The organization of chromosomes into territories plays an important role in a wide range of cellular processes including gene expression, transcription, and DNA repair. Current understanding has largely excluded the spatio-temporal dynamic fluctuations of the chromatin polymer. We combine in vivo chromatin motion analysis with mathematical modeling to elucidate the physical properties that underlie the formation and fluctuations of territories. Chromosome motion varies in predicted ways along the length of the chromosome, dependent on tethering at the centromere. Detachment of a tether upon inactivation of the centromere results in increased spatial mobility. A confined bead-spring chain tethered at both ends provides a mechanism to generate observed variations in local mobility as a function of distance from the tether. These predictions are realized in experimentally determined higher effective spring constants closer to the centromere. The dynamic fluctuations and territorial organization of chromosomes are, in part, dictated by tethering at the centromere.
Biological questions are increasingly being addressed using a wide range of quantitative analytical tools to examine protein complex composition. Knowledge of the absolute number of proteins present provides insights into organization, function, and maintenance and is used in mathematical modeling of complex cellular dynamics. In this chapter, we outline and describe three microscopy-based methods for determining absolute protein numbers—fluorescence correlation spectroscopy, stepwise photobleaching, and ratiometric comparison of fluorescence intensity to known standards. In addition, we discuss the various fluorescently labeled proteins that have been used as standards for both stepwise photobleaching and ratiometric comparison analysis. A detailed procedure for determining absolute protein number by ratiometric comparison is outlined in the second half of this chapter. Counting proteins by quantitative microscopy is a relatively simple yet very powerful analytical tool that will increase our understanding of protein complex composition.
The budding yeast kinetochore is ~68nm in length with a diameter slightly larger than a 25nm microtubule . The kinetochores from the 16 chromosomes are organized in a stereotypic cluster encircling central spindle microtubules. Quantitative analysis of the inner kinetochore cluster (Cse4, COMA) reveals structural features not apparent in singly attached kinetochores. The cluster of Cse4 containing kinetochores is physically larger perpendicular to the spindle axis relative to the cluster of Ndc80 molecules . If there were a single Cse4 (molecule or nucleosome) at the kinetochore attached to each microtubule plus-end, the cluster of Cse4 would appear geometrically identical to Ndc80. Thus, the structure of the inner kinetochore at the surface of the chromosomes remains unsolved.
We have used point fluorescence microscopy and statistical probability maps  to deduce the two dimensional mean position of representative components of the yeast kinetochore relative to the mitotic spindle in metaphase. Comparison of the experimental images to three-dimensional architectures from convolution of mathematical models reveals a pool of Cse4 radially displaced from Cse4 at the kinetochore and kinetochore microtubule plus-ends. The pool of displaced Cse4 can be experimentally depleted in mRNA processing pat1Δ or xrn1Δ mutants. The peripheral Cse4 molecules do not template outer kinetochore components. This study suggests an inner kinetochore plate at the centromere-microtubule interface in budding yeast and yields information on the number of Ndc80 molecules at the microtubule attachment site.
One of the most fundamental concepts of microscopy is that of resolution–the ability to clearly distinguish two objects as separate. Recent advances such as structured illumination microscopy (SIM) and point localization techniques including photoactivated localization microscopy (PALM), and stochastic optical reconstruction microscopy (STORM) strive to overcome the inherent limits of resolution of the modern light microscope. These techniques, however, are not always feasible or optimal for live cell imaging. Thus, in this review, we explore three techniques for extracting high resolution data from images acquired on a widefield microscope–deconvolution, model convolution, and Gaussian fitting. Deconvolution is a powerful tool for restoring a blurred image using knowledge of the point spread function (PSF) describing the blurring of light by the microscope, although care must be taken to ensure accuracy of subsequent quantitative analysis. The process of model convolution also requires knowledge of the PSF to blur a simulated image which can then be compared to the experimentally acquired data to reach conclusions regarding its geometry and fluorophore distribution. Gaussian fitting is the basis for point localization microscopy, and can also be applied to tracking spot motion over time or measuring spot shape and size. All together, these three methods serve as powerful tools for high-resolution imaging using widefield microscopy.
