Evolution of a dynamic cytoskeleton

CryoET of β-amyloid and tau within postmortem Alzheimer's disease brain

A defining pathological feature of most neurodegenerative diseases is the assembly of proteins into amyloid that form disease-specific structures1. In Alzheimer's disease, this is characterized by the deposition of β-amyloid and tau with disease-specific conformations. The in situ structure of amyloid in the human brain is unknown. Here, using cryo-fluorescence microscopy-targeted cryo-sectioning, cryo-focused ion beam-scanning electron microscopy lift-out and cryo-electron tomography, we determined in-tissue architectures of β-amyloid and tau pathology in a postmortem Alzheimer's disease donor brain. β-amyloid plaques contained a mixture of fibrils, some of which were branched, and protofilaments, arranged in parallel arrays and lattice-like structures. Extracellular vesicles and cuboidal particles defined the non-amyloid constituents of β-amyloid plaques. By contrast, tau inclusions formed parallel clusters of unbranched filaments. Subtomogram averaging a cluster of 136 tau filaments in a single tomogram revealed the polypeptide backbone conformation and filament polarity orientation of paired helical filaments within tissue. Filaments within most clusters were similar to each other, but were different between clusters, showing amyloid heterogeneity that is spatially organized by subcellular location. The in situ structural approaches outlined here for human donor tissues have applications to a broad range of neurodegenerative diseases.

A life off the beaten track in biomechanics: Imperfect elasticity, cytoskeletal glassiness, and epithelial unjamming

Textbook descriptions of elasticity, viscosity, and viscoelasticity fail to account for certain mechanical behaviors that typify soft living matter. Here, we consider three examples. First, strong empirical evidence suggests that within lung parenchymal tissues, the frictional stresses expressed at the microscale are fundamentally not of viscous origin. Second, the cytoskeleton (CSK) of the airway smooth muscle cell, as well as that of all eukaryotic cells, is more solid-like than fluid-like, yet its elastic modulus is softer than the softest of soft rubbers by a factor of 104-105. Moreover, the eukaryotic CSK expresses power law rheology, innate malleability, and fluidization when sheared. For these reasons, taken together, the CSK of the living eukaryotic cell is reminiscent of the class of materials called soft glasses, thus likening it to inert materials such as clays, pastes slurries, emulsions, and foams. Third, the cellular collective comprising a confluent epithelial layer can become solid-like and jammed, fluid-like and unjammed, or something in between. Esoteric though each may seem, these discoveries are consequential insofar as they impact our understanding of bronchospasm and wound healing as well as cancer cell invasion and embryonic development. Moreover, there are reasons to suspect that certain of these phenomena first arose in the early protist as a result of evolutionary pressures exerted by the primordial microenvironment. We have hypothesized, further, that each then became passed down virtually unchanged to the present day as a conserved core process. These topics are addressed here not only because they are interesting but also because they track the journey of one laboratory along a path less traveled by.

Enzyme-Instructed Self-Assembly of Peptides: From Concept to Representative Applications

Enzyme-instructed self-assembly, integrating enzymatic reaction and molecular self-assembly, has drawn noticeable attention over the last decade with the intension of being used in valuable applications. Recent advances in the field allow it possible to spatiotemporally control peptide self-assembly in cellular milieu, broadening the potential applications of peptide assemblies to cancer therapy and subcellular delivery. In this minireview, the concept of enzyme-instructed self-assembly of peptide, containing enzymatic trigger and spatiotemporal control, is described. Representative applications in cells are also discussed, followed by outlook on the field of enzyme-instructed self-assembly.

Cancer associated fibroblast: Mediators of tumorigenesis

It is well accepted that the tumor microenvironment plays a pivotal role in cancer onset, development, and progression. The majority of clinical interventions are designed to target either cancer or stroma cells. These emphases have been directed by one of two prevailing theories in the field, the Somatic Mutation Theory and the Tissue Organization Field Theory, which represent two seemingly opposing concepts. This review proposes that the two theories are mutually inclusive and should be concurrently considered for cancer treatments. Specifically, this review discusses the dynamic and reciprocal processes between stromal cells and extracellular matrices, using pancreatic cancer as an example, to demonstrate the inclusivity of the theories. Furthermore, this review highlights the functions of cancer associated fibroblasts, which represent the major stromal cell type, as important mediators of the known cancer hallmarks that the two theories attempt to explain.

