The fibrillar substructure of keratin filaments unraveled

Enhancing Mexican lime (Citrus aurantifolia cv.) shelf life with innovative edible coatings: xanthan gum edible coating enriched with Spirulina platensis and pomegranate seed oils

BACKGROUND

The Mexican lime (Citrus aurantifolia cv.), widely consumed in Iran and globally, is known for its high perishability. Edible coatings have emerged as a popular method to extend the shelf life of fruits, with xanthan gum-based coatings being particularly favored for their environmental benefits. This study aims to evaluate the effectiveness of an edible coating formulated from xanthan gum, enriched with Spirulina platensis (Sp) and pomegranate seed oil (PSO), in improving the quality and reducing the weight loss of Mexican lime fruit under conditions of 20 ± 2 °C and 50-60% relative humidity.

RESULTS

Based on the results, the application of coatings was generally effective in reducing fruit weight loss, with the least weight loss observed in the xanthan gum 0.2%+ Spirulina platensis extract (1%) treatment. Additionally, the levels of total phenols and flavonoids in the treated fruits exceeded those in the control group, with xanthan gum 0.2%+ Spirulina platensis extract (1%) and xanthan gum 0.2% exhibiting the highest concentrations of these compounds. The antioxidant capacity of the fruits was also enhanced by the coatings, surpassing that of the control group, with xanthan gum 0.2%+ Spirulina platensis extract (1%) achieving the highest levels. The treatments significantly suppressed the activity of the polyphenol oxidase (PPO) enzyme, with xanthan gum 0.2% demonstrating the most potent inhibitory effect. Furthermore, the treatments resulted in increased activities of catalase (CAT) and peroxidase (POD) enzymes compared to the control. Except for xanthan gum 0.2%+ pomegranate seed oil (0.05%), all treatments maintained the fruit's greenness (a*) more effectively than the control.

CONCLUSIONS

Peel browning is a major factor contributing to the decline in quality and shelf life of lime fruit. The application of 0.1% and 0.2% xanthan gum coatings, as well as a combination of 0.2% xanthan gum and Spirulina platensis extract, significantly inhibited PPO activity and enhanced the activity of CAT and POD and phenolic compound in Mexican lime fruits stored at of 20 ± 2 °C for 24 days. Consequently, these treatments comprehensively preserved lime fruit quality by significantly reducing browning, maintaining green color, and preserving internal quality parameters such as TA, thereby enhancing both visual appeal and overall fruit quality.

Structural determinants of intermediate filament mechanics

Intermediate filaments (IFs) are integral to the cell cytoskeleton, supporting cellular mechanical stability. Unlike other cytoskeletal components, the detailed structure of assembled IFs has yet to be resolved. This review highlights new insights, linking the complex IF hierarchical assembly to their mechanical properties and impact on cellular functions. While we focus on vimentin IFs, we draw comparisons to keratins, showcasing the distinctive structural and mechanical features that underlie their unique mechanical responses.

Keratin filament mechanics and energy dissipation are determined by metal-like plasticity

Cell mechanics are determined by an intracellular biopolymer network, including intermediate filaments that are expressed in a cell-type-specific manner. A prominent pair of intermediate filaments are keratin and vimentin, as they are expressed by non-motile and motile cells, respectively. Therefore, the differential expression of these proteins coincides with a change in cellular mechanics and dynamic properties of the cells. This observation raises the question of how the mechanical properties already differ on the single filament level. Here, we use optical tweezers and a computational model to compare the stretching and dissipation behavior of the two filament types. We find that keratin and vimentin filaments behave in opposite ways: keratin filaments elongate but retain their stiffness, whereas vimentin filaments soften but retain their length. This finding is explained by fundamentally different ways to dissipate energy: viscous sliding of subunits within keratin filaments and non-equilibrium α helix unfolding in vimentin filaments.

Physico-Chemical Characterization of Keratin from Wool and Chicken Feathers Extracted Using Refined Chemical Methods

In this work, the characteristic structure of keratin extracted from two different kinds of industrial waste, namely sheep wool and chicken feathers, using the sulfitolysis method to allow film deposition, has been investigated. The structural and microscopic properties have been studied by means of scanning electron microscopy (SEM), Raman spectroscopy, atomic force microscopy (AFM), and infrared (IR) spectroscopy. Following this, small-angle X-ray scattering (SAXS) analysis for intermediate filaments has been performed. The results indicate that the assembly character of the fiber can be obtained by using the most suitable extraction method, to respond to hydration, thermal, and redox agents. The amorphous part of the fiber and medium range structure is variously affected by the competition between polar bonds (reversible hydrogen bonds) and disulfide bonds (DB), the covalent irreversible ones, and has been investigated by using fine structural methods such as Raman and SAXS, which have depicted in detail the intermediate filaments of keratin from the two different animal origins. The preservation of the secondary structure of the protein obtained does offer a potential for further application of the waste-obtained keratin in polymer films and, possibly, biocomposites.

Effect of mutations on the hydrophobic interactions of the hierarchical molecular structure and mechanical properties of epithelial keratin 1/10

Human epithelial keratin is an intermediate filament protein that serves as a backbone to maintain the stability of the cell nucleus and mechanical stability of the whole cells. The present study focused on two point mutations, F231L and S233L, of the 1B domain of keratin K 1/10 related to the rare genetic skin disease palmoplantar keratoderma (PPK). We used molecular dynamics simulation to study the effects of the mutations on various hierarchical structures, including heterodimers, tetramers, and octamers of the K1/10 1B domain at the atomic scale. The initial results demonstrated that the wild type and mutant proteins were highly similar at the dimer level but had different microstructures and mechanics at a higher-level assembly. A decrease in the hydrophobic interactions and hydrogen bonds at the terminus resulted in weakened mechanical properties of the tetramer and octamer of the F231L mutant. The asymmetrical structure of the S233L tetramer with an uneven distribution of the hydrogen bonds decreased its mechanical properties. However, the S233L mutation provided extra hydrophobic interactions between these mutated amino acid residues in the octamer, leading to improved mechanical properties. The results of the present study provided a deeper understanding of how the differences in point mutations induced the changes in the configuration and mechanical properties at the molecular scale. The differences in these properties may influence keratin assembly at the microscopic scale and ultimately cause diseases at the macroscopic scale.