Recently in Cell, Fisher et al (2013) use high-resolution time-lapse imaging to peer into bacterial genome (nucleoid) structure. An elastic filament confined via an internal network, the nucleoid undergoes periodic fluctuations critical in relieving tension. Programmed tethers and their release highlight a primordial mechanical cycle for chromosome segregation.
The Sec14-like phosphatidylinositol transfer protein Sfh3 associates with bulk LDs in vegetative cells but targets to a neutral lipid hydrolase-rich LD pool during sporulation. Sfh3 inhibits LD utilization by a PtdIns-4-phosphate–dependent mechanism, and this inhibition prevents prospore membrane biogenesis in sporulating cells.
Lipid droplet (LD) utilization is an important cellular activity that regulates energy balance and release of lipid second messengers. Because fatty acids exhibit both beneficial and toxic properties, their release from LDs must be controlled. Here we demonstrate that yeast Sfh3, an unusual Sec14-like phosphatidylinositol transfer protein, is an LD-associated protein that inhibits lipid mobilization from these particles. We further document a complex biochemical diversification of LDs during sporulation in which Sfh3 and select other LD proteins redistribute into discrete LD subpopulations. The data show that Sfh3 modulates the efficiency with which a neutral lipid hydrolase-rich LD subclass is consumed during biogenesis of specialized membrane envelopes that package replicated haploid meiotic genomes. These results present novel insights into the interface between phosphoinositide signaling and developmental regulation of LD metabolism and unveil meiosis-specific aspects of Sfh3 (and phosphoinositide) biology that are invisible to contemporary haploid-centric cell biological, proteomic, and functional genomics approaches.
During mitosis, cohesin and condensin cross-link pericentromeres of different chromosomes to coordinate centromere attachment sites.
The mitotic segregation apparatus composed of microtubules and chromatin functions to faithfully partition a duplicated genome into two daughter cells. Microtubules exert extensional pulling force on sister chromatids toward opposite poles, whereas pericentric chromatin resists with contractile springlike properties. Tension generated from these opposing forces silences the spindle checkpoint to ensure accurate chromosome segregation. It is unknown how the cell senses tension across multiple microtubule attachment sites, considering the stochastic dynamics of microtubule growth and shortening. In budding yeast, there is one microtubule attachment site per chromosome. By labeling several chromosomes, we find that pericentromeres display coordinated motion and stretching in metaphase. The pericentromeres of different chromosomes exhibit physical linkage dependent on centromere function and structural maintenance of chromosomes complexes. Coordinated motion is dependent on condensin and the kinesin motor Cin8, whereas coordinated stretching is dependent on pericentric cohesin and Cin8. Linking of pericentric chromatin through cohesin, condensin, and kinetochore microtubules functions to coordinate dynamics across multiple attachment sites.
The mitotic chromatin spring is organized into a rosette of intramolecular loops of pericentric chromatin by condensin and cohesin. Model convolution reveals that condensin clusters along the spindle axis, while cohesin is dispersed radially along pericentromere loops.
In mitosis, the pericentromere is organized into a spring composed of cohesin, condensin, and a rosette of intramolecular chromatin loops. Cohesin and condensin are enriched in the pericentromere, with spatially distinct patterns of localization. Using model convolution of computer simulations, we deduce the mechanistic consequences of their spatial segregation. Condensin lies proximal to the spindle axis, whereas cohesin is radially displaced from condensin and the interpolar microtubules. The histone deacetylase Sir2 is responsible for the axial position of condensin, while the radial displacement of chromatin loops dictates the position of cohesin. The heterogeneity in distribution of condensin is most accurately modeled by clusters along the spindle axis. In contrast, cohesin is evenly distributed (barrel of 500-nm width × 550-nm length). Models of cohesin gradients that decay from the centromere or sister cohesin axis, as previously suggested, do not match experimental images. The fine structures of cohesin and condensin deduced with subpixel localization accuracy reveal critical features of how these complexes mold pericentric chromatin into a functional spring.
During mitosis, cohesin- and condensin-based pericentric chromatin loops function as a spring network to balance spindle microtubule force.