Enzyme-Instructed Self-Assembly for Cancer Therapy and Imaging

Enzymatic reactions and self-assembly are two fundamental attributes of cells. It is not surprising that one can use enzyme-instructed self-assembly (EISA)-the integration of enzymatic transformation and molecular self-assembly-to modulate the emergent properties of supramolecular assemblies for controlling cell behaviors. The exploration of EISA for developing cancer therapy and imaging has made considerable progress over the last five years. In this Topical Review, we discuss these exciting results and the future promise of EISA. After describing several key studies to illustrate the progress of EISA in developing cancer therapy, we discuss the use of EISA for molecular imaging. Then, we give the outlook of EISA for developing supramolecular anticancer medicine that inhibits multiple hallmark capabilities of cancer.

Single particle cryo-EM-an optimal tool to study cytoskeletal proteins

Cytoskeletal proteins play essential roles in many cellular processes. Knowledge of their structures is important to understand their function and regulation. Since cytoskeletal polymers are difficult to crystallize, cryo-EM has been the predominant method of choice to study their structures. Recent advances in the methodology have enabled reconstructions at near-atomic resolution. In this review, we focus on novel insights gained from high-resolution cryo-EM structures of cytoskeletal polymers. These include eukaryotic proteins such as F-actin and microtubules as well as their prokaryotic homologues. The unprecedented high-resolutions allow identifying small molecules, including nucleotides and drugs, as well as subtle changes at interfaces that are key to complex processes, such as nucleotide hydrolysis in microtubules and actin filaments. While major methodological advances have already promoted the structural analysis of cytoskeletal polymers, there are still specific methodological challenges to overcome and many scientific questions remain to be answered.

Structural differences explain diverse functions of Plasmodium actins

Actins are highly conserved proteins and key players in central processes in all eukaryotic cells. The two actins of the malaria parasite are among the most divergent eukaryotic actins and also differ from each other more than isoforms in any other species. Microfilaments have not been directly observed in Plasmodium and are presumed to be short and highly dynamic. We show that actin I cannot complement actin II in male gametogenesis, suggesting critical structural differences. Cryo-EM reveals that Plasmodium actin I has a unique filament structure, whereas actin II filaments resemble canonical F-actin. Both Plasmodium actins hydrolyze ATP more efficiently than α-actin, and unlike any other actin, both parasite actins rapidly form short oligomers induced by ADP. Crystal structures of both isoforms pinpoint several structural changes in the monomers causing the unique polymerization properties. Inserting the canonical D-loop to Plasmodium actin I leads to the formation of long filaments in vitro. In vivo, this chimera restores gametogenesis in parasites lacking actin II, suggesting that stable filaments are required for exflagellation. Together, these data underline the divergence of eukaryotic actins and demonstrate how structural differences in the monomers translate into filaments with different properties, implying that even eukaryotic actins have faced different evolutionary pressures and followed different paths for developing their polymerization properties.

Malaria parasites have two actin isoforms, which are among the most divergent within the actin family that comprises highly conserved proteins, essential in all eukaryotic cells. In Plasmodium, actin is indispensable for motility and, thus, the infectivity of the deadly parasite. Yet, actin filaments have not been observed in vivo in these pathogens. Here, we show that the two Plasmodium actins differ from each other in both monomeric and filamentous form and that actin I cannot replace actin II during male gametogenesis. Whereas the major isoform actin I cannot form stable filaments alone, the mosquito-stage-specific actin II readily forms long filaments that have dimensions similar to canonical actins. A chimeric actin I mutant that forms long filaments in vitro also rescues gametogenesis in parasites lacking actin II. Both Plasmodium actins rapidly hydrolyze ATP and form short oligomers in the presence of ADP, which is a fundamental difference to all other actins characterized to date. Structural and functional differences in the two Plasmodium actin isoforms compared both to each other and to canonical actins reveal how the polymerization properties of eukaryotic actins have evolved along different avenues.

