Understanding force-generating microtubule systems through in vitro reconstitution

EB3-informed dynamics of the microtubule stabilizing cap during stalled growth

Microtubule stability is known to be governed by a stabilizing GTP/GDP-Pi cap, but the exact relation between growth velocity, GTP hydrolysis, and catastrophes remains unclear. We investigate the dynamics of the stabilizing cap through in vitro reconstitution of microtubule dynamics in contact with microfabricated barriers, using the plus-end binding protein GFP-EB3 as a marker for the nucleotide state of the tip. The interaction of growing microtubules with steric objects is known to slow down microtubule growth and accelerate catastrophes. We show that the lifetime distributions of stalled microtubules, as well as the corresponding lifetime distributions of freely growing microtubules, can be fully described with a simple phenomenological 1D model based on noisy microtubule growth and a single EB3-dependent hydrolysis rate. This same model is furthermore capable of explaining both the previously reported mild catastrophe dependence on microtubule growth rates and the catastrophe statistics during tubulin washout experiments.

Mechanical confinement prevents ectopic platelet release

Blood platelets are produced by megakaryocytes (MKs), their parent cells, which are in the bone marrow. Once mature, MK pierces through the sinusoid vessel, and the initial protrusion further elongates as proplatelet or buds to release platelets. The mechanisms controlling the decision to initiate proplatelet and platelet formation are unknown. Here, we show that the mechanical properties of the microenvironment prevent proplatelet and platelet release in the marrow stroma while allowing this process in the bloodstream. Loss of marrow confinement following myelosuppression led to inappropriate proplatelet and platelet release into the extravascular space. We further used an inert viscoelastic hydrogel to evaluate the impact of compressive stress. Transcriptional analysis showed that culture in three-dimensional gel induced upregulation of genes related to the Rho-GTPase pathway. We found higher Rho-GTPase activation, myosin light chain phosphorylation and F-actin under mechanical constraints while proplatelet formation was inhibited. The use of latrunculin-A to decrease F-actin promoted microtubule-dependent budding and proplatelet extension inside the gel. Additionally, ex vivo exposure of intact bone marrow to latrunculin-A triggered proplatelet extensions in the interstitial space. In vivo, this confinement-mediated high intracellular tension is responsible for the formation of the peripheral zone, a unique actin-rich structure. Cytoskeleton reorganization induces the disappearance of the peripheral zone upon reaching a liquid milieu to facilitate proplatelet and platelet formation. Hence, our data provide insight into the mechanisms preventing ectopic platelet release in the marrow stroma. Identifying such pathways is especially important for understanding pathologies altering marrow mechanics such as chemotherapy or myelofibrosis.

Reconstitution of cytolinker-mediated crosstalk between actin and vimentin

Cell shape and motility are determined by the cytoskeleton, an interpenetrating network of actin filaments, microtubules, and intermediate filaments. The biophysical properties of each filament type individually have been studied extensively by cell-free reconstitution. By contrast, the interactions between the three cytoskeletal networks are relatively unexplored. They are coupled via crosslinkers of the plakin family such as plectin. These are challenging proteins for reconstitution because of their giant size and multidomain structure. Here we engineer a recombinant actin-vimentin crosslinker protein called 'ACTIF' that provides a minimal model system for plectin, recapitulating its modular design with actin-binding and intermediate filament-binding domains separated by a coiled-coil linker for dimerisation. We show by fluorescence and electron microscopy that ACTIF has a high binding affinity for vimentin and actin and creates mixed actin-vimentin bundles. Rheology measurements show that ACTIF-mediated crosslinking strongly stiffens actin-vimentin composites. Finally, we demonstrate the modularity of this approach by creating an ACTIF variant with the intermediate filament binding domain of Adenomatous Polyposis Coli. Our protein engineering approach provides a new cell-free system for the biophysical characterization of intermediate filament-binding crosslinkers and for understanding the mechanical synergy between actin and vimentin in mesenchymal cells.