Update of the keratin gene family: evolution, tissue-specific expression patterns, and relevance to clinical disorders

Intermediate filament (IntFil) genes arose during early metazoan evolution, to provide mechanical support for plasma membranes contacting/interacting with other cells and the extracellular matrix. Keratin genes comprise the largest subset of IntFil genes. Whereas the first keratin gene appeared in sponge, and three genes in arthropods, more rapid increases in keratin genes occurred in lungfish and amphibian genomes, concomitant with land animal-sea animal divergence (~ 440 to 410 million years ago). Human, mouse and zebrafish genomes contain 18, 17 and 24 non-keratin IntFil genes, respectively. Human has 27 of 28 type I "acidic" keratin genes clustered at chromosome (Chr) 17q21.2, and all 26 type II "basic" keratin genes clustered at Chr 12q13.13. Mouse has 27 of 28 type I keratin genes clustered on Chr 11, and all 26 type II clustered on Chr 15. Zebrafish has 18 type I keratin genes scattered on five chromosomes, and 3 type II keratin genes on two chromosomes. Types I and II keratin clusters-reflecting evolutionary blooms of keratin genes along one chromosomal segment-are found in all land animal genomes examined, but not fishes; such rapid gene expansions likely reflect sudden requirements for many novel paralogous proteins having divergent functions to enhance species survival following sea-to-land transition. Using data from the Genotype-Tissue Expression (GTEx) project, tissue-specific keratin expression throughout the human body was reconstructed. Clustering of gene expression patterns revealed similarities in tissue-specific expression patterns for previously described "keratin pairs" (i.e., KRT1/KRT10, KRT8/KRT18, KRT5/KRT14, KRT6/KRT16 and KRT6/KRT17 proteins). The ClinVar database currently lists 26 human disease-causing variants within the various domains of keratin proteins.

Assembly and recognition of keratins: A structural perspective

Keratins are one of the major components of cytoskeletal network and assemble into fibrous structures named intermediate filaments (IFs), which are important for maintaining the mechanical properties of cells and tissues. Over the past decades, evidence has shown that the functions of keratins go beyond providing mechanical support for cells, they interact with multiple cellular components and are widely involved in the pathways of cell proliferation, differentiation, motility and death. However, the structural details of keratins and IFs are largely missing and many questions remain regarding the mechanisms of keratin assembly and recognition. Here we briefly review the current structural models and assembly of keratins as well as the interactions of keratins with the binding partners, which may provide a structural view for understanding the mechanisms of keratins in the biological activities and the related diseases.

Structural heterogeneity of cellular K5/K14 filaments as revealed by cryo-electron microscopy

Keratin intermediate filaments are an essential and major component of the cytoskeleton in epithelial cells. They form a stable yet dynamic filamentous network extending from the nucleus to the cell periphery, which provides resistance to mechanical stresses. Mutations in keratin genes are related to a variety of epithelial tissue diseases. Despite their importance, the molecular structure of keratin filaments remains largely unknown. In this study, we analyzed the structure of keratin 5/keratin 14 filaments within ghost mouse keratinocytes by cryo-electron microscopy and cryo-electron tomography. By averaging a large number of keratin segments, we have gained insights into the helical architecture of the filaments. Two-dimensional classification revealed profound variations in the diameter of keratin filaments and their subunit organization. Computational reconstitution of filaments of substantial length uncovered a high degree of internal heterogeneity along single filaments, which can contain regions of helical symmetry, regions with less symmetry and regions with significant diameter fluctuations. Cross-section views of filaments revealed that keratins form hollow cylinders consisting of multiple protofilaments, with an electron dense core located in the center of the filament. These findings shed light on the complex and remarkable heterogenic architecture of keratin filaments, suggesting that they are highly flexible, dynamic cytoskeletal structures.

Neuroprotective effect of Betalain against AlCl3-induced Alzheimer's disease in Sprague Dawley Rats via putative modulation of oxidative stress and nuclear factor kappa B (NF-κB) signaling pathway

Alzheimer's disease (AD) is the most progressive form of neurodegenerative disease, which severely impairs cognitive function. Oxidative stress is identified to contribute to the mechanisms responsible for the pathogenesis of such neurodegenerative diseases. Aluminum is a potent neurotoxin for inducing oxidative stress associated with neurodegenerative diseases. The treatment for AD is limited; hence more treatment options are the need of the day. Betalain is known for its multitude of medicinal assets, including anti-inflammatory activity. Hence, this study was intended to investigate the possible protective effect of betalain against aluminum chloride (AlCl₃) induced AD on Sprague Dawley (SD) rats. AlCl₃ (100 mg/kg) was administrated orally to induce the AD in SD rats. The rats were supplemented with low and high betalain doses (10 mg/kg and 20 mg/kg) for four weeks. At the end of the experiment, the rats were subjected to behavioral examination and sacrificed to study the biochemical and histological parameters. The results showed attenuation of memory and learning capacity, suppression of lipid oxidation (MDA) through regulation of antioxidant content (SOD, CAT, and GSH) and inhibition of lactate dehydrogenase (LDH), nitric oxide (NO), acetylcholinesterase (AChE), and transmembrane protein (Na+K+ATPase) activity. In addition, the NF-ƙB associated mRNA expression (TNF-α IL-6, Il-1β, iNOS, COX-2) was decreased, as evidenced in histopathological results. The present investigation established that the betalain treatment ameliorated the AlCl₃ induced AD by modulating NF-κB pathway activation.

Recent insight into intermediate filament structure

Intermediate filaments (IFs) are key players in multiple cellular processes throughout human tissues. Their biochemical and structural properties are important for understanding filament assembly mechanisms, for interactions between IFs and binding partners, and for developing pharmacological agents that target IFs. IF proteins share a conserved coiled-coil central-rod domain flanked by variable N-terminal 'head' and C-terminal 'tail' domains. There have been several recent advances in our understanding of IF structure from the study of keratins, glial fibrillary acidic protein, and lamin. These include discoveries of (i) a knob-pocket tetramer assembly mechanism in coil 1B; (ii) a lamin-specific coil 1B insert providing a one-half superhelix turn; (iii) helical, yet flexible, linkers within the rod domain; and (iv) the identification of coil 2B residues required for mature filament assembly. Furthermore, the head and tail domains of some IFs contain low-complexity aromatic-rich kinked segments, and structures of IFs with binding partners show electrostatic surfaces are a major contributor to complex formation. These new data advance the connection between IF structure, pathologic mutations, and clinical diseases in humans.

