Ciliopathy patient variants reveal organelle-specific functions for TUBB4B in axonemal microtubules

Tubulin, one of the most abundant cytoskeletal building blocks, has numerous isotypes in metazoans encoded by different conserved genes. Whether these distinct isotypes form cell type- and context-specific microtubule structures is poorly understood. Based on a cohort of 12 patients with primary ciliary dyskinesia as well as mouse mutants, we identified and characterized variants in the TUBB4B isotype that specifically perturbed centriole and cilium biogenesis. Distinct TUBB4B variants differentially affected microtubule dynamics and cilia formation in a dominant-negative manner. Structure-function studies revealed that different TUBB4B variants disrupted distinct tubulin interfaces, thereby enabling stratification of patients into three classes of ciliopathic diseases. These findings show that specific tubulin isotypes have distinct and nonredundant subcellular functions and establish a link between tubulinopathies and ciliopathies.

A conserved CENP-E region mediates BubR1-independent recruitment to the outer corona at mitotic onset

The outer corona plays an essential role at the onset of mitosis by expanding to maximize microtubule attachment to kinetochores.1,2 The low-density structure of the corona forms through the expansion of unattached kinetochores. It comprises the RZZ complex, the dynein adaptor Spindly, the plus-end directed microtubule motor centromere protein E (CENP-E), and the Mad1/Mad2 spindle-assembly checkpoint proteins.3,4,5,6,7,8,9,10 CENP-E specifically associates with unattached kinetochores to facilitate chromosome congression,11,12,13,14,15,16 interacting with BubR1 at the kinetochore through its C-terminal region (2091-2358).17,18,19,20,21 We recently showed that CENP-E recruitment to BubR1 at the kinetochores is both rapid and essential for correct chromosome alignment. However, CENP-E is also recruited to the outer corona by a second, slower pathway that is currently undefined.19 Here, we show that BubR1-independent localization of CENP-E is mediated by a conserved loop that is essential for outer-corona targeting. We provide a structural model of the entire CENP-E kinetochore-targeting domain combining X-ray crystallography and Alphafold2. We reveal that maximal recruitment of CENP-E to unattached kinetochores critically depends on BubR1 and the outer corona, including dynein. Ectopic expression of the CENP-E C-terminal domain recruits the RZZ complex, Mad1, and Spindly, and prevents kinetochore biorientation in cells. We propose that BubR1-recruited CENP-E, in addition to its essential role in chromosome alignment to the metaphase plate, contributes to the recruitment of outer corona proteins through interactions with the CENP-E corona-targeting domain to facilitate the rapid capture of microtubules for efficient chromosome alignment and mitotic progression.

Potent microtubule-depolymerizing activity of a mitotic Kif18b-MCAK-EB network

The precise regulation of microtubule length during mitosis is essential to assemble and position the mitotic spindle and segregate chromosomes. The kinesin-13 Kif2C or MCAK acts as a potent microtubule depolymerase that diffuses short distances on microtubules, whereas the kinesin-8 Kif18b is a processive motor with weak depolymerase activity. However, the individual activities of these factors cannot explain the dramatic increase in microtubule dynamics in mitosis. Using in vitro reconstitution and single-molecule imaging, we demonstrate that Kif18b, MCAK and the plus-end tracking protein EB3 (also known as MAPRE3) act in an integrated manner to potently promote microtubule depolymerization at very low concentrations. We find that Kif18b can transport EB3 and MCAK and promotes their accumulation to microtubule plus ends through multivalent weak interactions. Together, our work defines the mechanistic basis for a cooperative Kif18b-MCAK-EB network at microtubule plus ends, that acts to efficiently shorten and regulate microtubules in mitosis, essential for correct chromosome segregation.

