Solid-state NMR studies of the dynamics and structure of mouse keratin intermediate filaments

Determination of methyl order parameters using solid state NMR under off magic angle spinning

Quantification of dipolar couplings in biological solids is important for the understanding of dynamic processes. Under Magic Angle Spinning (MAS), order parameters are normally obtained by recoupling of anisotropic interactions involving the application of radio frequency pulses. We have recently shown that amide backbone order parameters can be estimated accurately in a spin-echo experiment in case the rotor spinning angle is slightly mis-calibrated. In this work, we apply this method to determine methyl order parameters in a deuterated sample of the SH3 domain of chicken α-spectrin in which the methyl containing side chains valine and leucine are selectively protonated.

Externalized Keratin 8: A Target at the Interface of Microenvironment and Intracellular Signaling in Colorectal Cancer Cells

Accumulating evidence supports the remarkable presence at the membrane surface of cancer cells of proteins, which are normally expressed in the intracellular compartment. Although these proteins, referred to as externalized proteins, represent a highly promising source of accessible and druggable targets for cancer therapy, the mechanisms via which they impact cancer biology remain largely unexplored. The aim of this study was to expose an externalized form of cytokeratin 8 (eK8) as a key player of colorectal tumorigenesis and characterize its mode of action. To achieve this, we generated a unique antagonist monoclonal antibody (D-A10 MAb) targeting an eight-amino-acid-long domain of eK8, which enabled us to ascertain the pro-tumoral activity of eK8 in both KRAS-mutant and wild-type colorectal cancers (CRC). We showed that this pro-tumoral activity involves a bidirectional eK8-dependent control of caspase-mediated apoptosis in vivo and of the plasminogen-induced invasion process in cellulo. Furthermore, we demonstrated that eK8 is anchored at the plasma membrane supporting this dual function. We, therefore, identified eK8 as an innovative therapeutic target in CRC and provided a unique MAb targeting eK8 that displays anti-neoplastic activities that could be useful to treat CRC, including those harboring KRAS mutations.

Modeling the Structure of Keratin 1 and 10 Terminal Domains and their Misassembly in Keratoderma

The terminal domains of suprabasal keratins of the skin epithelium are very resistant to evidence-based structural analysis because of their inherent flexibility and lack of predictable structure. We present a model for the structure and interactions of the head and tail domains of epidermal keratins 1 and 10, based on all-atom 3D simulations of keratin primary amino acid sequences, and tyrosine phosphorylation predictions, extracted from published databases. We observed that keratin 1 and 10 end domains are likely to form a tetrameric terminal domain complex incorporating a reversibly extendable region potentially acting as a molecular spring. This structure is formed by intermolecular stacking of aromatic residues, which would spatially constrain the keratin 1/keratin 10 end domains to allow filament compaction and bundling, whilst also retaining extensibility to ensure flexibility of the keratin filament network in the differentiating epidermis. The tetrameric terminal domain complex model may also help to elucidate the effects of mutations in the end domains of suprabasal keratins and so contribute to understanding of the mechanisms leading to keratinopathies such as striate palmoplantar keratoderma, as reported in this study.

Electron paramagnetic resonance analysis of the vimentin tail domain reveals points of order in a largely disordered region and conformational adaptation upon filament assembly

Very little data have been reported that describe the structure of the tail domain of any cytoplasmic intermediate filament (IF) protein. We report here the results of studies using site directed spin labeling and electron paramagnetic resonance (SDSL-EPR) to explore the structure and dynamics of the tail domain of human vimentin in tetramers (protofilaments) and filaments. The data demonstrate that in contrast to the vimentin head and rod domains, the tail domains are not closely apposed in protofilaments. However, upon assembly into intact IFs, several sites, including positions 445, 446, 451, and 452, the conserved "beta-site," become closely apposed, indicating dynamic changes in tail domain structure that accompany filament elongation. No evidence is seen for coiled-coil structure within the region studied, in either protofilaments or assembled filaments. EPR analysis also establishes that more than half of the tail domain is very flexible in both the assembly intermediate and the intact IF. However, by positioning the spin label at distinct sites, EPR is able to identify both the rod proximal region and sites flanking the beta-site motif as rigid locations within the tail. The rod proximal region is well assembled at the tetramer stage with only slight changes occurring during filament elongation. In contrast, at the beta site, the polypeptide backbone transitions from flexible in the assembly intermediate to much more rigid in the intact IF. These data support a model in which the distal tail domain structure undergoes significant conformational change during filament elongation and final assembly.