3D Morphology, ultrastructure and development of Ceratomyxa puntazzi stages: first insights into the mechanisms of motility and budding in the Myxozoa

Free, amoeboid movement of organisms within media as well as substrate-dependent cellular crawling processes of cells and organisms require an actin cytoskeleton. This system is also involved in the cytokinetic processes of all eukaryotic cells. Myxozoan parasites are known for the disease they cause in economical important fishes. Usually, their pathology is related to rapid proliferation in the host. However, the sequences of their development are still poorly understood, especially with regard to pre-sporogonic proliferation mechanisms. The present work employs light microscopy (LM), electron microscopy (SEM, TEM) and confocal laser scanning microscopy (CLSM) in combination with specific stains (Nile Red, DAPI, Phalloidin), to study the three-dimensional morphology, motility, ultrastructure and cellular composition of Ceratomyxa puntazzi, a myxozoan inhabiting the bile of the sharpsnout seabream.

Our results demonstrate the occurrence of two C. puntazzi developmental cycles in the bile, i.e. pre-sporogonic proliferation including frequent budding as well as sporogony, resulting in the formation of durable spore stages and we provide unique details on the ultrastructure and the developmental sequence of bile inhabiting myxozoans. The present study describes, for the first time, the cellular components and mechanisms involved in the motility of myxozoan proliferative stages, and reveals how the same elements are implicated in the processes of budding and cytokinesis in the Myxozoa. We demonstrate that F-actin rich cytoskeletal elements polarize at one end of the parasites and in the filopodia which are rapidly de novo created and re-absorbed, thus facilitating unidirectional parasite motility in the bile. We furthermore discover the myxozoan mechanism of budding as an active, polarization process of cytokinesis, which is independent from a contractile ring and thus differs from the mechanism, generally observed in eurkaryotic cells. We hereby demonstrate that CLSM is a powerful tool for myxozoan research with a great potential for exploitation, and we strongly recommend its future use in combination with in vivo stains.

Classically activated macrophages use stable microtubules for matrix metalloproteinase-9 (MMP-9) secretion

As major effector cells of the innate immune response, macrophages must adeptly migrate from blood to infected tissues. Endothelial transmigration is accomplished by matrix metalloproteinase (MMP)-induced degradation of basement membrane and extracellular matrix components. The classical activation of macrophages with LPS and IFN-γ causes enhanced microtubule (MT) stabilization and secretion of MMPs. Macrophages up-regulate MMP-9 expression and secretion upon immunological challenge and require its activity for migration during the inflammatory response. However, the dynamics of MMP-9 production and intracellular distribution as well as the mechanisms responsible for its trafficking are unknown. Using immunofluorescent imaging, we localized intracellular MMP-9 to small Golgi-derived cytoplasmic vesicles that contained calreticulin and protein-disulfide isomerase in activated RAW 264.7 macrophages. We demonstrated vesicular organelles of MMP-9 aligned along stable subsets of MTs and showed that selective modulation of MT dynamics contributes to the enhanced trafficking of MMP-9 extracellularly. We found a Rab3D-dependent association of MMP-9 vesicles with the molecular motor kinesin, whose association with the MT network was greatly enhanced after macrophage activation. Finally, we implicated kinesin 5B and 3B isoforms in the effective trafficking of MMP-9 extracellularly.

Interactions between laminin receptor and the cytoskeleton during translation and cell motility

Human laminin receptor acts as both a component of the 40S ribosomal subunit to mediate cellular translation and as a cell surface receptor that interacts with components of the extracellular matrix. Due to its role as the cell surface receptor for several viruses and its overexpression in several types of cancer, laminin receptor is a pathologically significant protein. Previous studies have determined that ribosomes are associated with components of the cytoskeleton, however the specific ribosomal component(s) responsible has not been determined. Our studies show that laminin receptor binds directly to tubulin. Through the use of siRNA and cytoskeletal inhibitors we demonstrate that laminin receptor acts as a tethering protein, holding the ribosome to tubulin, which is integral to cellular translation. Our studies also show that laminin receptor is capable of binding directly to actin. Through the use of siRNA and cytoskeletal inhibitors we have shown that this laminin receptor-actin interaction is critical for cell migration. These data indicate that interactions between laminin receptor and the cytoskeleton are vital in mediating two processes that are intimately linked to cancer, cellular translation and migration.