Measuring and modeling forces generated by microtubules

Tubulins are essential proteins, which are conserved across all eukaryotic species. They polymerize to form microtubules, cytoskeletal components of paramount importance for cellular mechanics. The microtubules combine an extraordinarily high flexural rigidity and a non-equilibrium behavior, manifested in their intermittent assembly and disassembly. These chemically fueled dynamics allow microtubules to generate significant pushing and pulling forces at their ends to reposition intracellular organelles, remodel membranes, bear compressive forces, and transport chromosomes during cell division. In this article, we review classical and recent studies, which have allowed the quantification of microtubule-generated forces. The measurements, to which we owe most of the quantitative information about microtubule forces, were carried out in biochemically reconstituted systems in vitro. We also discuss how mathematical and computational modeling has contributed to the interpretations of these results and shaped our understanding of the mechanisms of force production by tubulin polymerization and depolymerization.

Plasmodium microtubule-binding protein EB1 is critical for partitioning of nuclei in male gametogenesis

Sexual reproduction of the malaria parasites is critical for their transmission to a mosquito vector. Several signaling molecules, such as kinases and phosphatases, are known to regulate this process. We previously demonstrated that Plasmodium falciparum (Pf) Ca2+-dependent protein kinase 4 (CDPK4) and serine/arginine-rich protein kinase 1 (SRPK1) are critical for axoneme formation during male gametogenesis, with genetic deletion of either gene causing a complete block in parasite transmission to the mosquito. A comparative phospho-proteome analysis of Pfcdpk4- and RNA-seq analysis of Pfsrpk1- gametocytes showed that these kinases regulate similar biological processes linked to both microtubule (MT) dynamics and cell motility. One of these proteins was a nuclear MT-associated End Binding protein 1 (EB1), which was hypophosphorylated in Pfcdpk4- gametocytes. To study the functional relevance of EB1, we created gene deletion parasites for EB1. We further demonstrate that Pfeb1- parasites like WT NF54 parasites proliferate normally as asexuals and undergo gametocytogenesis and gametogenesis. Strikingly, these parasites suffer a severe defect in nuclear segregation and partitioning of nuclei into emerging microgametes. Further genetic crosses utilizing male- and female-sterile parasites revealed that Pfeb1- parasites only suffer a male fertility defect. Overall, our study reveals an essential function for PfEB1 in male gamete nuclear segregation and suggests a potential therapeutic avenue in the design of transmission-blocking drugs to prevent malaria transmission from humans to mosquito. IMPORTANCE Gametogenesis and subsequent gamete fusion are central to successful transmission of the malaria parasites to a female Anopheles mosquito vector and completion of the sexual phase of the parasite life cycle. Male gametogenesis involves the formation of axonemes inside male gametes from male gametocytes via active cytoskeleton remodeling. The tubulin and tubulin-binding proteins are, thus, attractive anti-malarial drug targets. In the present study, we demonstrate that a microtubule-binding protein PfEB1 is essential for male gamete fertility, specifically for the inheritance of nuclei from activated male gametocytes. Targeting PfEB1 function may provide new avenues into designing interventions to prevent malaria transmission and disease spread.

Repurposing the mitotic machinery to drive cellular elongation and chromatin reorganisation in Plasmodium falciparum gametocytes

The sexual stage gametocytes of the malaria parasite, Plasmodium falciparum, adopt a falciform (crescent) shape driven by the assembly of a network of microtubules anchored to a cisternal inner membrane complex (IMC). Using 3D electron microscopy, we show that a non-mitotic microtubule organizing center (MTOC), embedded in the parasite's nuclear membrane, orients the endoplasmic reticulum and the nascent IMC and seeds cytoplasmic microtubules. A bundle of microtubules extends into the nuclear lumen, elongating the nuclear envelope and capturing the chromatin. Classical mitotic machinery components, including centriolar plaque proteins, Pfcentrin-1 and -4, microtubule-associated protein, End-binding protein-1, kinetochore protein, PfNDC80 and centromere-associated protein, PfCENH3, are involved in the nuclear microtubule assembly/disassembly process. Depolymerisation of the microtubules using trifluralin prevents elongation and disrupts the chromatin, centromere and kinetochore organisation. We show that the unusual non-mitotic hemispindle plays a central role in chromatin organisation, IMC positioning and subpellicular microtubule formation in gametocytes.