The basic keratin 10-binding domain of the virulence-associated pneumococcal serine-rich protein PsrP adopts a novel MSCRAMM fold

Streptococcus pneumoniae is a major human pathogen, and a leading cause of disease and death worldwide. Pneumococcal invasive disease is triggered by initial asymptomatic colonization of the human upper respiratory tract. The pneumococcal serine-rich repeat protein (PsrP) is a lung-specific virulence factor whose functional binding region (BR) binds to keratin-10 (KRT10) and promotes pneumococcal biofilm formation through self-oligomerization. We present the crystal structure of the KRT10-binding domain of PsrP (BR_187–385) determined to 2.0 Å resolution. BR_187–385 adopts a novel variant of the DEv-IgG fold, typical for microbial surface components recognizing adhesive matrix molecules adhesins, despite very low sequence identity. An extended β-sheet on one side of the compressed, two-sided barrel presents a basic groove that possibly binds to the acidic helical rod domain of KRT10. Our study also demonstrates the importance of the other side of the barrel, formed by extensive well-ordered loops and stabilized by short β-strands, for interaction with KRT10.

Characterization of stratum corneum molecular dynamics by natural-abundance ¹³C solid-state NMR

Despite the enormous potential for pharmaceutical applications, there is still a lack of understanding of the molecular details that can contribute to increased permeability of the stratum corneum (SC). To investigate the influence of hydration and heating on the SC, we record the natural-abundance (13)C signal of SC using polarization transfer solid-state NMR methods. Resonance lines from all major SC components are assigned. Comparison of the signal intensities obtained with the INEPT and CP pulse sequences gives information on the molecular dynamics of SC components. The majority of the lipids are rigid at 32°C, and those lipids co-exist with a small pool of mobile lipids. The ratio between mobile and rigid lipids increases with hydration. An abrupt change of keratin filament dynamics occurs at RH = 80-85%, from completely rigid to a structure with rigid backbone and mobile protruding terminals. Heating has a strong effect on the lipid mobility, but only a weak influence on the keratin filaments. The results provide novel molecular insight into how the SC constituents are affected by hydration and heating, and improve the understanding of enhanced SC permeability, which is associated with elevated temperatures and SC hydration.

Unique amino acid signatures that are evolutionarily conserved distinguish simple-type, epidermal and hair keratins

Keratins (Ks) consist of central α-helical rod domains that are flanked by non-α-helical head and tail domains. The cellular abundance of keratins, coupled with their selective cell expression patterns, suggests that they diversified to fulfill tissue-specific functions although the primary structure differences between them have not been comprehensively compared. We analyzed keratin sequences from many species: K1, K2, K5, K9, K10, K14 were studied as representatives of epidermal keratins, and compared with K7, K8, K18, K19, K20 and K31, K35, K81, K85, K86, which represent simple-type (single-layered or glandular) epithelial and hair keratins, respectively. We show that keratin domains have striking differences in their amino acids. There are many cysteines in hair keratins but only a small number in epidermal keratins and rare or none in simple-type keratins. The heads and/or tails of epidermal keratins are glycine and phenylalanine rich but alanine poor, whereas parallel domains of hair keratins are abundant in prolines, and those of simple-type epithelial keratins are enriched in acidic and/or basic residues. The observed differences between simple-type, epidermal and hair keratins are highly conserved throughout evolution. Cysteines and histidines, which are infrequent keratin amino acids, are involved in de novo mutations that are markedly overrepresented in keratins. Hence, keratins have evolutionarily conserved and domain-selectively enriched amino acids including glycine and phenylalanine (epidermal), cysteine and proline (hair), and basic and acidic (simple-type epithelial), which reflect unique functions related to structural flexibility, rigidity and solubility, respectively. Our findings also support the importance of human keratin 'mutation hotspot' residues and their wild-type counterparts.