Understanding molecular mechanisms and predicting phenotypic effects of pathogenic tubulin mutations

Cells rely heavily on microtubules for several processes, including cell division and molecular trafficking. Mutations in the different tubulin-α and -β proteins that comprise microtubules have been associated with various diseases and are often dominant, sporadic and congenital. While the earliest reported tubulin mutations affect neurodevelopment, mutations are also associated with other disorders such as bleeding disorders and infertility. We performed a systematic survey of tubulin mutations across all isotypes in order to improve our understanding of how they cause disease, and increase our ability to predict their phenotypic effects. Both protein structural analyses and computational variant effect predictors were very limited in their utility for differentiating between pathogenic and benign mutations. This was even worse for those genes associated with non-neurodevelopmental disorders. We selected tubulin-α and -β disease mutations that were most poorly predicted for experimental characterisation. These mutants co-localise to the mitotic spindle in HeLa cells, suggesting they may exert dominant-negative effects by altering microtubule properties. Our results show that tubulin mutations represent a blind spot for current computational approaches, being much more poorly predicted than mutations in most human disease genes. We suggest that this is likely due to their strong association with dominant-negative and gain-of-function mechanisms.

Reconstitution of an active human CENP-E motor

CENP-E is a large kinesin motor protein which plays pivotal roles in mitosis by facilitating chromosome capture and alignment, and promoting microtubule flux in the spindle. So far, it has not been possible to obtain active human CENP-E to study its molecular properties. Xenopus CENP-E motor has been characterized in vitro and is used as a model motor; however, its protein sequence differs significantly from human CENP-E. Here, we characterize human CENP-E motility in vitro. Full-length CENP-E exhibits an increase in run length and longer residency times on microtubules when compared to CENP-E motor truncations, indicating that the C-terminal microtubule-binding site enhances the processivity when the full-length motor is active. In contrast with constitutively active human CENP-E truncations, full-length human CENP-E has a reduced microtubule landing rate in vitro, suggesting that the non-motor coiled-coil regions self-regulate motor activity. Together, we demonstrate that human CENP-E is a processive motor, providing a useful tool to study the mechanistic basis for how human CENP-E drives chromosome congression and spindle organization during human cell division.

The C-terminal helix of BubR1 is essential for CENP-E-dependent chromosome alignment

During cell division, misaligned chromosomes are captured and aligned by motors before their segregation. The CENP-E motor is recruited to polar unattached kinetochores to facilitate chromosome alignment. The spindle checkpoint protein BubR1 (also known as BUB1B) has been reported as a CENP-E interacting partner, but the extent to which BubR1 contributes to CENP-E localization at kinetochores has remained controversial. Here we define the molecular determinants that specify the interaction between BubR1 and CENP-E. The basic C-terminal helix of BubR1 is necessary but not sufficient for CENP-E interaction, and a minimal key acidic patch on the kinetochore-targeting domain of CENP-E is also essential. We then demonstrate that BubR1 is required for the recruitment of CENP-E to kinetochores to facilitate chromosome alignment. This BubR1-CENP-E axis is critical for alignment of chromosomes that have failed to congress through other pathways and recapitulates the major known function of CENP-E. Overall, our studies define the molecular basis and the function for CENP-E recruitment to BubR1 at kinetochores during mammalian mitosis.This article has an associated First Person interview with the first author of the paper.

The role of protein complexes in human genetic disease

Many human genetic disorders are caused by mutations in protein-coding regions of DNA. Taking protein structure into account has therefore provided key insight into the molecular mechanisms underlying human genetic disease. Although most studies have focused on the intramolecular effects of mutations, the critical role of the assembly of proteins into complexes is being increasingly recognized. Here, we review multiple ways in which consideration of protein complexes can help us to understand and explain the effects of pathogenic mutations. First, we discuss disorders caused by mutations that perturb intersubunit interactions in homomeric and heteromeric complexes. Second, we address how protein complex assembly can facilitate a dominant-negative mechanism, whereby mutated subunits can disrupt the activity of wild-type protein. Third, we show how mutations that change protein expression levels can lead to damaging stoichiometric imbalances. Finally, we review how mutations affecting different subunits of the same heteromeric complex often cause similar diseases, whereas mutations in different interfaces of the same subunit can cause distinct phenotypes.