The mechanical forces that shape our senses

Developing organs are shaped, in part, by physical interaction with their environment in the embryo. In recent years, technical advances in live-cell imaging and material science have greatly expanded our understanding of the mechanical forces driving organ formation. Here, we provide a broad overview of the types of forces generated during embryonic development and then focus on a subset of organs underlying our senses: the eyes, inner ears, nose and skin. The epithelia in these organs emerge from a common origin: the ectoderm germ layer; yet, they arrive at unique and complex forms over developmental time. We discuss exciting recent animal studies that show a crucial role for mechanical forces in, for example, the thickening of sensory placodes, the coiling of the cochlea and the lengthening of hair. Finally, we discuss how microfabricated organoid systems can now provide unprecedented insights into the physical principles of human development.

Bending-torsional elasticity and energetics of the plus-end microtubule tip

SignificanceThe mechanochemical basis of microtubule growth, which is essential for the normal function and division of eukaryotic cells, has remained elusive and controversial, despite extensive work. In particular, recent findings have created the paradox that the microtubule plus-end tips look very similar during both growing and shrinking phases, thereby challenging the traditional textbook picture. Our large-scale atomistic simulations resolve this paradox and explain microtubule growth and shrinkage dynamics as a process governed by energy barriers between protofilament conformations, the heights of which are in turn fine-tuned by different nucleotide states, thus implementing an information-driven Brownian ratchet.

Cross-linkers at growing microtubule ends generate forces that drive actin transport

Complex cellular processes such as cell migration require coordinated remodeling of both the actin and the microtubule cytoskeleton. The two networks for instance exert forces on each other via active motor proteins. Here we show that, surprisingly, coupling via passive cross-linkers can also result in force generation. We specifically study the transport of actin filaments by growing microtubule ends. We show by cell-free reconstitution experiments, computer simulations, and theoretical modeling that this transport is driven by the affinity of the cross-linker for the chemically distinct microtubule tip region. Our work predicts that growing microtubules could potentially rapidly relocate newly nucleated actin filaments to the leading edge of the cell and thus boost migration.

The actin and microtubule cytoskeletons form active networks in the cell that can contract and remodel, resulting in vital cellular processes such as cell division and motility. Motor proteins play an important role in generating the forces required for these processes, but more recently the concept of passive cross-linkers being able to generate forces has emerged. So far, these passive cross-linkers have been studied in the context of separate actin and microtubule systems. Here, we show that cross-linkers also allow actin and microtubules to exert forces on each other. More specifically, we study single actin filaments that are cross-linked to growing microtubule ends, using in vitro reconstitution, computer simulations, and a minimal theoretical model. We show that microtubules can transport actin filaments over large (micrometer-range) distances and find that this transport results from two antagonistic forces arising from the binding of cross-linkers to the overlap between the actin and microtubule filaments. The cross-linkers attempt to maximize the overlap between the actin and the tip of the growing microtubules, creating an affinity-driven forward condensation force, and simultaneously create a competing friction force along the microtubule lattice. We predict and verify experimentally how the average transport time depends on the actin filament length and the microtubule growth velocity, confirming the competition between a forward condensation force and a backward friction force. In addition, we theoretically predict and experimentally verify that the condensation force is of the order of 0.1 pN. Thus, our results reveal an active mechanism for local actin remodeling by growing microtubules that relies on passive cross-linkers.

