtubule ends and the cortex, vesicle translocation following search and capture, microtubule disassembly and the delivery of signals to the cortex that govern actin assembly and cell polarity.

Asymmetric spindle positioning

When a spindle is positioned asymmetrically in a dividing cell, the resulting daughter cells are unequal in size. Asymmetric spindle positioning is driven by regulated forces that can pull or push a spindle. The physical and molecular mechanisms that can position spindles asymmetrically have been studied in several systems, and some themes have begun to emerge from recent research. Recent work in budding yeast has presented a model for how cytoskeletal motors and cortical capture molecules can function in orienting and positioning a spindle. The temporal regulation of microtubule-based pulling forces that move a spindle has been examined in one animal system. Although the spindle positioning force generators have not been identified in most animal systems, the forces have been found to be regulated by both PAR polarity proteins and G-protein signaling pathways in more than one animal system.

Microtubule regulation in mitosis: tubulin phosphorylation by the cyclin-dependent kinase Cdk1

The activation of the cyclin-dependent kinase Cdk1 at the transition from interphase to mitosis induces important changes in microtubule dynamics. Cdk1 phosphorylates a number of microtubule- or tubulin-binding proteins but, hitherto, tubulin itself has not been detected as a Cdk1 substrate. Here we show that Cdk1 phosphorylates beta-tubulin both in vitro and in vivo. Phosphorylation occurs on Ser172 of beta-tubulin, a site that is well conserved in evolution. Using a phosphopeptide antibody, we find that a fraction of the cell tubulin is phosphorylated during mitosis, and this tubulin phosphorylation is inhibited by the Cdk1 inhibitor roscovitine. In mitotic cells, phosphorylated tubulin is excluded from microtubules, being present in the soluble tubulin fraction. Consistent with this distribution in cells, the incorporation of Cdk1-phosphorylated tubulin into growing microtubules is impaired in vitro. Additionally, EGFP-beta3-tubulin(S172D/E) mutants that mimic phosphorylated tubulin are unable to incorporate into microtubules when expressed in cells. Modeling shows that the presence of a phosphoserine at position 172 may impair both GTP binding to beta-tubulin and interactions between tubulin dimers. These data indicate that phosphorylation of tubulin by Cdk1 could be involved in the regulation of microtubule dynamics during mitosis.