A rapid computational approach identifies SPICE1 as an Aurora kinase substrate

Aurora kinases play a major role in mitosis by regulating diverse substrates. Defining their critical downstream targets is important in understanding Aurora kinase function. Here we have developed an unbiased computational approach to identify new Aurora kinase substrates based on phosphorylation site clustering, protein localization, protein structure, and species conservation. We validate the microtubule-associated proteins Clasp2, Elys, tubulin tyrosine ligase-like polyglutamylase residues 330-624 and spindle and centriole associated protein 1, residues 549-855 (SPICE1), as Aurora A and B kinases substrates in vitro. We also demonstrate that SPICE1 localization is regulated by Aurora kinases during mitosis. In the absence of Aurora kinase activity, SPICE1 remains at centrioles but does not target to the spindle. Similarly, a nonphosphorylatable SPICE1 mutant no longer localizes to the spindle. Finally, we show that misregulating SPICE1 phosphorylation results in abnormal centriole number, spindle multipolarity, and chromosome alignment defects. Overall, our work indicates that temporal and spatial Aurora kinase-mediated regulation of SPICE1 is important for correct chromosome segregation. In addition, our work provides a database-search tool that enables rapid identification of Aurora kinase substrates.

The depolymerase activity of MCAK shows a graded response to Aurora B kinase phosphorylation through allosteric regulation

Kinesin-13 motors regulate precise microtubule dynamics and limit microtubule length throughout metazoans by depolymerizing microtubule ends. Recently, the kinesin-13 motor family member MCAK (also known Kif2C) has been proposed to undergo large conformational changes during its catalytic cycle, as it switches from being in solution to being bound to microtubules. Here, we reveal that MCAK has a compact conformation in solution through crosslinking and electron microscopy experiments. When MCAK is bound to the microtubule ends, it adopts an extended conformation with the N-terminus and neck region of MCAK interacting with the microtubule. Interestingly, the region of MCAK that interacts with the microtubule is the region phosphorylated by Aurora B and contains an end binding (EB) protein-binding motif. The level of phosphorylation of the N-terminus results in a graded microtubule depolymerase activity. Here, we show that the N-terminus of MCAK forms a platform to integrate Aurora B kinase downstream signals and in response fine-tunes its depolymerase activity during mitosis. We propose that this allosteric control mechanism allows decoupling of the N-terminus from the motor domain of MCAK to allow MCAK depolymerase activity at kinetochores.

Summary: The kinesin-13 MCAK has a compact conformation in solution but is extended when bound to microtubules. Aurora B phosphorylation of MCAK inhibits depolymerase activity by disrupting its extended conformation.

Microtubule end tethering of a processive kinesin-8 motor Kif18b is required for spindle positioning

Mitotic spindle positioning specifies the plane of cell division during anaphase. Spindle orientation and positioning are therefore critical to ensure symmetric division in mitosis and asymmetric division during development. The control of astral microtubule length plays an essential role in positioning the spindle. In this study, using gene knockout, we show that the kinesin-8 Kif18b controls microtubule length to center the mitotic spindle at metaphase. Using in vitro reconstitution, we reveal that Kif18b is a highly processive plus end-directed motor that uses a C-terminal nonmotor microtubule-binding region to accumulate at growing microtubule plus ends. This region is regulated by phosphorylation to spatially control Kif18b accumulation at plus ends and is essential for Kif18b-dependent spindle positioning and regulation of microtubule length. Finally, we demonstrate that Kif18b shortens microtubules by increasing the catastrophe rate of dynamic microtubules. Overall, our work reveals that Kif18b uses its motile properties to reach microtubule ends, where it regulates astral microtubule length to ensure spindle centering.