Crystal Structure of Keratin 1/10(C401A) 2B Heterodimer Demonstrates a Proclivity for the C-Terminus of Helix 2B to Form Higher Order Molecular Contacts

We previously determined the crystal structure of the wild-type keratin 1/10 helix 2B heterodimer at 3.3 Å resolution. We proposed that the resolution of the diffraction data was limited due to the crystal packing effect from keratin 10 (K10) residue Cys401. Cys401K10 formed a disulfide-linkage with Cys401 from another K1/10 heterodimer, creating an "X-shaped" structure and a loose crystal packing arrangement. We hypothesized that mutation of Cys401K10 to alanine would eliminate the disulfide-linkage and improve crystal packing thereby increasing resolution of diffraction and enabling a more accurate side chain electron density map. Indeed, when a K10 Cys401Ala 2B mutant was paired with its native keratin 1 (K1) 2B heterodimer partner its x-ray crystal structure was determined at 2.07 Å resolution; the structure does not contain a disulfide linkage. Superposition of the K1/K10(Cys401Ala) 2B structure onto the wild-type K1/10 2B heterodimer structure had a root-mean-square-deviation of 1.88 Å; the variability in the atomic positions reflects the dynamic motion expected in this filamentous coiled-coil complex. The electrostatic, hydrophobic, and contour features of the molecular surface are similar to the lower resolution wild-type structure. We postulated that elimination of the disulfide linkage in the K1/K10(Cys401Ala) 2B structure could allow for the 2B heterodimers to bind/pack in the A22 tetramer configuration associated with mature keratin intermediate filament assembly. Analysis of the crystal packing revealed a half-staggered anti-parallel tetrameric complex of 2B heterodimers; however, their register is not consistent with models of the A22 mode of tetrameric alignment or prior biochemical cross-linking studies.

Lateral Subunit Coupling Determines Intermediate Filament Mechanics

The cytoskeleton is a composite network of three types of protein filaments, among which intermediate filaments (IFs) are the most extensible ones. Two very important IFs are keratin and vimentin, which have similar molecular architectures but different mechanical behaviors. Here we compare the mechanical response of single keratin and vimentin filaments using optical tweezers. We show that the mechanics of vimentin strongly depends on the ionic strength of the buffer and that its force-strain curve suggests a high degree of cooperativity between subunits. Indeed, a computational model indicates that in contrast to keratin, vimentin is characterized by strong lateral subunit coupling of its charged monomers during unfolding of α helices. We conclude that cells can tune their mechanics by differential use of keratin versus vimentin.

Human keratin 1/10-1B tetramer structures reveal a knob-pocket mechanism in intermediate filament assembly

To characterize keratin intermediate filament assembly mechanisms at atomic resolution, we determined the crystal structure of wild-type human keratin-1/keratin-10 helix 1B heterotetramer at 3.0 Å resolution. It revealed biochemical determinants for the A₁₁ mode of axial alignment in keratin filaments. Four regions on a hydrophobic face of the K1/K10-1B heterodimer dictated tetramer assembly: the N-terminal hydrophobic pocket (defined by L227^K1, Y230^K1, F231^K1, and F234^K1), the K10 hydrophobic stripe, K1 interaction residues, and the C-terminal anchoring knob (formed by F314^K1 and L318^K1). Mutation of both knob residues to alanine disrupted keratin 1B tetramer and full-length filament assembly. Individual knob residue mutant F314A^K1, but not L318A^K1, abolished 1B tetramer formation. The K1-1B knob/pocket mechanism is conserved across keratins and many non-keratin intermediate filaments. To demonstrate how pathogenic mutations cause skin disease by altering filament assembly, we additionally determined the 2.39 Å structure of K1/10-1B containing a S233L^K1 mutation linked to epidermolytic palmoplantar keratoderma. Light scattering and circular dichroism measurements demonstrated enhanced aggregation of K1^S233L/K10-1B in solution without affecting secondary structure. The K1^S233L/K10-1B octamer structure revealed S233L^K1 causes aberrant hydrophobic interactions between 1B tetramers.

Helical twist direction in the macrofibrils of keratin fibres is left handed

Macrofibrils, the main structural features within the cortical cells of mammalian hair shafts, are long composite bundles of keratin intermediate filaments (KIFs) embedded in a matrix of keratin-associated proteins. The KIFs can be helically arranged around the macrofibril central axis, making a cylinder within which KIF helical angle relative to macrofibril axis increases approximately linearly from macrofibril centre to edge. Mesophase-based self-assembly has been implicated in the early formation of macrofibrils, which first appear as liquid-crystal tactoids in the bulb of hair follicles. Formation appears to be driven initially by interactions between pre-keratinized KIFs. Differences in the nature of these KIF-KIF interactions could result in all macrofibrils being internally twisted in a single handedness, or a 50:50 mixture of handedness within each cortical cell. We data-mined 41 electron tomograms containing three-dimensional macrofibril data from previously published studies of hair and wool. In all 644 macrofibrils examined we found that within each tomogram all macrofibrils had the same handedness. We concluded that earlier reports of left- and right-handed macrofibrils were due to artefacts of imaging or data processing. A handedness marker was used to confirm (using re-imaged sections from earlier studies) that, in both human and sheep, all macrofibrils are left-handed around the macrofibril axis. We conclude that this state is universal within mammalian hair. This also supports the conclusion that the origin of macrofibril twist is the expression of chiral twisting forces between adjacent KIFs, rather than mesophase splay and bending forces relaxing to twisting forces acting within a confined space.

Engineering peroxiredoxin 3 to facilitate control over self-assembly

Oligomeric proteins are abundant in nature and are useful for a range of nanotechnological applications; however, a key requirement in using these proteins is controlling when and how they form oligomeric assemblies. Often, protein oligomerisation is triggered by various cellular signals, allowing for controllable oligomerisation. An example of this is human peroxiredoxin 3 (Prx), a stable protein that natively forms dimers, dodecameric rings, stacks, and tubes in response to a range of environmental stimuli. Although we know the key environmental stimuli for switching between different oligomeric states of Prx, we still have limited molecular knowledge and control over the formation and size of the protein's stacks and tubes. Here, we have generated a range of Prx mutants with either a decreased or knocked out ability to stack, and used both imaging and solution studies to show that Prx stacks through electrostatic interactions that are stabilised by a hydrogen bonding network. Furthermore, we show that altering the length of the polyhistidine tag will alter the length of the Prx stacks, with longer polyhistidine tags giving longer stacks. Finally, we have analysed the effect a variety of heavy metals have on the oligomeric state of Prx, wherein small transition metals like nickel enhances Prx stacking, while larger positively charged metals like tungstate ions can prevent Prx stacking. This work provides further structural characterisation of Prx, to enhance its use as a platform from which to build protein nanostructures for a variety of applications.