Investigation of the content and of the distribution of chemical elements in human nails by SRXRF

The purpose of this investigation is to analyze 20 nails in individuals (and several persons) for the definition of how chemical elements distribute from nail to nail. The aim was to determine whether it will be rightful to take only one nail for the elemental analysis for the diagnostic of human state in future or not? Another purpose of the research is to analyze the elemental content of nails in temporal dynamic (in several persons). Analytical determinations of 20 nails of nine donors (healthy persons), nails of both hands and both feet were carried out. The analysis was performed by SRXRF. Symmetry of the elemental distribution in nails of right and left hands and right and left feet was found. The analysis of the distribution of chemical elements on the total area of a nail (55 points) was performed. The nail cutaway reflects adequately the distribution of several chemical elements over the nail plate area. In this study the elemental concentrations in nails of three donors in a 6-month period was determined. This study found the content of the chemical elements in donors' nails changes with time, individually.

Morphology and molecular mobility of fibrous hard alpha-keratins by 1H, 13C, and 129Xe NMR

The morphology and molecular mobility changes of the side chains for hard alpha-keratin due to oxidative and reductive/oxidative treatments for temperatures around the DSC denaturation peak were investigated by (1)H, (13)C, and (129)Xe NMR spectroscopy and (1)H spin diffusion. Proton wide-line spectra were used to obtain the phase composition (rigid, interface, and amorphous fractions) and molecular dynamics of each phase. Proton spin diffusion experiments using a double-quantum filter and initial rate approximation were employed to obtain the dependence of the rigid domain sizes on chemical treatments and denaturation temperatures. A drastic reduction in the rigid domain thickness takes place for the reductive/oxidative treatment. The keratin mobility gradient in the interfacial region at different denaturation temperatures was measured for hard alpha-keratin from (1)H spin diffusion data. (13)C CPMAS spectra were used to provide a detailed examination of the effects of the chemical treatments especially on the disulfide bonds. Thermally polarized (129)Xe spectra suggest the existence of voids in the hard alpha-keratin induced by the reductive and oxidative treatment. The surface of the hard alpha-keratin fiber surface is probed by the laser hyperpolarized (129)Xe. A qualitative model describing the changes induced in hard alpha-keratin protein by chemical transformation was developed and could be correlated with the changes in domain thickness, phase composition, and molecular dynamics.

Accurate measurement of methyl 13C chemical shifts by solid-state NMR for the determination of protein side chain conformation: the influenza a M2 transmembrane peptide as an example

The use of side chain methyl (13)C chemical shifts for the determination of the rotameric conformation of Val and Leu residues in proteins by solid-state NMR spectroscopy is described. Examination of the solution NMR stereospecifically assigned methyl groups shows significant correlation between the difference in the two methyl carbons' chemical shifts and the side chain conformation. It is found that alpha-helical and beta-sheet backbones cause different side chain methyl chemical shift trends. In alpha-helical Leu's, a relatively large absolute methyl (13)C shift difference of 2.89 ppm is found for the most populated mt rotamer (chi(1) = -60 degrees, chi(2) = 180 degrees), while a much smaller value of 0.73 ppm is found for the next populated tp rotamer (chi(1) = 180 degrees, chi(2) = 60 degrees). For alpha-helical Val residues, the dominant t rotamer (chi(1) = 180 degrees) has more downfield Cgamma2 chemical shifts than Cgamma1 by 1.71 ppm, while the next populated m rotamer (chi(1) = -60 degrees) shows the opposite trend of more downfield Cgamma1 chemical shift by 1.23 ppm. These significantly different methyl (13)C chemical shifts exist despite the likelihood of partial rotameric averaging at ambient temperature. We show that these conformation-dependent methyl (13)C chemical shifts can be utilized for side chain structure determination once the methyl (13)C resonances are accurately measured by double-quantum (DQ) filtered 2D correlation experiments, most notably the dipolar DQ to single-quantum (SQ) correlation technique. The advantage of the DQ-SQ correlation experiment over simple 2D SQ-SQ correlation experiments is demonstrated on the transmembrane peptide of the influenza A M2 proton channel. The methyl chemical shifts led to predictions of the side chain rotameric states for several Val and Leu residues in this tetrameric helical bundle. The predicted Val rotamers were further verified by dipolar correlation experiments that directly measure the chi(1) torsion angles. It was found that the chemical-shift-predicted side chain conformations are fully consistent with the direct torsion angle results; moreover, the methyl (13)C chemical shifts are sensitive to approximately 5 degrees changes in the chi(1) torsion angle due to drug binding.