Microtubule Organization Is Essential for Maintaining Cellular Morphology and Function

Microtubules (MTs) are highly dynamic polymers essential for a wide range of cellular physiologies, such as acting as directional railways for intracellular transport and position, guiding chromosome segregation during cell division, and controlling cell polarity and morphogenesis. Evidence has established that maintaining microtubule (MT) stability in neurons is vital for fundamental cellular and developmental processes, such as neurodevelopment, degeneration, and regeneration. To fulfill these diverse functions, the nervous system employs an arsenal of microtubule-associated proteins (MAPs) to control MT organization and function. Subsequent studies have identified that the disruption of MT function in neurons is one of the most prevalent and important pathological features of traumatic nerve damage and neurodegenerative diseases and that this disruption manifests as a reduction in MT polymerization and concomitant deregulation of the MT cytoskeleton, as well as downregulation of microtubule-associated protein (MAP) expression. A variety of MT-targeting agents that reverse this pathological condition, which is regarded as a therapeutic opportunity to intervene the onset and development of these nervous system abnormalities, is currently under development. Here, we provide an overview of the MT-intrinsic organization process and how MAPs interact with the MT cytoskeleton to promote MT polymerization, stabilization, and bundling. We also highlight recent advances in MT-targeting therapeutic agents applied to various neurological disorders. Together, these findings increase our current understanding of the function and regulation of MT organization in nerve growth and regeneration.

Multiscale mechanics and temporal evolution of vimentin intermediate filament networks

The cytoskeleton, an intricate network of protein filaments, motor proteins, and cross-linkers, largely determines the mechanical properties of cells. Among the three filamentous components, F-actin, microtubules, and intermediate filaments (IFs), the IF network is by far the most extensible and resilient to stress. We present a multiscale approach to disentangle the three main contributions to vimentin IF network mechanics-single-filament mechanics, filament length, and interactions between filaments-including their temporal evolution. Combining particle tracking, quadruple optical trapping, and computational modeling, we derive quantitative information on the strength and kinetics of filament interactions. Specifically, we find that hydrophobic contributions to network mechanics enter mostly via filament-elongation kinetics, whereas electrostatics have a direct influence on filament-filament interactions.

Vimentin intermediate filaments stabilize dynamic microtubules by direct interactions

The cytoskeleton determines cell mechanics and lies at the heart of important cellular functions. Growing evidence suggests that the manifold tasks of the cytoskeleton rely on the interactions between its filamentous components-actin filaments, intermediate filaments, and microtubules. However, the nature of these interactions and their impact on cytoskeletal dynamics are largely unknown. Here, we show in a reconstituted in vitro system that vimentin intermediate filaments stabilize microtubules against depolymerization and support microtubule rescue. To understand these stabilizing effects, we directly measure the interaction forces between individual microtubules and vimentin filaments. Combined with numerical simulations, our observations provide detailed insight into the physical nature of the interactions and how they affect microtubule dynamics. Thus, we describe an additional, direct mechanism by which cells establish the fundamental cross talk of cytoskeletal components alongside linker proteins. Moreover, we suggest a strategy to estimate the binding energy of tubulin dimers within the microtubule lattice.

Cytoskeletal-based mechanisms differently regulate in vivo and in vitro proplatelet formation

Platelets are produced by bone marrow megakaryocytes through cytoplasmic protrusions, named native proplatelets (nPPT), into blood vessels. Proplatelets also refer to protrusions observed in megakaryocyte culture (cultured proplatelets [cPPT]) which are morphologically different. Contrary to cPPT, the mechanisms of nPPT formation are poorly understood. We show here in living mice that nPPT elongation is in equilibrium between protrusion and retraction forces mediated by myosin-IIA. We also found, using wild-type and b1-tubulin-deficient mice, that microtubule behavior differs between cPPT and nPPT, being absolutely required in vitro, while less critical in vivo. Remarkably, microtubule depolymerization in myosin-deficient mice did not affect nPPT elongation. We then calculated that blood Stokes’ forces may be sufficient to promote nPPT extension, independently of myosin and microtubules. Together, we propose a new mechanism for nPPT extension that might explain contradictions between severely affected cPPT production and moderate platelet count defects in some patients and animal models.