Mutation-induced alterations of intra-filament subunit organization in vimentin filaments revealed by SAXS

Vimentin intermediate filaments constitute a distinct filament system in mesenchymal cells that is instrumental for cellular mechanics and migration. In vitro, the rod-like monomers assemble in a multi-step, salt-dependent manner into micrometer long biopolymers. To disclose the underlying mechanisms further, we employed small angle X-ray scattering on two recombinant vimentin variants, whose assembly departs at strategic points from the normal assembly route: (i) vimentin with a tyrosine to leucine change at position 117; (ii) vimentin missing the non-α-helical carboxyl-terminal domain. Y117L vimentin assembles into unit-length filaments (ULFs) only, whereas ΔT vimentin assembles into filaments containing a higher number of tetramers per cross section than normal vimentin filaments. We show that the shape and inner structure of these mutant filaments is significantly altered. ULFs assembled from Y117L vimentin contain more, less tightly bundled vimentin tetramers, and ΔT vimentin filaments preserve the number density despite the higher number of tetramers per filament cross-section.

Viscoelastic properties of vimentin originate from nonequilibrium conformational changes

Structure and dynamics of living matter rely on design principles fundamentally different from concepts of traditional material science. Specialized intracellular filaments in the cytoskeleton permit living systems to divide, migrate, and grow with a high degree of variability and durability. Among the three filament systems, microfilaments, microtubules, and intermediate filaments (IFs), the physical properties of IFs and their role in cellular mechanics are the least well understood. We use optical trapping of individual vimentin filaments to investigate energy dissipation, strain history dependence, and creep behavior of stretched filaments. By stochastic and numerical modeling, we link our experimental observations to the peculiar molecular architecture of IFs. We find that individual vimentin filaments display tensile memory and are able to dissipate more than 70% of the input energy. We attribute these phenomena to distinct nonequilibrium folding and unfolding of α helices in the vimentin monomers constituting the filaments.

The Molecular Revolution in Cutaneous Biology: Keratin Genes and their Associated Disease: Diversity, Opportunities, and Challenges

The abundance of keratin proteins and the filaments they form in surface epithelia has long been appreciated. This said, the remarkable diversity of keratin proteins and the notion that they are encoded by one of the largest gene families in the human genome has come to the fore relatively recently, coinciding with the sequencing of whole genomes. This complexity has generated some practical challenges, notably in terms of nomenclature and tractability. More importantly, however, studies of keratin have seeded the discovery of the genetic basis for a large number of genodermatoses and continue to provide a unique perspective on and insight into epithelial cells and tissues, whether normal or diseased.

Intermediate Filaments: Structure and Assembly

Proteins of the intermediate filament (IF) supergene family are ubiquitous structural components that comprise, in a cell type-specific manner, the cytoskeleton proper in animal tissues. All IF proteins show a distinctly organized, extended α-helical conformation prone to form two-stranded coiled coils, which are the basic building blocks of these highly flexible, stress-resistant cytoskeletal filaments. IF proteins are highly charged, thus representing versatile polyampholytes with multiple functions. Taking vimentin, keratins, and the nuclear lamins as our prime examples, we present an overview of their molecular and structural parameters. These, in turn, document the ability of IF proteins to form distinct, highly diverse supramolecular assemblies and biomaterials found, for example, at the inner nuclear membrane, throughout the cytoplasm, and in highly complex extracellular appendages, such as hair and nails, of vertebrate organisms. Ultimately, our aim is to set the stage for a more rational understanding of the immediate effects that missense mutations in IF genes have on cellular functions and for their far-reaching impact on the development of the numerous IF diseases caused by them.

Complementary roles of specific cysteines in keratin 14 toward the assembly, organization, and dynamics of intermediate filaments in skin keratinocytes

We recently showed that inter-keratin disulfide bonding plays an important role in the assembly, organization, and dynamics of keratin intermediate filaments in skin keratinocytes. In particular, cysteine 367 located in the central α-helical rod domain of keratin 14 is necessary for the formation of a stable perinuclear network of keratin filaments (with type II partner keratin 5) in skin keratinocytes analyzed by static and live cell imaging. Here, we show that two additional cysteine residues located in the non-helical head domain of K14, Cys-4 and Cys-40, also participate in inter-keratin disulfide bonding and tandemly play a key role complementary to that of Cys-367 in the assembly, organization, and dynamics of keratin filaments in skin keratinocytes in primary culture. Analysis of K14 variants with single or multiple substitutions of cysteine residues points to a spatial and temporal hierarchy in how Cys-4/Cys-40 and Cys-367 regulate keratin assembly in vitro and filament dynamics in live keratinocytes in culture. Our findings substantiate the importance and complexity of a novel determinant, namely inter-keratin disulfide bonding, for the regulation of several aspects of keratin filaments in surface epithelia.

A role for disulfide bonding in keratin intermediate filament organization and dynamics in skin keratinocytes

Disulfide bonds involving cysteine 367 in K14 play a crucial role in the assembly, dynamics, and organization of K14-containing filaments in epidermal keratinocytes.

We recently reported that a trans-dimer, homotypic disulfide bond involving Cys367 in keratin 14 (K14) occurs in an atomic-resolution structure of the interacting K5/K14 2B domains and in keratinocyte cell lines. Here we show that a sizable fraction of the K14 and K5 protein pools participates in interkeratin disulfide bonding in primary cultures of mouse skin keratinocytes. By comparing the properties of wild-type K14 with a completely cysteine-free variant thereof, we found that K14-dependent disulfide bonding limited filament elongation during polymerization in vitro but was necessary for the genesis of a perinuclear-concentrated network of keratin filaments, normal keratin cycling, and the sessile behavior of the nucleus and whole cell in keratinocytes studied by live imaging. Many of these phenotypes were rescued when analyzing a K14 variant harboring a single Cys residue at position 367. These findings establish disulfide bonding as a novel and important mechanism regulating the assembly, intracellular organization, and dynamics of K14-containing intermediate filaments in skin keratinocytes.

The effects of solvent composition on the affinity of a peptide towards hair keratin: experimental and molecular dynamics data

Molecular dynamics simulations with a developed hair protofibril model demonstrated the ability to improve peptide uptake by hair shafts.