Mechanisms of Mitotic Kinase Regulation: A Structural Perspective

Protein kinases are major regulators of mitosis, with over 30% of the mitotic proteome phosphorylated on serines, threonines and tyrosines. The human genome encodes for 518 kinases that have a structurally conserved catalytic domain and includes about a dozen of cell division specific ones. Yet each kinase has unique structural features that allow their distinct substrate recognition and modes of regulation. These unique regulatory features determine their accurate spatio-temporal activation critical for correct progression through mitosis and are exploited for therapeutic purposes. In this review, we will discuss the principles of mitotic kinase activation and the structural determinants that underlie functional specificity.

Dynein at kinetochores: Making the connection

McHugh and Welburn highlight Mosalaganti et al.’s and Gama et al.’s work revealing the molecular mechanism of dynein recruitment to kinetochores.​

Dynein removes the checkpoint proteins from kinetochores once chromosomes are bioriented. In this issue, Gama et al. (2017. J. Cell Biol.https://doi.org/10.1083/jcb.201610108) and Mosalaganti et al. (2017. J. Cell Biol.https://doi.org/10.1083/jcb.201611060) reveal the molecular basis for how dynein and its adaptor protein Spindly are recruited to the ROD–Zw10–Zwilch complex in the fibrous corona of unattached kinetochores.

Inhibition of ectopic microtubule assembly by the kinesin-13 KLP-7 prevents chromosome segregation and cytokinesis defects in oocytes

In most species, oocytes lack centrosomes. Accurate meiotic spindle assembly and chromosome segregation - essential to prevent miscarriage or developmental defects - thus occur through atypical mechanisms that are not well characterized. Using quantitative in vitro and in vivo functional assays in the C. elegans oocyte, we provide novel evidence that the kinesin-13 KLP-7 promotes destabilization of the whole cellular microtubule network. By counteracting ectopic microtubule assembly and disorganization of the microtubule network, this function is strictly required for spindle organization, chromosome segregation and cytokinesis in meiotic cells. Strikingly, when centrosome activity was experimentally reduced, the absence of KLP-7 or the mammalian kinesin-13 protein MCAK (KIF2C) also resulted in ectopic microtubule asters during mitosis in C. elegans zygotes or HeLa cells, respectively. Our results highlight the general function of kinesin-13 microtubule depolymerases in preventing ectopic, spontaneous microtubule assembly when centrosome activity is defective or absent, which would otherwise lead to spindle microtubule disorganization and aneuploidy.

Molecular architecture of the Dam1 complex-microtubule interaction

Mitosis is a highly regulated process that allows the equal distribution of the genetic material to the daughter cells. Chromosome segregation requires the formation of a bipolar mitotic spindle and assembly of a multi-protein structure termed the kinetochore to mediate attachments between condensed chromosomes and spindle microtubules. In budding yeast, a single microtubule attaches to each kinetochore, necessitating robustness and processivity of this kinetochore-microtubule attachment. The yeast kinetochore-localized Dam1 complex forms a direct interaction with the spindle microtubule. In vitro, the Dam1 complex assembles as a ring around microtubules and couples microtubule depolymerization with cargo movement. However, the subunit organization within the Dam1 complex, its higher-order oligomerization and how it interacts with microtubules remain under debate. Here, we used chemical cross-linking and mass spectrometry to define the architecture and subunit organization of the Dam1 complex. This work reveals that both the C termini of Duo1 and Dam1 subunits interact with the microtubule and are critical for microtubule binding of the Dam1 complex, placing Duo1 and Dam1 on the inside of the ring structure. Integrating this information with available structural data, we provide a coherent model for how the Dam1 complex self-assembles around microtubules.