Inter-cellular variation in DNA content of Entamoeba histolytica originates from temporal and spatial uncoupling of cytokinesis from the nuclear cycle

Accumulation of multiple copies of the genome in a single nucleus and several nuclei in a single cell has previously been noted in Entamoeba histolytica, contributing to the genetic heterogeneity of this unicellular eukaryote. In this study, we demonstrate that this genetic heterogeneity is an inherent feature of the cell cycle of this organism. Chromosome segregation occurs on a variety of novel microtubular assemblies including multi-polar spindles. Cytokinesis in E. histolytica is completed by the mechanical severing of a thin cytoplasmic bridge, either independently or with the help of neighboring cells. Importantly, cytokinesis is uncoupled from the nuclear division cycle, both temporally and spatially, leading to the formation of unequal daughter cells. Sorting of euploid and polyploid cells showed that each of these sub-populations acquired heterogeneous DNA content upon further growth. Our study conclusively demonstrates that genetic heterogeneity originates from the unique mode of cell division events in this protist.

Proliferating eukaryotic cells regulate their DNA synthesis, chromosome segregation, and cell division with great precision so that daughter cells are genetically identical. Our study demonstrates that in proliferating cells of the protist pathogen Entamoeba histolytica re-duplication of DNA followed by segregation on atypical and diverse microtubular structures is frequently observed. In this parasite, cell division is erratic, so that each daughter cell may contain one or more nuclei and sometimes no nuclei. This uncoupling of cell cycle events and survival of daughter cells with unequal DNA contents leads to genetic heterogeneity in E. histolytica. Our study highlights the inherent plasticity of the Entamoeba genome and the ability of this protist to survive in the absence of strict regulatory mechanisms that are a hallmark of the eukaryotic cell cycle.

Tubulin and its prokaryotic homologue FtsZ: a structural and functional comparison

Microtubules are one of the three primary constituents of the eukaryotic cytoskeleton and are constructed from the protein tubulin. FtsZ is a close structural homologue of tubulin within prokaryotes, and plays an important structural role during cell division. This article compares what is known about the structures that these two homologues are able to form in vivo and in vitro and examines the evidence that the water in the immediate vicinity of the structures, particularly in microtubules, may play an important role in their formation and stability. The article then examines evidence that this hydration layer might help our understanding of how the structures formed by tubulin and FtsZ are stabilised by associated proteins and selected cations. The article then considers recent studies of the charge distribution and dipole moments of tubulin and extends this work to include the electrostatic characteristics of FtsZ. There is then an examination of the ways in which the electrostatic similarities and differences between the two proteins might be related to the similarities and differences in the filamentary structures that they form.

The alteration of a mechanical property of bone cells during the process of changing from osteoblasts to osteocytes

Osteocytes acquire their stellate shape during the process of changing from osteoblasts in bone. Throughout this process, dynamic cytoskeletal changes occur. In general, changes of the cytoskeleton affect cellular mechanical properties. Mechanical properties of living cells are connected with their biological functions and physiological processes. In this study, we for the first time analyzed elastic modulus, a mechanical property of bone cells. Bone cells in embryonic chick calvariae and in isolated culture were identified using fluorescently labeled phalloidin and OB7.3, a chick osteocyte-specific monoclonal antibody, and then observed by confocal laser scanning microscopy. The elastic modulus of living cells was analyzed with atomic force microscopy. To examine the consequences of focal adhesion formation on the elastic modulus, cells were pretreated with GRGDS and GRGES, and then the elastic modulus of the cells was analyzed. Focal adhesions in the cells were visualized by immunofluorescence of vinculin. From fluorescence images, we could distinguish osteoblasts, osteoid osteocytes and mature osteocytes both in vivo and in vitro. The elastic modulus of peripheral regions of cells in all three populations was significantly higher than in their nuclear regions. The elastic modulus of the peripheral region of osteoblasts was 12053+/-934 Pa, that of osteoid osteocytes was 7971+/-422 Pa and that of mature osteocytes was 4471+/-198 Pa. These results suggest that the level of elastic modulus of bone cells was proportional to the stage of changing from osteoblasts to osteocytes. The focal adhesion area of osteoblasts was significantly higher than that of osteocytes. The focal adhesion area of osteoblasts was decreased after treatment with GRGDS, however, that of osteocytes was not. The elastic modulus of osteoblasts and osteoid osteocytes were decreased after treatment with GRGDS. However, that of mature osteocytes was not changed. There were dynamic changes in the mechanical property of elastic modulus and in focal adhesions of bone cells.