Interaction of plectin with keratins 5 and 14: dependence on several plectin domains and keratin quaternary structure

Plectin, a cytolinker of the plakin family, anchors the intermediate filament (IF) network formed by keratins 5 and 14 (K5/K14) to hemidesmosomes, junctional adhesion complexes in basal keratinocytes. Genetic alterations of these proteins cause epidermolysis bullosa simplex (EBS) characterized by disturbed cytoarchitecture and cell fragility. The mechanisms through which mutations located after the documented plectin IF-binding site, composed of the plakin-repeat domain (PRD) B5 and the linker, as well as mutations in K5 or K14, lead to EBS remain unclear. We investigated the interaction of plectin C terminus, encompassing four domains, the PRD B5, the linker, the PRD C, and the C extremity, with K5/K14 using different approaches, including a rapid and sensitive fluorescent protein-binding assay, based on enhanced green fluorescent protein-tagged proteins (FluoBACE). Our results demonstrate that all four plectin C-terminal domains contribute to its association with K5/K14 and act synergistically to ensure efficient IF binding. The plectin C terminus predominantly interacted with the K5/K14 coil 1 domain and bound more extensively to K5/K14 filaments compared with monomeric keratins or IF assembly intermediates. These findings indicate a multimodular association of plectin with K5/K14 filaments and give insights into the molecular basis of EBS associated with pathogenic mutations in plectin, K5, or K14 genes.

Stretching desmin filaments with receding meniscus reveals large axial tensile strength

Desmin forms the intermediate filament system of muscle cells where it plays important role in maintaining mechanical integrity and elasticity. Although the importance of intermediate-filament elasticity in cellular mechanics is being increasingly recognized, the molecular basis of desmin's elasticity is not fully understood. We explored desmin elasticity by molecular combing with forces calculated to be as large as 4nN. Average filament contour length increased 1.55-fold axial on average. Molecular combing together with EGTA-treatment caused the fragmentation of the filament into short, 60 to 120-nm-long and 4-nm-wide structures. The fragments display a surface periodicity of 38nm, suggesting that they are composed of laterally attached desmin dimers. The axis of the fragments may deviate significantly from that of the overstretched filament, indicating that they have a large orientational freedom in spite of being axially interconnected. The emergence of protofibril fragments thus suggests that the interconnecting head or tail domains of coiled-coil desmin dimers are load-bearing elements during axial stretch.

Self-consistent field theory for the interactions between keratin intermediate filaments

Background

Keratins are important structural proteins found in skin, hair and nails. Keratin Intermediate Filaments are major components of corneocytes, nonviable horny cells of the Stratum Corneum, the outermost layer of skin. It is considered that interactions between unstructured domains of Keratin Intermediate Filaments are the key factor in maintaining the elasticity of the skin.

Results

We have developed a model for the interactions between keratin intermediate filaments based on self-consistent field theory. The intermediate filaments are represented by charged surfaces, and the disordered terminal domains of the keratins are represented by charged heteropolymers grafted to these surfaces. We estimate the system is close to a charge compensation point where the heteropolymer grafting density is matched to the surface charge density. Using a protein model with amino acid resolution for the terminal domains, we find that the terminal chains can mediate a weak attraction between the keratin surfaces. The origin of the attraction is a combination of bridging and electrostatics. The attraction disappears when the system moves away from the charge compensation point, or when excess small ions and/or NMF-representing free amino acids are added.

Conclusions

These results are in concordance with experimental observations, and support the idea that the interaction between keratin filaments, and ultimately in part the elastic properties of the keratin-containing tissue, is controlled by a combination of the physico-chemical properties of the disordered terminal domains and the composition of the medium in the inter-filament region.

Plasticity of intermediate filament subunits

Intermediate filaments (IFs) assembled in vitro from recombinantly expressed proteins have a diameter of 8–12 nm and can reach several micrometers in length. IFs assemble from a soluble pool of subunits, tetramers in the case of vimentin. Upon salt addition, the subunits form first unit length filaments (ULFs) within seconds and then assembly proceeds further by end-to-end fusion of ULFs and short filaments. So far, IF subunits have mainly been observed by electron microscopy of glycerol sprayed and rotary metal shadowed specimens. Due to the shear forces during spraying the IF subunits appear generally as straight thin rods. In this study, we used atomic force microscopy (AFM), cryo-electron microscopy (cryo-EM) combined with molecular modeling to investigate the conformation of the subunits of vimentin, desmin and keratin K5/K14 IFs in various conditions. Due to their anisotropic shape the subunits are difficult to image at high resolution by cryo-EM. In order to enhance contrast we used a cryo-negative staining approach. The subunits were clearly identified as thin, slightly curved rods. However the staining agent also forced the subunits to aggregate into two-dimensional networks of dot-like structures. To test this conformational change further, we imaged dried unfixed subunits on mica by AFM revealing a mixture of extended and dot-like conformations. The use of divalent ions such as calcium and magnesium, as well as glutaraldehyde exposure favored compact conformations over elongated ones. These experimental results as well as coarse-grained molecular dynamics simulations of a vimentin tetramer highlight the plasticity of IF subunits.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Three-dimensional cryo-electron microscopy on intermediate filaments

Together with microtubules and actin filaments (F-actin), intermediate filaments (IFs) form the cytoskeleton of metazoan cells. However, unlike the other two entities that are extremely conserved, IFs are much more diverse and are grouped into five different families. In contrast to microtubules and F-actin, IFs do not exhibit a polarity, which may be the reason that no molecular motors travel along them. The molecular structure of IFs is less well resolved than that of the other cytoskeletal systems. This is partially due to their functional variability, tissue-specific expression, and their intrinsic structural properties. IFs are composed mostly of relatively smooth protofibrils formed by antiparallel arranged α-helical coiled-coil bundles flanked by small globular domains at either end. These features make them difficult to study by various electron microscopy methods or atomic force microscopy (AFM). Furthermore, the elongated shape of monomeric or dimeric IF units interferes with the formation of highly ordered three-dimensional (3-D) crystals suitable for atomic resolution crystallography. So far, most of the data we currently have on IF macromolecular structures come from electron microscopy of negatively stained samples, and fragmented α-helical coiled-coil units solved by X-ray diffraction. In addition, AFM allows the observation of the dynamic states of IFs in solution and delivers a new view into the assembly properties of IFs. Here, we discuss the applicability of cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET) for the field. Both methods are strongly related and have only recently been applied to IFs. However, cryo-EM revealed distinct new features within IFs that have not been seen before, and cryo-ET adds a 3-D view of IFs revealing the path and number of protofilaments within the various IF assemblies.

Self-organization of keratin intermediate filaments into cross-linked networks

Keratins, the largest subgroup of intermediate filament (IF) proteins, form a network of 10-nm filaments built from type I/II heterodimers in epithelial cells. A major function of keratin IFs is to protect epithelial cells from mechanical stress. Like filamentous actin, keratin IFs must be cross-linked in vitro to achieve the high level of mechanical resilience characteristic of live cells. Keratins 5 and 14 (K5 and K14), the main pairing occurring in the basal progenitor layer of epidermis and related epithelia, can readily self-organize into large filament bundles in vitro and in vivo. Here, we show that filament self-organization is mediated by multivalent interactions involving distinct regions in K5 and K14 proteins. Self-organization is determined independently of polymerization into 10-nm filaments, but involves specific type I–type II keratin complementarity. We propose that self-organization is a key determinant of the structural support function of keratin IFs in vivo.