Two mechanisms drive pronuclear migration in mouse zygotes

A new life begins with the unification of the maternal and paternal chromosomes upon fertilization. The parental chromosomes first become enclosed in two separate pronuclei near the surface of the fertilized egg. The mechanisms that then move the pronuclei inwards for their unification are only poorly understood in mammals. Here, we report two mechanisms that act in concert to unite the parental genomes in fertilized mouse eggs. The male pronucleus assembles within the fertilization cone and is rapidly moved inwards by the flattening cone. Rab11a recruits the actin nucleation factors Spire and Formin-2 into the fertilization cone, where they locally nucleate actin and further accelerate the pronucleus inwards. In parallel, a dynamic network of microtubules assembles that slowly moves the male and female pronuclei towards the cell centre in a dynein-dependent manner. Both mechanisms are partially redundant and act in concert to unite the parental pronuclei in the zygote's centre.

Modified heptapeptide from tau binds both tubulin and microtubules

Background

Microtubules are the major cytoskeletal component in eukaryotes which are essential for a large spectrum of cellular activities. Monitoring the behavior of microtubules is helpful for a better understanding of the regulatory mechanism governing microtubule architecture and microtubule‐based activities. Here, we characterized the binding capability of a modified heptapeptide from tau to both tubulin and microtubules and sought to develop it as a fluorescent peptide for monitoring microtubules.

Methods

To deliver the fluorescent peptide into the cells, a cell‐penetrating peptide was conjugated to the modified heptapeptide from tau and synthesized. The affinity of the modified heptapeptide was determined by microscale thermophoresis. The microtubule labeling ability was determined by adding the peptide into the polymerized microtubule solutions or cultured HeLa cells.;

Results

Affinity determination revealed that the tau‐derived peptide specifically bound to tubulin. In addition, the peptide was able to label polymerized microtubules in solution, although no obvious microtubule filaments were observed clearly in living cells, probably due to the inadequate affinity.

Conclusions

These results suggest that using a peptide‐based strategy for imaging microtubules might be plausible and attempts to improve its affinity is warranted in the future.

Microtubules pull the strings: disordered sequences as efficient couplers of microtubule-generated force

Microtubules are dynamic polymers that grow and shrink through addition or loss of tubulin subunits at their ends. Microtubule ends generate mechanical force that moves chromosomes and cellular organelles, and provides mechanical tension. Recent literature describes a number of proteins and protein complexes that couple dynamics of microtubule ends to movements of their cellular cargoes. These 'couplers' are quite diverse in their microtubule-binding domains (MTBDs), while sharing similarity in function, but a systematic understanding of the principles underlying their activity is missing. Here, I review various types of microtubule couplers, focusing on their essential activities: ability to follow microtubule ends and capture microtubule-generated force. Most of the couplers require presence of unstructured positively charged sequences and multivalency in their microtubule-binding sites to efficiently convert the microtubule-generated force into useful connection to a cargo. An overview of the microtubule features supporting end-tracking and force-coupling, and the experimental methods to assess force-coupling properties is also provided.

Cytoplasmic Dynein Functions in Planar Polarization of Basal Bodies within Ciliated Cells

Despite common consensus about the importance of planar cell polarity (PCP) proteins in tissue orientation, little is known about the mechanisms used by PCP proteins to promote planar polarization of cytoskeletons within individual cells. One PCP protein Fzd6 asymmetrically localizes to the apical cell membrane of multi-ciliated ependymal cells lining the lateral ventricular (LV) wall on the side that contacts cerebrospinal fluid flow. Individual ependymal cells have planar polarized microtubules that connect ciliary basal bodies (BBs) with the cell cortex of the Fzd side to coordinate cilia orientation. Here, we report that cytoplasmic dynein is anchored to the cell cortex of the Fzd side via an adapter protein Daple that regulates microtubule dynamics. Asymmetric localization of cortical dynein generates a pulling force on dynamic microtubules connected to BBs, which in turn orients BBs toward the Fzd side. This is required for coordinated cilia orientation on the LV wall.