Acanthamoeba castellanii: identification and distribution of actin cytoskeleton

The presence of the cytoskeleton of Acanthamoeba castellanii was observed by means of cryo-electronmicroscopy and immunofluorescence techniques. This structure is formed largely by fibers and networks of actin located mainly in cytoplasmic locomotion structures as lamellipodia and as well as in various endocytic structures. In addition, the comparison between total actin content in whole extracts among different amoebae was made. The molecular weight of actin in A. castellanii was 44 kDa, and 45 kDa for Naegleria fowleri and Entamoeba histolytica.

The push and pull of the bacterial cytoskeleton

A crucial function for eukaryotic cytoskeletal filaments is to organize the intracellular space: facilitate communication across the cell and enable the active transport of cellular components. It was assumed for many years that the small size of the bacterial cell eliminates the need for a cytoskeleton, because simple diffusion of proteins is rapid over micron-scale distances. However, in the last decade, cytoskeletal proteins have indeed been found to exist in bacteria where they have an important role in organizing the bacterial cell. Here, we review the progress that has been made towards understanding the mechanisms by which bacterial cytoskeletal proteins influence cellular organization. These discoveries have advanced our understanding of bacterial physiology and provided insight into the evolution of the eukaryotic cytoskeleton.

Maturation of Human Neutrophil Phagosomes Includes Incorporation of Molecular Chaperones and Endoplasmic Reticulum Quality Control Machinery

A Human neutrophils are an essential component of the innate immune response. Although significant progress has been made toward understanding mechanisms of phagocytosis and microbicidal activity, a comprehensive analysis of proteins comprising neutrophil phagosomes has not been conducted. To that end, we used subcellular proteomics to identify proteins associated with human neutrophil phagosomes following receptor-mediated phagocytosis. Proteins (n = 411 spots) resolved from neutrophil phagosome fractions were identified by MALDI-TOF MS and/or LC-MS/MS analysis. Those associated with phagocytic vacuoles originated from multiple subcellular compartments, including the cytosol, plasma membrane, specific and azurophilic granules, and cytoskeleton. Unexpectedly several enzymes typically associated with mitochondria were identified in phagosome fractions. Furthermore proteins characteristic of the endoplasmic reticulum, including 11 molecular chaperones, were resolved from phagosome preparations. Confocal microscopy confirmed that proteins representing these major subcellular compartments were enriched on phagosomes of intact neutrophils. Notably calnexin and glucose-regulated protein 78 co-localized with gp91(phox) in human neutrophils and were thus likely delivered to phagosomes by fusion of specific granules. We conclude that neutrophil phagosomes have heretofore unrecognized complexity and function, which includes potential for antigen processing events.

Second thoughts on septation by the fission yeast, Schizosaccharomyces pombe: pull vs. push mechanisms with an appendix--dimensional modelling of the flat and variable septa