Intermediate filament scaffolds fulfill mechanical, organizational, and signaling functions in the cytoplasm

Intermediate filaments (IFs) are cytoskeletal polymers whose protein constituents are encoded by a large family of differentially expressed genes. Owing in part to their properties and intracellular organization, IFs provide crucial structural support in the cytoplasm and nucleus, the perturbation of which causes cell and tissue fragility and accounts for a large number of genetic diseases in humans. A number of additional roles, nonmechanical in nature, have been recently uncovered for IF proteins. These include the regulation of key signaling pathways that control cell survival, cell growth, and vectorial processes including protein targeting in polarized cellular settings. As this discovery process continues to unfold, a rationale for the large size of this family and the context-dependent regulation of its members is finally emerging.

Structural analysis of vimentin and keratin intermediate filaments by cryo-electron tomography

Intermediate filaments are a large and structurally diverse group of cellular filaments that are classified into five different groups. They are referred to as intermediate filaments (IFs) because they are intermediate in diameter between the two other cytoskeletal filament systems that is filamentous actin and microtubules. The basic building block of IFs is a predominantly alpha-helical rod with variable length globular N- and C-terminal domains. On the ultra-structural level there are two major differences between IFs and microtubules or actin filaments: IFs are non-polar, and they do not exhibit large globular domains. IF molecules associate via a coiled-coil interaction into dimers and higher oligomers. Structural investigations into the molecular building plan of IFs have been performed with a variety of biophysical and imaging methods such as negative staining and metal-shadowing electron microscopy (EM), mass determination by scanning transmission EM, X-ray crystallography on fragments of the IF stalk and low-angle X-ray scattering. The actual packing of IF dimers into a long filament varies between the different families. Typically the dimers form so called protofibrils that further assemble into a filament. Here we introduce new cryo-imaging methods for structural investigations of IFs in vitro and in vivo, i.e., cryo-electron microscopy and cryo-electron tomography, as well as associated techniques such as the preparation and handling of vitrified sections of cellular specimens.

Dissecting the 3-D structure of vimentin intermediate filaments by cryo-electron tomography

Vimentin polymerizes via complex lateral interactions of coiled-coil dimers into long, flexible filaments referred to as intermediate filaments (IFs). Intermediate in diameter between microtubules and microfilaments, IFs constitute the third cytoskeletal filament system of metazoan cells. Here we investigated the molecular basis of the 3-D architecture of vimentin IFs by cryo-electron microscopy (cryo-EM) as well as cryo-electron tomography (Cryo-ET) 3-D reconstruction. We demonstrate that vimentin filaments in cross-section exhibit predominantly a four-stranded protofibrilar organization with a right-handed supertwist with a helical pitch of about 96 nm. Compact filaments imaged by cryo-EM appear surprisingly straight and hence appear very stiff. In addition, IFs exhibited an increased flexibility at sites of partial unraveling. This is in strong contrast to chemically fixed, negatively stained preparations of vimentin filaments that generally exhibit smooth bending without untwisting. At some point along the filament unraveling may be triggered and propagates in a cooperative manner so that long stretches of filaments appear to have unraveled rapidly in a coordinated fashion.

Monitoring intermediate filament assembly by small-angle x-ray scattering reveals the molecular architecture of assembly intermediates

Intermediate filaments (IFs), along with microtubules, microfilaments, and associated cross-bridging proteins, constitute the cytoskeleton of metazoan cells. While crystallographic data on the dimer representing the elementary IF "building block" have recently become available, little structural detail is known about both the mature IF architecture and its assembly pathway. Here, we have applied solution small-angle x-ray scattering to investigate the in vitro assembly of a 53-kDa human IF protein vimentin at pH 8.4 by systematically varying the ionic strength conditions, and complemented these experiments by electron microscopy and analytical ultracentrifugation. While a vimentin solution in 5 mM Tris.HCl (pH 8.4) contains predominantly tetramers, addition of 20 mM NaCl induces further lateral assembly evidenced by the shift of the sedimentation coefficient and yields a distinct octameric intermediate. Four octamers eventually associate into unit-length filaments (ULFs) that anneal longitudinally. Based on the small-angle x-ray scattering experiments supplemented by crystallographic data and additional structural constraints, 3D molecular models of the vimentin tetramer, octamer, and ULF were constructed. Within each of the three oligomers, the adjacent dimers are aligned exclusively in an approximately half-staggered antiparallel A(11) mode with a distance of 3.2-3.4 nm between their axes. The ULF appears to be a dynamic and a relatively loosely packed structure with a roughly even mass distribution over its cross-section.

Exploring the mechanical properties of single vimentin intermediate filaments by atomic force microscopy

Intermediate filaments (IFs), together with actin filaments and microtubules, compose the cytoskeleton. Among other functions, IFs impart mechanical stability to cells when exposed to mechanical stress and act as a support when the other cytoskeletal filaments cannot keep the structural integrity of the cells. Here we present a study on the bending properties of single vimentin IFs in which we used an atomic force microscopy (AFM) tip to elastically deform single filaments hanging over a porous membrane. We obtained a value for the bending modulus of non-stabilized IFs between 300 MPa and 400 MPa. Our results together with previous ones suggest that IFs present axial sliding between their constitutive building blocks and therefore have a bending modulus that depends on the filament length. Measurements of glutaraldehyde-stabilized filaments were also performed to reduce the axial sliding between subunits and therefore provide a lower limit estimate of the Young's modulus of the filaments. The results show an increment of two to three times in the bending modulus for the stabilized IFs with respect to the non-stabilized ones, suggesting that the Young's modulus of vimentin IFs should be around 900 MPa or higher.