The C-terminal region of the motor protein MCAK controls its structure and activity through a conformational switch

The precise regulation of microtubule dynamics is essential during cell division. The kinesin-13 motor protein MCAK is a potent microtubule depolymerase. The divergent non-motor regions flanking the ATPase domain are critical in regulating its targeting and activity. However, the molecular basis for the function of the non-motor regions within the context of full-length MCAK is unknown. Here, we determine the structure of MCAK motor domain bound to its regulatory C-terminus. Our analysis reveals that the MCAK C-terminus binds to two motor domains in solution and is displaced allosterically upon microtubule binding, which allows its robust accumulation at microtubule ends. These results demonstrate that MCAK undergoes long-range conformational changes involving its C-terminus during the soluble to microtubule-bound transition and that the C-terminus-motor interaction represents a structural intermediate in the MCAK catalytic cycle. Together, our work reveals intrinsic molecular mechanisms underlying the regulation of kinesin-13 activity.

DOI:http://dx.doi.org/10.7554/eLife.06421.001

Within a cell, there is a scaffold-like structure called the cytoskeleton that provides shape and structural support, and acts as a transport network for the movement of molecules around the cell. This scaffold contains highly dynamic polymers called microtubules that are made from a protein called tubulin. The constant growth and shrinking of the ends of the microtubules is essential to rebuild and adapt the cytoskeleton according to the needs of the cell.

A protein called MCAK belongs to a family of motor proteins that can move along microtubules. It generally binds to the ends of the microtubules to shorten them. Previous studies have found that a single MCAK protein binds to another MCAK protein to form a larger molecule known as a dimer. Part of the MCAK protein forms a so-called motor domain, which enables this protein to bind to the microtubules. One end of the protein, known as the C-terminus, controls the activity of this motor domain. However, it is not clear how this works.

Talapatra et al. have now revealed the three-dimensional structure of MCAK's motor domain with the C-terminus using a technique called X-ray crystallography. The experiments show that the C-terminus binds to the motor domain, which promotes the formation of the dimers. A short stretch of amino acids—the building blocks of proteins—in the C-terminus interacts with two motor molecules. This ‘motif’ is also found in other similar proteins from a variety of animals. However, once MCAK binds to a microtubule, the microtubule triggers the release of the C-terminus from the motor domain. This allows MCAK to bind more strongly to the microtubule.

The experiments also show that the binding of the C-terminus to the motor domain alters the ability of MCAK to associate with microtubules, which encourages the protein to reach the ends of the polymers. Future work is required to see whether other motor proteins work in a similar way.

DOI:http://dx.doi.org/10.7554/eLife.06421.002

Length-dependent anisotropic scaling of spindle shape

Spindle length varies dramatically across species and during early development to segregate chromosomes optimally. Both intrinsic factors, such as regulatory molecules, and extrinsic factors, such as cytoplasmic volume, determine spindle length scaling. However, the properties that govern spindle shape and whether these features can be modulated remain unknown. Here, we analyzed quantitatively how the molecular players which regulate microtubule dynamics control the kinetics of spindle formation and shape. We find that, in absence of Clasp1 and Clasp2, spindle assembly is biphasic due to unopposed inward pulling forces from the kinetochore-fibers and that kinetochore-fibers also alter spindle geometry. We demonstrate that spindle shape scaling is independent of the nature of the molecules that regulate dynamic microtubule properties, but is dependent on the steady-state metaphase spindle length. The shape of the spindle scales anisotropically with increasing length. Our results suggest that intrinsic mechanisms control the shape of the spindle to ensure the efficient capture and alignment of chromosomes independently of spindle length.

The molecular basis for kinesin functional specificity during mitosis

Microtubule-based motor proteins play key roles during mitosis to assemble the bipolar spindle, define the cell division axis, and align and segregate the chromosomes. The majority of mitotic motors are members of the kinesin superfamily. Despite sharing a conserved catalytic core, each kinesin has distinct functions and localization, and is uniquely regulated in time and space. These distinct behaviors and functional specificity are generated by variations in the enzymatic domain as well as the non-conserved regions outside of the kinesin motor domain and the stalk. These flanking regions can directly modulate the properties of the kinesin motor through dimerization or self-interactions, and can associate with extrinsic factors, such as microtubule or DNA binding proteins, to provide additional functional properties. This review discusses the recently identified molecular mechanisms that explain how the control and functional specification of mitotic kinesins is achieved. © 2013 Wiley Periodicals, Inc.