The correlation of contraction by an actomyosin band with the closing of the septum of dividing cells of the fission yeast, Schizosaccharomyces pombe, cannot suggest cause-and-effect because contraction would be apparent whether the membrane enveloping the centripetally closing septum were pulled or were pushed. Thus the common observation of contraction is not critical. Diagrams of published electron micrographs of dividing wild-type fission yeasts illustrate variable (tilted) septal images that are counterintuitive to a pull model. Circumference calculations based on those images suggest that some variable forms might be only 6% closed even though their two-dimensional profiles would be 50% closed, if they were not tilted. Development of multiseptate forms of cdc4-8 and cdc4-377 temperature sensitive mutants incubated at their restrictive temperature was followed. These multiseptate forms are shown to have functional (functional in terms of generating divided uninucleate cytoplasts) but grotesque septa which are formed in the absence of actomyosin bands. By contrast, the myosin of the plant phragmoplast is not properly oriented for contractility, and Dictyostelium (attached cells) and Saccharomyces (mutants) have been shown to divide in the absence of myosin II, just as S. pombe does (above). Hence contractility, the essence of a pull model for septum closure, would seem to be non-essential. Other, non-contractile mechanisms of myosin are emphasized, and a push model becomes a rational default hypothesis. The essence of push models is that their synthesis/assembly mechanisms are driving force sufficient for septum closure.

Proteomic characterization of harvested pseudopodia with differential gel electrophoresis and specific antibodies

Malignant gliomas (astrocytomas) are lethal tumors that invade the brain. Invasive cell migration is initiated by extension of pseudopodia into interstitial spaces. In this study, U87 glioma cells formed pseudopodia in vitro as cells pushed through 3 microm pores of polycarbonate membranes. Harvesting pseudopodia in a novel two-step method provided material for proteomic analysis. Differences in the protein profiles of pseudopodia and whole cells were found using differential gel electrophoresis (DIGE) and immunoblotting. Proteins from two-dimensional (2D) gels with M(R)'s of 20-100 kDa and pI's of 3.0-10.0 were identified by peptide mass fingerprinting analysis using mass spectrometry. For DIGE, lysates of pseudopodia and whole cells were each labeled with electrophilic forms of fluorescent dyes, Cy3 or Cy5, and analyzed as mixtures. Analysis was repeated with reciprocal labeling. Differences in protein distributions were detected by manual inspection and computer analysis. Topographical digital maps of the scanned gels were used for algorithmic spot matching, normalization of background, quantifying spot differences, and elimination of artifacts. Pseudopodial proteins in Coomassie-stained 2D gels included isoforms of glycolytic enzymes as the largest group, seven of 24 proteins. Peptide mass fingerprint analysis of DIGE gels demonstrated increased isoforms of annexin (Anx) I, AnxII, enolase, pyruvate kinase, and aldolase, and decreased mitochondrial manganese superoxide dismutase and transketolase in pseudopodia. Specific antibodies showed restricted immunoreactivity of the hepatocyte growth factor (HGF) alpha chain to pseudopodia, indicating localization of its active form. Met (the HGF receptor), actin, and total AnxI were increased in pseudopodial lysates on immunoblots. Increased constituents of the pseudopodial proteome in glioma cells, identified in this study as actin, HGF, Met, and isoforms of AnxI, AnxII, and several glycolytic enzymes, represent therapeutic targets to consider for suppression of tumor invasion.

Variations on a theme: the many modes of cytokinesis

Animal cell division is believed to be mediated primarily by the 'purse-string' mechanism, which entails furrowing of the equatorial region, driven by the interaction of actin and myosin II filaments within contractile rings. However, myosin II-null Dictyostelium cells on substrates divide efficiently in a cell cycle-coupled manner. This process, termed cytokinesis B, appears to be driven by polar traction forces. Data in the literature can be interpreted as suggesting that adherent higher animal cells also use a cytokinesis B-like mechanism for cytokinesis. An additional chemotaxis-based cytokinesis that involves a 'midwife' cell has also been reported. Collectively, these findings demonstrate an unexpected diversity of mechanisms by which animal cells carry out cytokinesis.

Multiple Parallelisms in Animal Cytokinesis

The process of cytokinesis in animal cells is usually presented as a relatively simple picture: A cleavage plane is first positioned in the equatorial region by the astral microtubules of the anaphase mitotic apparatus, and a contractile ring made up of parallel filaments of actin and myosin II is formed and encircles the cortex at the division site. Active sliding between the two filament systems constricts the perimeter of the cortex, leading to separation of two daughter cells. However, recent studies in both animal cells and lower eukaryotic model organisms have demonstrated that cytokinesis is actually far more complex. It is now obvious that the three key processes of cytokinesis, cleavage plane determination, equatorial furrowing, and scission, are driven by different mechanisms in different types of cells. In some cases, moreover, multiple pathways appear to have redundant functions in a single cell type. In this review, we present a novel hypothesis that incorporates recent observations on the activities of mitotic microtubules and the biochemistry of Rho-type GTPase proteins and postulates that two different sets of microtubules are responsible for the two known mechanisms of cleavage plane determination and also for two distinct mechanisms of equatorial furrowing.