Nanomechanical properties of desmin intermediate filaments

Desmin intermediate filaments play important role in the mechanical integrity and elasticity of muscle cells. The mechanisms of how desmin contributes to cellular mechanics are little understood. Here, we explored the nanomechanics of desmin by manipulating individual filaments with atomic force microscopy. In complex, hierarchical force responses we identified recurring features which likely correspond to distinct properties and structural transitions related to desmin's extensibility and elasticity. The most frequently observed feature is an initial unbinding transition that corresponds to the removal of approximately 45-nm-long coiled-coil dimers from the filament surface with 20-60 pN forces in usually two discrete steps. In tethers longer than 60 nm we most often observed force plateaus studded with bumps spaced approximately 16 nm apart, which are likely caused by a combination of protofilament unzipping, dimer-dimer sliding and coiled-coil-domain unfolding events. At high stresses and strains non-linear, entropic elasticity was dominant, and sometimes repetitive sawtooth force transitions were seen which might arise because of slippage within the desmin protofilament. A model is proposed in which mechanical yielding is caused by coiled-coil domain unfolding and dimer-dimer sliding/slippage, and strain hardening by the entropic elasticity of partially unfolded protofilaments.

In vivo formation steps of the hard alpha-keratin intermediate filament along a hair follicle: evidence for structural polymorphism

Several aspects of the intermediate filaments' molecular architecture remain mysterious despite decades of study. The growth process and the final architecture may depend on the physical, chemical, and biochemical environment. Aiming at clarifying this issue, we have revisited the structure of the human hair follicle by means of X-ray microdiffraction. We conclude that the histology-based growth zones along the follicle are correlated to the fine architecture of the filaments deduced from X-ray microdiffraction. Our analysis reveals the existence of two major polymorph intermediate filament architectures. Just above the bulb, the filaments are characterized by a diameter of 100 Angstroms and a low-density core. The following zone upwards is characterized by the lateral aggregation of the filaments into a compact network of filaments, by a contraction of their diameter (to 75 Angstroms) and by the setting up of a long-range longitudinal ordering. In the upper zone, the small structural change associated with the tissue hardening likely concerns the terminal domains. The architecture of the intermediate filament in the upper zones could be specific to hard alpha-keratin whilst the other architecture found in the lower zone could be representative for intermediate filaments in a different environment.

Exploring the mechanical behavior of single intermediate filaments

Intermediate filaments (IFs) are structural elements of eukaryotic cells with distinct mechanical properties. Tissue integrity is severely impaired, in particular in skin and muscle, when IFs are either absent or malfunctioning due to mutations. Our knowledge on the mechanical properties of IFs is mainly based on tensile testing of macroscopic fibers and on the rheology of IF networks. At the single filament level, the only piece of data available is a measure of the persistence length of vimentin IFs. Here, we have employed an atomic force microscopy (AFM) based protocol to directly probe the mechanical properties of single cytoplasmic IFs when adsorbed to a solid support in physiological buffer environment. Three IF types were studied in vitro: recombinant murine desmin, recombinant human keratin K5/K14 and neurofilaments isolated from rat brains, which are composed of the neurofilament triplet proteins NF-L, NF-M and NF-H. Depending on the experimental conditions, the AFM tip was used to laterally displace or to stretch single IFs on the support they had been adsorbed to. Upon applying force, IFs were stretched on average 2.6-fold. The maximum stretching that we encountered was 3.6-fold. A large reduction of the apparent filament diameter was observed concomitantly. The observed mechanical properties therefore suggest that IFs may indeed function as mechanical shock absorbers in vivo.

Characterization of the in vitro co-assembly process of the intermediate filament proteins vimentin and desmin: mixed polymers at all stages of assembly

We have investigated the co-assembly properties of the intermediate filament (IF) proteins vimentin and desmin. First, the soluble complexes formed by both proteins separately in 5 mM Tris-HCl, pH 8.4, were characterized by analytical ultracentrifugation. In both cases, s-values of around 5 S were obtained corresponding to the formation of tetramers. However, at pH 7.5 and in the presence of 1 mM EDTA, both proteins behaved quite differently; whereas vimentin sedimented at 7.2 S, desmin assembled into much larger complexes of about 13 S. A mixture of equimolar amounts of vimentin and desmin in 8 M urea yielded, after reconstitution into 5 mM Tris-HCl, pH 7.5, and 1 mM EDTA, complexes exhibiting a sharp peak at 10.9 S. This intermediate s-value indicated that co-assembly into a distinct new set of complexes had occurred. As judged by electron microscopy and viscometry, these mixtures assembled into IFs with characteristics similar to those of pure vimentin and desmin. Furthermore, when vimentin and desmin tetramers were mixed in 5 mM Tris-HCl, pH 8.4, and subsequently subjected to IF assembly conditions, again "hybrid" filaments were obtained. Most interestingly, after 10 min of assembly, mass-per-length (MPL) measurements by scanning transmission electron microscopy yielded IFs with an MPL-peak value of 36 +/- 5 kDa/nm, hence closer to that of vimentin IFs (33 +/- 4 kDa/nm) than to that of desmin IFs (48 +/- 8 kDa/nm). Finally, when unit length-filaments (ULF) of vimentin and desmin were mixed and assembled further, the diameters of individual mature IFs formed exhibited a significantly higher degree of width inhomogeneity along their length than vimentin and desmin IFs as might be expected for a modular mode of assembly. Last but not least, atomic force microscopy provided further direct evidence that desmin IFs are able to fuse end-to-end with vimentin IFs. In summary, we have shown that vimentin and desmin are able to co-assemble at the dimer, tetramer, ULF and even the mature IF level.

Investigation of the morphology of intermediate filaments adsorbed to different solid supports

Morphologically, glutaraldehyde-fixed and -dried intermediate filaments (IFs) appear flexible, and with a width of 8-12 nm when observed by electron microscopy. Sometimes, the filaments are even unraveled on the carbon-coated grid and reveal a protofilamentous architecture. In this study, we have used atomic force microscopy to further investigate the morphology of IFs in a more physiological environment. First, we have imaged hydrated glutaraldehyde-fixed IFs adsorbed to a graphite support. In such conditions, human vimentin and desmin IFs appeared compact with a height of 5-8 nm and revealed either a beading repeat or a helical morphology. Second, we have analyzed the architecture of hydrated vimentin, desmin, and neurofilament IFs adsorbed to mica, graphite, and hydrophilic glass without the presence of fixative. On mica, vimentin IFs had a height of only 3-5 nm, whereas desmin IFs appeared as 8-10 nm height filaments with a helical twist. Neurofilaments were 10-12 nm in height with a pronounced 30-50 nm beading along their length. On graphite, the different IFs were either not adsorbing properly or their architecture was modified yielding, for example, broad, flattened filaments. Finally, hydrophilic glass was the surface which seemed to best preserve the architecture of the three IFs, even if, in some cases, unraveled vimentin filaments were observed on this support. These results are straightening the idea that mature IFs are dynamic polymers in vitro and that IFs can be distinguished from each others by their physicochemical properties.