The mechanisms by which sister chromatids maintain biorientation on the metaphase spindle are critical to the fidelity of chromosome segregation. Active force interplay exists between predominantly extensional microtubule-based spindle forces and restoring forces from chromatin. These forces regulate tension at the kinetochore that silences the spindle assembly checkpoint to ensure faithful chromosome segregation. Depletion of pericentric cohesin or condensin has been shown to increase the mean and variance of spindle length, which have been attributed to a softening of the linear chromatin spring. Models of the spindle apparatus with linear chromatin springs that match spindle dynamics fail to predict the behavior of pericentromeric chromatin in wild-type and mutant spindles. We demonstrate that a nonlinear spring with a threshold extension to switch between spring states predicts asymmetric chromatin stretching observed in vivo. The addition of cross-links between adjacent springs recapitulates coordination between pericentromeres of neighboring chromosomes.
Genetic interactions reveal the functional relationships between pairs of genes. In this study, we describe a method for the systematic generation and quantitation of triple mutants, termed Triple Mutant Analysis (TMA). We have used this approach to interrogate partially redundant pairs of genes in S. cerevisiae, including ASF1 and CAC1, two histone chaperones. After subjecting asf1Δ cac1Δ to TMA, we found that the Swi/Snf Rdh54 protein, compensates for the absence of Asf1 and Cac1. Rdh54 more strongly associates with the chromatin apparatus and the pericentromeric region in the double mutant. Moreover, Asf1 is responsible for the synthetic lethality observed in cac1Δ strains lacking the HIRA-like proteins. A similar TMA was carried out after deleting both CLB5 and CLB6, cyclins that regulate DNA replication, revealing a strong functional connection to chromosome segregation. This approach can reveal functional redundancies that cannot be uncovered using traditional double mutant analyses.
The influx of physicists to the realm of biology around 1940 represented the birth of molecular biology. Now, with the sequencing of thousands of genomes and the promise of the $1,000 human genome, we find ourselves returning to physics. The cell is a foreign place, one that requires concepts from physics and statistical mechanics to gain a basic understanding.
Dicentric chromosomes undergo breakage in mitosis, resulting in chromosome deletions, duplications, and translocations. In this study, we map chromosome break sites of dicentrics in Saccharomyces cerevisiae by a mitotic recombination assay. The assay uses a diploid strain in which one homolog has a conditional centromere in addition to a wild-type centromere, and the other homolog has only the wild-type centromere; the conditional centromere is inactive when cells are grown in galactose and is activated when the cells are switched to glucose. In addition, the two homologs are distinguishable by multiple single-nucleotide polymorphisms (SNPs). Under conditions in which the conditional centromere is activated, the functionally dicentric chromosome undergoes double-stranded DNA breaks (DSBs) that can be repaired by mitotic recombination with the homolog. Such recombination events often lead to loss of heterozygosity (LOH) of SNPs that are centromere distal to the crossover. Using a PCR-based assay, we determined the position of LOH in multiple independent recombination events to a resolution of ∼4 kb. This analysis shows that dicentric chromosomes have recombination breakpoints that are broadly distributed between the two centromeres, although there is a clustering of breakpoints within 10 kb of the conditional centromere.
Saccharomyces cerevisiae; dicentric chromosomes; mitotic crossovers; loss of heterozygosity; break-induced replication
Tension sensing of bi-oriented chromosomes is essential for the fidelity of chromosome segregation. The spindle assembly checkpoint (SAC) conveys lack of tension or attachment to the anaphase promoting complex. Components of the SAC (Bub1) phosphorylate histone H2A (S121) and recruit the protector of cohesin, Shugoshin (Sgo1) to the inner centromere. How the chromatin structural modifications of the inner centromere are integrated into the tension sensing mechanisms and the checkpoint are not known.
We have identified a Bub1/Sgo1 dependent structural change in the geometry and dynamics of kinetochores and the pericentric chromatin upon reduction of microtubule dynamics. The cluster of inner kinetochores contract while the pericentric chromatin and cohesin that encircle spindle microtubules undergo a radial expansion. Despite its increased spatial distribution, the pericentric chromatin is less dynamic. The change in dynamics is due to histone H2A phosphorylation and Sgo1 recruitment to the pericentric chromatin, rather than microtubule dynamics.