Myosins and cell dynamics in cellular slime molds

Myosin is a mechanochemical transducer and serves as a motor for various motile activities such as cell migration, cytokinesis, maintenance of cell shape, phagocytosis, and morphogenesis. Nonmuscle myosin in vivo does not either stay static at specific subcellular regions or construct highly organized structures, such as sarcomere in skeletal muscle cells. The cellular slime mold Dictyostelium discoideum is an ideal "model organism" for the investigation of cell movement and cytokinesis. The advantages of this organism prompted researchers to carry out pioneering cell biological, biochemical, and molecular genetic studies on myosin II, which resulted in elucidation of many fundamental features of function and regulation of this most abundant molecular motor. Furthermore, recent molecular biological research has revealed that many unconventional myosins play various functions in vivo. In this article, how myosins are organized and regulated in a dynamic manner in Dictyostelium cells is reviewed and discussed.

Mathematical Modeling Suggests Cooperative Interactions between a Disordered Polyvalent Ligand and a Single Receptor Site

BACKGROUND

The CDK inhibitor Sic1 must be phosphorylated on at least six sites in order to allow its recognition by the SCF ubiquitin ligase subunit Cdc4. However, because Cdc4 appears to have only a single phospho-epitope binding site, the apparent cooperative dependence on the number of phosphorylation sites in Sic1 cannot be accounted for by traditional thermodynamic models of cooperativity.

RESULTS

We develop a general kinetic model, which predicts an unexpected multiplicative increase in affinity as a function of ligand sites. This effect, termed allovalency, derives from a high local concentration of interaction sites moving independently of each other. Modeling of this interaction by a first exit time approach indicates that the probability of ligand rebinding increases exponentially with the number of sites. This type of interaction is relatively immune to loss of any one site and may be easily tuned to any given threshold by adjusting the properties of individual sites.

CONCLUSIONS

The allovalency model suggests that a previously undescribed mechanism may underlie certain cooperative interactions. The widespread occurrence of flexible polyvalent ligands in biological systems suggests that this principle may be broadly applicable.

How did cells get their size?

Cells exercise size homeostasis, and the origins of their ability to do so is the topic of this essay. Before there were cells, there were protocells. The most basic questions about protocells as objects are: What were they made of, and how big were they? Asking how big they were implies that the answer to the first part includes a boundary. The best candidate for that boundary is a self-assembling lipid bilayer. Therefore, protocells are defined here as Darwinian liposomes-bilayer vesicles with mutable on-board replicases linked to phenotypes. Because liposomes undergo spontaneous fission and fusion, and are subject to osmotic forces, size regulation in the earliest protocells would essentially have been liposome physics. For successful protocells, averting osmotic lysis would have been the first order of business. However, from the outset size mattered too, because of sex and reproduction (i.e., genome mixing and genome copying in entities with phenotypes). Protocell fission and fusion would have blended seamlessly into protocell sex and reproduction, making any gene product that furnished control over protocell size changes doubly adaptive. A recurrent theme is the feedback role of bilayer tension in protocell size control. Ways in which primitive peptides and their aggregates (e.g., channels) might have allowed liposomes to gain improved volume and surface area homeostasis are suggested. Domain-swapped proteins that polymerize as filaments are discussed as the origin of cytoskeleton structures that diversify and stabilize liposome shapes and sizes. Throughout, attention is paid to the question of set points for cell size.