The intermediate filament architecture as determined by X-ray diffraction modeling of hard alpha-keratin

Despite investigation since the 1950s, the molecular architecture of intermediate filaments has not yet been fully elucidated. Reliable information about the longitudinal organization of the molecules within the filaments and about the lateral interfilament packing is now available, which is not the case for the transverse architecture. Interesting results were recently obtained from in vitro microscopy observations and cross-linking of keratin, desmin, and vimentin analyses. The structural features that emerge from these analyses could not be fully representative of the in vivo architecture because intermediate filaments are subject to polymorphism. To bring new light to the transverse intermediate filament architecture, we have analyzed the x-ray scattering equatorial profile of human hair. Its comparison with simulated profiles from atomic models of a real sequence has allowed results to be obtained that are representative of hard alpha-keratin intermediate filaments under in vivo conditions. In short, the alpha-helical coiled coils, which are characteristic of the central rod of intermediate filament dimers, are straight and not supercoiled into oligomers; the radial density across the intermediate filament section is fairly uniform; the coiled coils are probably assembled into tetrameric oligomers, and finally the oligomer positions and orientations are not regularly ordered. These features are discussed in terms of filament self-assembling and structural variability.

Morphological analysis of glutaraldehyde-fixed vimentin intermediate filaments and assembly-intermediates by atomic force microscopy

Atomic force microscopy (AFM) was used to study the morphology of vimentin intermediate filaments (IFs) and their assembly intermediates. At each time after initiation of IF assembly in vitro of recombinant mouse vimentin, the sample was fixed with 0.1% glutaraldehyde and then applied to AFM analysis. When mature vimentin IFs were imaged in air on mica, they appeared to have a width of approximately 28 nm, a height of approximately 4 nm and a length of several micrometers. Taking into account the probe tip's distortion effect, the exact width was evaluated to be approximately 25 nm, suggesting that the filaments flatten on the substrate rather than be cylindrical with a diameter of approximately 10 nm. Vimentin IFs in air clearly demonstrated approximately 21-nm repeating patterns along the filament axis. The three-dimensional profiles of vimentin IFs indicated that the characteristic patterns were presented by repeating segments with a convex surface. The repeating patterns close to 21 nm were also observed by AFM analysis in a physiological solution condition, suggesting that the segments along the filaments are an intrinsic substructure of vimentin IFs. In the course of IF assembly, assembly intermediates were analyzed in air. Many short filaments with a full-width and an apparent length of approximately 78 nm (evaluated length approximately 69 nm) were observed immediately after initiation of the assembly reaction. Interestingly, the short full-width filaments appeared to be composed of the four segments. Further incubation enabled the short full-width filaments to anneal longitudinally into longer filaments with a distinct elongation step of approximately 40 nm, which corresponds to the length of the two segments. To explain these observations, we propose a vimentin IF formation model in which vimentin dimers are supercoiling around the filament axis.

Intermediate filaments: molecular structure, assembly mechanism, and integration into functionally distinct intracellular Scaffolds

The superfamily of intermediate filament (IF) proteins contains at least 65 distinct proteins in man, which all assemble into approximately 10 nm wide filaments and are principal structural elements both in the nucleus and the cytoplasm with essential scaffolding functions in metazoan cells. At present, we have only circumstantial evidence of how the highly divergent primary sequences of IF proteins lead to the formation of seemingly similar polymers and how this correlates with their function in individual cells and tissues. Point mutations in IF proteins, particularly in lamins, have been demonstrated to lead to severe, inheritable multi-systemic diseases, thus underlining their importance at several functional levels. Recent structural work has now begun to shed some light onto the complex fine tuning of structure and function in these fibrous, coiled coil forming multidomain proteins and their contribution to cellular physiology and gene regulation.

Molecular and biophysical characterization of assembly-starter units of human vimentin

We have developed an assembly protocol for the intermediate filament (IF) protein vimentin based on a phosphate buffer system, which enables the dynamic formation of authentic IFs. The advantage of this physiological buffer is that analysis of the subunit interactions by chemical cross-linking of internal lysine residues becomes feasible. By this system, we have analyzed the potential interactions of the coiled-coil rod domains with one another, which are assumed to make a crucial contribution to IF formation and stability. We show that headless vimentin, which dimerizes under low salt conditions, associates into tetramers of the A(22)-type configuration under assembly conditions, indicating that one of the effects of increasing the ionic strength is to favor coil 2-coil 2 interactions. Furthermore, in order to obtain insight into the molecular interactions that occur during the first phase of assembly of full-length vimentin, we employed a temperature-sensitive variant of human vimentin, which is arrested at the "unit-length filament" (ULF) state at room temperature, but starts to elongate upon raising the temperature to 37 degrees C. Most importantly, we demonstrate by cross-linking analysis that ULF formation predominantly involves A(11)-type dimer-dimer interactions. The presence of A(22) and A(12) cross-linking products in mature IFs, however, indicates that major rearrangements do occur during the longitudinal annealing and radial compaction steps of IF assembly.

Assessing the flexibility of intermediate filaments by atomic force microscopy

Eukaryotic cells contain three cytoskeletal filament systems that exhibit very distinct assembly properties, supramolecular architectures, dynamic behaviour and mechanical properties. Microtubules and microfilaments are relatively stiff polar structures whose assembly is modulated by the state of hydrolysis of the bound nucleotide. In contrast, intermediate filaments (IFs) are more flexible apolar structures assembled from a approximately 45 nm long coiled-coil dimer as the elementary building block. The differences in flexibility that exist among the three filament systems have been described qualitatively by comparing electron micrographs of negatively stained dehydrated filaments and by directly measuring the persistence length of F-actin filaments (approximately 3-10 microm) and microtubules (approximately 1-8 mm) by various physical methods. However, quantitative data on the persistence length of IFs are still missing. Toward this goal, we have carried out atomic force microscopy (AFM) in physiological buffer to characterise the morphology of individual vimentin IFs adsorbed to different solid supports. In addition, we compared these images with those obtained by transmission electron microscopy (TEM) of negatively stained dehydrated filaments. For each support, we could accurately measure the apparent persistence length of the filaments, yielding values ranging between 0.3 microm and 1 microm. Making simple assumptions concerning the adsorption mechanism, we could estimate the persistence length of an IF in a dilute solution to be approximately 1 microm, indicating that the lower measured values reflect constraints induced by the adsorption process of the filaments on the corresponding support. Based on our knowledge of the structural organisation and mechanical properties of IFs, we reason that the lower persistence length of IFs compared to that of F-actin filaments is caused by the presence of flexible linker regions within the coiled-coil dimer and by postulating the occurrence of axial slipping between dimers within IFs.