Bub1 and Sgo1 act as a rheostat to regulate the chromatin spring and maintain force balance. Through Histone H2A S121 phosphorylation and recruitment of Sgo1, Bub1 kinase softens the chromatin spring in response to changes in microtubule dynamics. The geometric alteration of all 16 kinetochores and pericentric chromatin reflect global changes in the pericentromeric region and provide mechanisms for mechanically amplifying damage at a single kinetochore microtubule.
Dynamics of histones under tension in the pericentromere depends on RSC and ISW2 chromatin remodeling. The underlying pericentromeric chromatin forms a platform that is required to maintain kinetochore structure when under spindle-based tension.
Nucleosome positioning is important for the structural integrity of chromosomes. During metaphase the mitotic spindle exerts physical force on pericentromeric chromatin. The cell must adjust the pericentromeric chromatin to accommodate the changing tension resulting from microtubule dynamics to maintain a stable metaphase spindle. Here we examine the effects of spindle-based tension on nucleosome dynamics by measuring the histone turnover of the chromosome arm and the pericentromere during metaphase in the budding yeast Saccharomyces cerevisiae. We find that both histones H2B and H4 exhibit greater turnover in the pericentromere during metaphase. Loss of spindle-based tension by treatment with the microtubule-depolymerizing drug nocodazole or compromising kinetochore function results in reduced histone turnover in the pericentromere. Pericentromeric histone dynamics are influenced by the chromatin-remodeling activities of STH1/NPS1 and ISW2. Sth1p is the ATPase component of the Remodels the Structure of Chromatin (RSC) complex, and Isw2p is an ATP-dependent DNA translocase member of the Imitation Switch (ISWI) subfamily of chromatin-remodeling factors. The balance between displacement and insertion of pericentromeric histones provides a mechanism to accommodate spindle-based tension while maintaining proper chromatin packaging during mitosis.
Quantitative measurement of the number of Cse4, CBF3, and Ndc80 proteins at kinetochores reveals a 2.5–3-fold increased copy number relative to prior estimates.
Cse4 is the budding yeast homologue of CENP-A, a modified histone H3 that specifies the base of kinetochores in all eukaryotes. Budding yeast is unique in having only one kinetochore microtubule attachment site per centromere. The centromere is specified by CEN DNA, a sequence-specific binding complex (CBF3), and a Cse4-containing nucleosome. Here we compare the ratio of kinetochore proximal Cse4-GFP fluorescence at anaphase to several standards including purified EGFP molecules in vitro to generate a calibration curve for the copy number of GFP-fusion proteins. Our results yield a mean of ∼5 Cse4s, ∼3 inner kinetochore CBF3 complexes, and ∼20 outer kinetochore Ndc80 complexes. Our calibrated measurements increase 2.5–3-fold protein copy numbers at eukaryotic kinetochores based on previous ratio measurements assuming two Cse4s per budding yeast kinetochore. All approximately five Cse4s may be associated with the CEN nucleosome, but we show that a mean of three Cse4s could be located within flanking nucleosomes at random sites that differ between chromosomes.
Fidelity during chromosome segregation is essential to prevent aneuploidy. The proteins and chromatin at the centromere form a unique site for kinetochore attachment and allow the cell to sense and correct errors during chromosome segregation. Centromeric chromatin is characterized by distinct chromatin organization, epigenetics, centromere-associated proteins and histone variants. These include the histone H3 variant centromeric protein A (CENPA), the composition and deposition of which have been widely investigated. Studies have examined the structural and biophysical properties of the centromere and have suggested that the centromere is not simply a ‘landing pad’ for kinetochore formation, but has an essential role in mitosis by assembling and directing the organization of the kinetochore.
The kinetochore is the protein machine built at the centromere that integrates mechanical force and chemical energy from dynamic microtubules into directed chromosome motion. The kinetochore also provides a powerful signaling function that is able to alter the properties of the spindle checkpoint and initiate a signal transduction cascade that leads to inhibition of the anaphase promoting complex and cell cycle arrest. Together, the kinetochore accomplishes the feat of chromosome segregation with unparalleled accuracy. Errors in segregation lead to Down’s syndrome, the most frequent inherited birth defect, pregnancy loss, and cancer. Over a century after the discovery of the kinetochore, an architectural map comprising greater than 100 proteins is emerging. Understanding the architecture and physical biology of the key components provides new insights into how this fascinating machine moves genomes.