The microtubule-binding protein Cep170 promotes the targeting of the kinesin-13 depolymerase Kif2b to the mitotic spindle

The N-terminus of the kinesin-13 family (Kif2a, Kif2b, Kif2c) is the primary localization determinant. However, the C-terminus of Kif2b associates with Cep170 and Cep170R to create targeting specificity. Cep170 has microtubule-binding properties in vitro and provides a second microtubule-binding site to Kif2b to target it to the spindle.

Microtubule dynamics are essential throughout mitosis to ensure correct chromosome segregation. Microtubule depolymerization is controlled in part by microtubule depolymerases, including the kinesin-13 family of proteins. In humans, there are three closely related kinesin-13 isoforms (Kif2a, Kif2b, and Kif2c/MCAK), which are highly conserved in their primary sequences but display distinct localization and nonoverlapping functions. Here we demonstrate that the N-terminus is a primary determinant of kinesin-13 localization. However, we also find that differences in the C-terminus alter the properties of kinesin-13, in part by facilitating unique protein–protein interactions. We identify the spindle-localized proteins Cep170 and Cep170R (KIAA0284) as specifically associating with Kif2b. Cep170 binds to microtubules in vitro and provides Kif2b with a second microtubule-binding site to target it to the spindle. Thus the intrinsic properties of kinesin-13s and extrinsic factors such as their associated proteins result in the diversity and specificity within the kinesin-13 depolymerase family.

Aurora B phosphorylates spatially distinct targets to differentially regulate the kinetochore-microtubule interface

Accurate chromosome segregation requires carefully regulated interactions between kinetochores and microtubules, but how plasticity is achieved to correct diverse attachment defects remains unclear. Here we demonstrate that Aurora B kinase phosphorylates three spatially distinct targets within the conserved outer kinetochore KNL1/Mis12 complex/Ndc80 complex (KMN) network, the key player in kinetochore-microtubule attachments. The combinatorial phosphorylation of the KMN network generates graded levels of microtubule-binding activity, with full phosphorylation severely compromising microtubule binding. Altering the phosphorylation state of each protein causes corresponding chromosome segregation defects. Importantly, the spatial distribution of these targets along the kinetochore axis leads to their differential phosphorylation in response to changes in tension and attachment state. In total, rather than generating exclusively binary changes in microtubule binding, our results suggest a mechanism for the tension-dependent fine-tuning of kinetochore-microtubule interactions.

The human kinetochore Ska1 complex facilitates microtubule depolymerization-coupled motility

Mitotic chromosome segregation requires that kinetochores attach to microtubule polymers and harness microtubule dynamics to drive chromosome movement. In budding yeast, the Dam1 complex couples kinetochores with microtubule depolymerization. However, a metazoan homolog of the Dam1 complex has not been identified. To identify proteins that play a corresponding role at the vertebrate kinetochore-microtubule interface, we isolated a three subunit human Ska1 complex, including the previously uncharacterized protein Rama1 that localizes to the outer kinetochore and spindle microtubules. Depletion of Ska1 complex subunits severely compromises proper chromosome segregation. Reconstituted Ska1 complex possesses two separable biochemical activities: direct microtubule binding through the Ska1 subunit, and microtubule-stimulated oligomerization imparted by the Rama1 subunit. The full Ska1 complex forms assemblies on microtubules that can facilitate the processive movement of microspheres along depolymerizing microtubules. In total, these results demonstrate a critical role for the Ska1 complex in interacting with dynamic microtubules at the outer kinetochore.