•

Daple anchors cytoplasmic dynein to the cell cortex of ependymal cells on LV wall

•

Cytoplasmic dynein is anchored to the Fzd6/Dvl1/Daple side of the cell cortex

•

Cytoplasmic dynein functions include BB positioning and orientation

•

Cortex-anchored dynein generates a pulling force on microtubules connected to BBs

The Generation of Dynein Networks by Multi-Layered Regulation and Their Implication in Cell Division

Cytoplasmic dynein-1 (hereafter referred to as dynein) is a major microtubule-based motor critical for cell division. Dynein is essential for the formation and positioning of the mitotic spindle as well as the transport of various cargos in the cell. A striking feature of dynein is that, despite having a wide variety of functions, the catalytic subunit is coded in a single gene. To perform various cellular activities, there seem to be different types of dynein that share a common catalytic subunit. In this review, we will refer to the different kinds of dynein as “dyneins.” This review attempts to classify the mechanisms underlying the emergence of multiple dyneins into four layers. Inside a cell, multiple dyneins generated through the multi-layered regulations interact with each other to form a network of dyneins. These dynein networks may be responsible for the accurate regulation of cellular activities, including cell division. How these networks function inside a cell, with a focus on the early embryogenesis of Caenorhabditis elegans embryos, is discussed, as well as future directions for the integration of our understanding of molecular layering to understand the totality of dynein’s function in living cells.

Anopheles aquasalis transcriptome reveals autophagic responses to Plasmodium vivax midgut invasion

Background

Elimination of malaria depends on mastering transmission and understanding the biological basis of Plasmodium infection in the vector. The first mosquito organ to interact with the parasite is the midgut and its transcriptomic characterization during infection can reveal effective antiplasmodial responses able to limit the survival of the parasite. The vector response to Plasmodium vivax is not fully characterized, and its specificities when compared with other malaria parasites can be of fundamental interest for specific control measures.

Methods

Experimental infections were performed using a membrane-feeding device. Three groups were used: P. vivax-blood-fed, blood-fed on inactivated gametocytes, and unfed mosquitoes. Twenty-four hours after feeding, the mosquitoes were dissected and the midgut collected for transcriptomic analysis using RNAseq. Nine cDNA libraries were generated and sequenced on an Illumina HiSeq2500. Readings were checked for quality control and analysed using the Trinity platform for de novo transcriptome assembly. Transcript quantification was performed and the transcriptome was functionally annotated. Differential expression gene analysis was carried out. The role of the identified mechanisms was further explored using functional approaches.

Results

Forty-nine genes were identified as being differentially expressed with P. vivax infection: 34 were upregulated and 15 were downregulated. Half of the P. vivax-related differentially expressed genes could be related to autophagy; therefore, the effect of the known inhibitor (wortmannin) and activator (spermidine) was tested on the infection outcome. Autophagic activation significantly reduced the intensity and prevalence of infection. This was associated with transcription alterations of the autophagy regulating genes Beclin, DRAM and Apg8.

Conclusions

Our data indicate that P. vivax invasion of An. aquasalis midgut epithelium triggers an autophagic response and its activation reduces infection. This suggests a novel mechanism that mosquitoes can use to fight Plasmodium infection.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3506-8) contains supplementary material, which is available to authorized users.