Motile plant cell body: a "bug" within a "cage"

Analysis of the cytoskeleton in morphogenetically active plant cells allows us to propose a unified concept for the structural organization of eukaryotic cells. Their cytoarchitecture is determined by two principal structural complexes: nucleus-microtubule-based cell bodies ("bugs") and plasma-membrane-F-actin-based cell periphery complexes ("cages"). There are dynamic interactions between each of these entities in response to extracellular and intracellular signals. In the case of the cell body, these signals determine its polarization, rotation and migration. Interactions between cell body and cell periphery complexes determine cell growth polarity and morphogenesis throughout the eukaryotic kingdom.

Cytokinesis without myosin II

The ability of substrate-anchored Dictyostelium cells to divide without myosin II has opened the possibility of analysing the formation of cleavage furrows in the absence of a contractile ring made of filamentous myosin and actin. Similar possibilities exist in mutants of budding yeast and, less strictly, also in drug-treated mammalian cells. Myosin-II-independent activities in Dictyostelium include the microtubule-induced programming of the cell surface into ruffling areas and regions that are converted into a concave furrow, as well as the translocation of cortexillins and cross-linked membrane proteins towards the cleavage furrow. A centripetal flow of actin filaments followed by their disassembly in the cleavage furrow is proposed to underlie the translocation.

Relative microelastic mapping of living cells by atomic force microscopy

The spatial and temporal changes of the mechanical properties of living cells reflect complex underlying physiological processes. Following these changes should provide valuable insight into the biological importance of cellular mechanics and their regulation. The tip of an atomic force microscope (AFM) can be used to indent soft samples, and the force versus indentation measurement provides information about the local viscoelasticity. By collecting force-distance curves on a time scale where viscous contributions are small, the forces measured are dominated by the elastic properties of the sample. We have developed an experimental approach, using atomic force microscopy, called force integration to equal limits (FIEL) mapping, to produce robust, internally quantitative maps of relative elasticity. FIEL mapping has the advantage of essentially being independent of the tip-sample contact point and the cantilever spring constant. FIEL maps of living Madine-Darby canine kidney (MDCK) cells show that elasticity is uncoupled from topography and reveal a number of unexpected features. These results present a mode of high-resolution visualization in which the contrast is based on the mechanical properties of the sample.

Microtubule polymerization dynamics

The polymerization dynamics of microtubules are central to their biological functions. Polymerization dynamics allow microtubules to adopt spatial arrangements that can change rapidly in response to cellular needs and, in some cases, to perform mechanical work. Microtubules utilize the energy of GTP hydrolysis to fuel a unique polymerization mechanism termed dynamic instability. In this review, we first describe progress toward understanding the mechanism of dynamic instability of pure tubulin and then discuss the function and regulation of microtubule dynamic instability in living cells.

Prosome cytodistribution relative to desmin and actin filaments in dividing C2.7 myoblasts and during myotube formation in vitro

Prosomes constitute the multicatalytic proteinase (MCP) core of the 26S proteasomes, but were first observed as subcomplexes of untranslated mRNP; this suggests that they play a putative role in the control of protein biosynthesis in addition to their catabolic enzymatic function. In previous investigations it was shown that some prosomes colocalize with the intermediate filaments (IF) of the cytoskeleton, of the cytokeratin type in epithelial cells, and of the vimentin type in fibroblasts. Studies on adult rat muscle carried out with prosome-specific monoclonal antibodies (p-mAbs) have shown, surprisingly, that specific types of prosomes predominantly occupy a particular zone in between the M and the Z lines of the sarcomeric structure. The data presented here show that the subunit composition of prosomes changes when the dividing C2.7 myoblasts fuse into myotubes. We show furthermore that, in dividing C2.7 myoblasts, prosomes colocalize with the desmin network as well as with that of actin, in a distribution that changes with the subunit pattern of the prosomes investigated by individual p-mAbs. Surprisingly, when myogenic fusion is induced, specific types of prosomes move first to the nuclei; later on, they reappear in the cytoplasm. There, superimposing initially onto the reorganizing desmin filaments that run from one pole of the prefusion myoblast to the other, prosomes gradually colocalize with the actin fibers in the fusing myotubes, finally forming a "pearl on a string" pattern. These results are discussed in relation to parallel observations of prosome distribution between the actin and IF networks not only in epithelial cells but also in fusing muscle satellite cells, which made it possible to monitor the complete buildup of the sarcomeric structure.