Temperature-dependent dielectric properties of slightly hydrated horn keratin

With an aim to reveal the mechanism of protein-water interaction in a predominantly two phase model protein system this study investigates the frequency and temperature dependence of dielectric constant epsilon' and loss factor epsilon'' in cow horn keratin in the frequency range 30 Hz to 3 MHz and temperature range 30-200 degrees C at two levels of hydration. These two levels of hydration were achieved by exposing the sample to air at 50% relative humidity (RH) at ambient temperature and by evacuating the sample for 72 h at 105 degrees C. A low frequency dispersion (LFD) and an intermediate frequency alpha-dispersion were the two main dielectric responses observed in the air-dried sample. The LFD and the high frequency arm of the alpha-dispersion followed the same fractional power law of frequency. Within the framework of percolation cluster model these dispersions, respectively have been attributed to percolation of protons between and within the clusters of hydrogen-bonded water molecules bound to polar or ionizable protein components. The alpha-dispersion peak, which results from intra-cluster charge percolation conformed to Cole-Cole modified Debye equation. Temperature dependence of the dielectric constant in the air-dried sample exhibited peaks at 120 and 155 degrees C which have been identified as temperatures of onset of release of water bound to polar protein components in the amorphous and crystalline regions, respectively. An overall rise in the permittivity was observed above 175 degrees C, which has been identified as the onset of chain melting in the crystalline region of the protein.

Side chain and backbone dynamics of phospholamban in phospholipid bilayers utilizing 2H and 15N solid-state NMR spectroscopy

2H and 15N solid-state NMR spectroscopic techniques were used to investigate both the side chain and backbone dynamics of wild-type phospholamban (WT-PLB) and its phosphorylated form (P-PLB) incorporated into 1-palmitoyl-2-oleoyl-sn-glycerophosphocholine (POPC) phospholipid bilayers. 2H NMR spectra of site-specific CD3-labeled WT-PLB (at Leu51, Ala24, and Ala15) in POPC bilayers were similar under frozen conditions (-25 degrees C). However, significant differences in the line shapes of the 2H NMR spectra were observed in the liquid crystalline phase at and above 0 degrees C. The 2H NMR spectra indicate that Leu51, located toward the lower end of the transmembrane (TM) helix, shows restricted side chain motion, implying that it is embedded inside the POPC lipid bilayer. Additionally, the line shape of the 2H NMR spectrum of CD3-Ala24 reveals more side chain dynamics, indicating that this residue (located in the upper end of the TM helix) has additional backbone and internal side chain motions. 2H NMR spectra of both WT-PLB and P-PLB with CD3-Ala15 exhibit strong isotropic spectral line shapes. The dynamic isotropic nature of the 2H peak can be attributed to side chain and backbone motions to residues located in an aqueous environment outside the membrane. Also, the spectra of 15N-labeled amide WT-PLB at Leu51 and Leu42 residues showed only a single powder pattern component indicating that these two 15N-labeled residues located in the TM helix are motionally restricted at 25 degrees C. Conversely, 15N-labeled amide WT-PLB at Ala11 located in the cytoplasmic domain showed both powder and isotropic components at 25 degrees C. Upon phosphorylation, the mobile component contribution increases at Ala11. The 2H and 15N NMR data indicate significant backbone motion for the cytoplasmic domain of WT-PLB when compared to the transmembrane section.

Comparative sequence analysis and radiation hybrid mapping of the canine keratin 10 gene

The type I keratin, K10, is expressed in epidermal keratinocytes undergoing terminal differentiation to form the stratum corneum, a barrier essential for life. In order to facilitate the study of keratinization disorders in the dog, the sequence and mapping of KRT10 is reported. The coding region of KRT10 is 1707 bp and is comprised of eight exons. Although the length of KRT10 has been reported to be polymorphic in humans, this was not observed in the eight domestic dog breeds studied, although one wild canid displayed a size difference. The structure and sequence of this gene is highly conserved across mammalian species. Canine K10 had an 86% amino acid identity with the human gene. KRT10 was localized to the on-going canine radiation hybrid map to chromosome 9 in the type I keratin gene cluster.