MTrack: Automated Detection, Tracking, and Analysis of Dynamic Microtubules

Microtubules are polar, dynamic filaments fundamental to many cellular processes. In vitro reconstitution approaches with purified tubulin are essential to elucidate different aspects of microtubule behavior. To date, deriving data from fluorescence microscopy images by manually creating and analyzing kymographs is still commonplace. Here, we present MTrack, implemented as a plug-in for the open-source platform Fiji, which automatically identifies and tracks dynamic microtubules with sub-pixel resolution using advanced objection recognition. MTrack provides automatic data interpretation yielding relevant parameters of microtubule dynamic instability together with population statistics. The application of our software produces unbiased and comparable quantitative datasets in a fully automated fashion. This helps the experimentalist to achieve higher reproducibility at higher throughput on a user-friendly platform. We use simulated data and real data to benchmark our algorithm and show that it reliably detects, tracks, and analyzes dynamic microtubules and achieves sub-pixel precision even at low signal-to-noise ratios.

Microtubule Plus End Dynamics - Do We Know How Microtubules Grow?: Cells boost microtubule growth by promoting distinct structural transitions at growing microtubule ends

Microtubules form a highly dynamic filament network in all eukaryotic cells. Individual microtubules grow by tubulin dimer subunit addition and frequently switch between phases of growth and shortening. These unique dynamics are powered by GTP hydrolysis and drive microtubule network remodeling, which is central to eukaryotic cell biology and morphogenesis. Yet, our knowledge of the molecular events at growing microtubule ends remains incomplete. Here, recent ultrastructural, biochemical and cell biological data are integrated to develop a realistic model of growing microtubule ends comprised of structurally distinct but biochemically overlapping zones. Proteins that recognize microtubule lattice conformations associated with specific tubulin guanosine nucleotide states may independently control major structural transitions at growing microtubule ends. A model is proposed in which tubulin dimer addition and subsequent closure of the MT wall are optimized in cells to achieve rapid physiological microtubule growth.

Mechanistic three-dimensional model to study centrosome positioning in the interphase cell

During the interphase in mammalian cells, the position of the centrosome is actively maintained at a small but finite distance away from the nucleus. The perinuclear positioning of the centrosome is crucial for cellular trafficking and progression into mitosis. Although the literature suggests that the contributions of the microtubule-associated forces bring the centrosome to the center of the cell, the position of the centrosome was merely investigated in the absence of the nucleus. Upon performing a coarse-grained simulation study with mathematical analysis, we show that the combined effect of the forces due to the cell cortex and the nucleus facilitate the centrosome positioning. Our study also demonstrates that in the absence of nucleus-based forces, the centrosome collapses on the nucleus due to cortical forces. Depending upon the magnitudes of the cortical forces and the nucleus-based forces, the centrosome appears to stay at various distances away from the nucleus. Such null force regions are found to be stable as well as unstable fixed points. This study uncovers a set of redundant schemes that the cell may adopt to produce the required cortical and nucleus-based forces stabilizing the centrosome at a finite distance away from the nucleus.

Reduced Radiation Damage in Transmission Electron Microscopy of Proteins in Graphene Liquid Cells

Liquid-phase electron microscopy (LPEM) is capable of imaging native (unstained) protein structure in liquid, but the achievable spatial resolution is limited by radiation damage. This damaging effect is more pronounced when targeting small molecular features than for larger structures. The matter is even more complicated because the critical dose that a sample can endure before radiation damage not only varies between proteins but also critically depends on the experimental conditions. Here, we examined the effect of the electron beam on the observed protein structure for optimized conditions using a liquid sample enclosure assembled from graphene sheets. It has been shown that graphene can reduce the damaging effect of electrons on biological materials. We used radiation sensitive microtubule proteins and investigated the radiation damage on these structures as a function of the spatial frequencies of the observed features with transmission electron microscopy (TEM). Microtubule samples were also examined using cryo-electron microscopy (cryo-TEM) for comparison. We used an electron flux of 11 ± 1-16 ± 1 e^-/Ųs and obtained a series of images from the same sample region. Our results show that graphene-encapsulated microtubules can maintain their structural features of spatial frequencies of up to 0.20 nm^-1 (5 nm), reflecting protofilaments for electron densities of up to 7.2 ± 1.4 × 10² e^-/Ų, an order of magnitude higher than measured for frozen microtubules in amorphous ice.