Toward a molecular structure of the eukaryotic kinetochore

Chromosome segregation in eukaryotes requires a large molecular assembly termed the kinetochore to attach chromosomes to spindle microtubules. Recent work has made substantial progress in defining the composition and activities of the kinetochore, but much remains to be learned about its macromolecular structure. This commentary discusses recent insights into structural features of the kinetochore, how these inform our understanding of its biological function, and the key challenges for the future.

Architecture of the Dam1 kinetochore ring complex and implications for microtubule-driven assembly and force-coupling mechanisms

The Dam1 kinetochore complex is essential for chromosome segregation in budding yeast. This ten-protein complex self-assembles around microtubules, forming ring-like structures that move with depolymerizing microtubule ends, a mechanism with implications for cellular function. Here we used EM-based single-particle and helical analyses to define the architecture of the Dam1 complex at 30-A resolution and the self-assembly mechanism. Ring oligomerization seems to be facilitated by a conformational change upon binding to microtubules, suggesting that the Dam1 ring is not preformed, but self-assembles around kinetochore microtubules. The C terminus of the Dam1p protein, where most of the Aurora kinase Ipl1 phosphorylation sites reside, is in a strategic location to affect oligomerization and interactions with the microtubule. One of Ipl1's roles might be to fine-tune the coupling of the microtubule interaction with the conformational change required for oligomerization, with phosphorylation resulting in ring breakdown.

How tyrosine 15 phosphorylation inhibits the activity of cyclin-dependent kinase 2-cyclin A

Inhibition of cyclin-dependent kinase 1 (CDK1) activity by Tyr-15 phosphorylation directly regulates entry into mitosis and is an important element in the control of the unperturbed cell cycle. Active site phosphorylation of other members of the CDK family that regulate cell cycle progression instates checkpoints that are fundamental to eukaryotic cell cycle regulation. Kinetic and crystallographic analyses of CDK2-cyclin A complexes reveal that this inhibitory mechanism operates through steric blockade of peptide substrate binding and through the creation of an environment that favors a non-productive conformation of the terminal group of ATP. By contrast, tyrosine phosphorylation of CDK2 alters neither its Km for ATP nor its significant intrinsic ATPase activity. Tyr-15-phosphorylated CDK2 retains trace protein phosphorylation activity that should be considered in quantitative and qualitative cell cycle models.

Molecular basis for the recognition of phosphorylated and phosphoacetylated histone h3 by 14-3-3

Phosphorylation of histone H3 is implicated in transcriptional activation and chromosome condensation, but its immediate molecular function has remained obscure. By affinity chromatography of nuclear extracts against modified H3 tail peptides, we identified 14-3-3 isoforms as proteins that bind these tails in a strictly phosphorylation-dependent manner. Acetylation of lysines 9 and 14 does not impede 14-3-3 binding to serine 10-phosphorylated H3 tails. In vivo, 14-3-3 is inducibly recruited to c-fos and c-jun nucleosomes upon gene activation, concomitant with H3 phosphoacetylation. We have determined the structures of 14-3-3zeta complexed with serine 10-phosphorylated or phosphoacetylated H3 peptides. These reveal a distinct mode of 14-3-3/phosphopeptide binding and provide a structural understanding for the lack of effect of acetylation at lysines 9 and 14 on this interaction. 14-3-3 isoforms thus represent a class of proteins that mediate the effect of histone phosphorylation at inducible genes.

Inhibition of the cell cycle with chemical inhibitors: a targeted approach

Genetic links between deregulation of the cell cycle and cancer are well established. There have been significant recent developments both in our understanding of the molecular mechanisms that control cell cycle progression and in methods for protein structure determination at atomic resolution. These advances have allowed the rational design of small molecules that modulate the cell cycle by competing for sites of protein-protein or protein-ATP interactions. There is considerable optimism that these compounds, a selection of which are here reviewed, will become clinically significant drugs.