Conditional temperature-sensitive (ts) mutations are valuable reagents for studying essential genes in the yeast Saccharomyces cerevisiae. We constructed 787 ts strains, covering 497 (~45%) of the 1,101 essential yeast genes, with ~30% of the genes represented by multiple alleles. All of the alleles are integrated into their native genomic locus in the S288C common reference strain and are linked to a kanMX selectable marker, allowing further genetic manipulation by synthetic genetic array (SGA)–based, high-throughput methods. We show two such manipulations: barcoding of 440 strains, which enables chemical-genetic suppression analysis, and the construction of arrays of strains carrying different fluorescent markers of subcellular structure, which enables quantitative analysis of phenotypes using high-content screening. Quantitative analysis of a GFP-tubulin marker identified roles for cohesin and condensin genes in spindle disassembly. This mutant collection should facilitate a wide range of systematic studies aimed at understanding the functions of essential genes.
The continuity of duplex DNA is generally considered a prerequisite for chromosome continuity. However, as previously shown in yeast as well as human cells, the introduction of a double-strand break (DSB) does not generate a chromosome break (CRB) in yeast or human cells. The transition from DSB to CRB was found to be under limited control by the tethering function of the RAD50/MRE11/XRS2 (MRX) complex. Using a system for differential fluorescent marking of both sides of an endonuclease-induced DSB in single cells, we found that nearly all DSBs are converted to CRBs in cells lacking both exonuclease 1 (EXO1) activity and MRX complex. Thus, it appears that some feature of exonuclease processing or resection at a DSB is critical for maintaining broken chromosome ends in close proximity. In addition, we discovered a thermal sensitive (cold) component to CRB formation in an MRX mutant that has implications for chromosome end mobility and/or end-processing.
double-strand break repair; chromosome break; exonuclease 1; MRX; fluorescent imaging
During mitosis, spindle microtubule force is balanced by the combined activities of the cohesin and condensin SMC complexes and intramolecular pericentric chromatin loops.
Sister chromatid cohesion provides the mechanistic basis, together with spindle microtubules, for generating tension between bioriented chromosomes in metaphase. Pericentric chromatin forms an intramolecular loop that protrudes bidirectionally from the sister chromatid axis. The centromere lies on the surface of the chromosome at the apex of each loop. The cohesin and condensin structural maintenance of chromosomes (SMC) protein complexes are concentrated within the pericentric chromatin, but whether they contribute to tension-generating mechanisms is not known. To understand how pericentric chromatin is packaged and resists tension, we map the position of cohesin (SMC3), condensin (SMC4), and pericentric LacO arrays within the spindle. Condensin lies proximal to the spindle axis and is responsible for axial compaction of pericentric chromatin. Cohesin is radially displaced from the spindle axis and confines pericentric chromatin. Pericentric cohesin and condensin contribute to spindle length regulation and dynamics in metaphase. Together with the intramolecular centromere loop, these SMC complexes constitute a molecular spring that balances spindle microtubule force in metaphase.
The mitotic spindle is a structure that forms during mitosis to help ensure that each daughter cell receives a full complement of genetic material. In metaphase, the spindle contains microtubules that nucleate inward from two opposing poles. Chromosomes are attached to plus-ends of these microtubules via protein structures called kinetochores. The centromere is the specific region of kinetochore attachment on the chromosome. Chromatin surrounding the centromere (pericentric chromatin) is subject to microtubule-based forces and is commonly modeled as a linear spring, where the force that it exerts is proportional to the distance that it is stretched. We have incorporated physically based models of chromatin to create more accurate and predictive models of the spindle. In addition, using fluorescence microscopy and motion analysis of fluorescently labeled chromatin spots we discovered that pericentric chromatin is restrained relative to free diffusive motion. The characterization of chromatin is crucial to understand mitotic spindle stability and to understand the cell cycle checkpoint regulating anaphase onset.
pericentric chromatin; worm-like chain; chromatin dynamics; modeling of chromatin