Microdissection of the sequence and structure of intermediate filament chains

A large number of intermediate filament (IF) chains have now been sequenced. From these data, it has been possible to deduce the main elements of the secondary structure, especially those lying within the central rod domain of the molecule. These conclusions, allied to results obtained from crosslinking studies, have shown that at least four unique but related structures are adopted by the class of structures known generically as intermediate filaments: (1) epidermal and reduced trichocyte keratin; (2) oxidized trichocyte keratin; (3) desmin, vimentin, neurofilaments, and related Type III and IV proteins; and (4) lamin molecules. It would be expected that local differences in sequences of the proteins in these four groups would occur, and that this would ultimately relate to assembly. Site-directed mutagenesis and theoretical methods have now made it possible to investigate these ideas further. In particular, new data have been obtained that allow the role played by some individual amino acids or a short stretch of sequence to be determined. Among the observations catalogued here are the key residues involved in intra- and interchain ionic interactions, as well as those involved in stabilizing some modes of molecular aggregation; the structure and role of subdomains in the head and tail domains; the repeat sequences occurring along the length of the chain and their structural significance; trigger motifs in coiled-coil segments; and helix initiation and termination motifs that terminate the rod domain. Much more remains to be done, not least of which is gaining an increased understanding of the many subtle differences that exist between different IF chains at the sequence level.

Evidence for novel functions of the keratin tail emerging from a mutation causing ichthyosis hystrix

Unraveling the molecular basis of inherited disorders of epithelial fragility has led to understanding of the complex structure and function of keratin intermediate filaments. Keratins are organized as a central alpha-helical rod domain flanked by nonhelical, variable end domains. Pathogenic mutations in 19 different keratin genes have been identified in sequences corresponding to conserved regions at the beginning and end of the rod. These areas have been recognized as zones of overlap between aligned keratin proteins and are thought to be crucial for proper assembly of keratin intermediate filaments. Consequently, all keratin disorders of skin, hair, nail, and mucous membranes caused by mutations in rod domain sequences are characterized by perinuclear clumping of fragmented keratin intermediate filaments, thus compromising mechanical strength and cell integrity. We report here the first mutation in a keratin gene (KRT1) that affects the variable tail domain (V2) and results in a profoundly different abnormality of the cytoskeletal architecture leading to a severe form of epidermal hyperkeratosis known as ichthyosis hystrix Curth-Macklin. Structural analyses disclosed a failure in keratin intermediate filament bundling, retraction of the cytoskeleton from the nucleus, and failed translocation of loricrin to the desmosomal plaques. These data provide the first in vivo evidence for the crucial role of a keratin tail domain in supramolecular keratin intermediate filament organization and barrier formation.

The influence of storage time on the temperature dependence of the dc electrical conductivity of horn keratin

The direct current electrical conductivity of horn keratin was measured as a function of temperature, in the temperature range 290-480 K, with a constant heating rate, for samples stored for periods of 2-3 weeks and 5 months. The activation energy of charge conducting process was calculated. The longer storage time reduced the water content, and the electrical conductivity, but increased the activation energy at 290-320 K.

Structure of water, proteins, and lipids in intact human skin, hair, and nail

Raman spectroscopy is a nondestructive analytical method for determining the structure and conformation of molecular compounds. It does not require sample preparation or pretreatment. Recently, near-infrared Fourier transform Raman spectroscopy has emerged as being specially suited for investigations of biologic material. In this study, we obtained near-infrared Fourier transform Raman spectra of intact human skin, hair, nail, and stratum corneum. We disclosed major spectral differences in conformational behavior of lipids and proteins between normal skin, hair, and nail. The amide I and III band location indicated that the majority of proteins in all samples have the same secondary alpha-helix structure. Positions of (S-S) stretching bands of proteins revealed a higher stability of the disulfide bonds in the hair and the nail. Analysis of vibrations of protein -CH groups showed that in the hair and the nail the proteins are apparently highly folded, interacting with the surroundings only to a small degree. The position of lipid specific peaks in spectra of hair, nail, and stratum corneum suggested a highly ordered, lamellar crystalline lipid structure. A greater lipid fluidity was found in whole skin. Assessment of the structure of water clusters revealed that mainly bound water is present in the human skin, stratum corneum, and nail. In conclusion, structural changes of water, proteins, and lipids in intact skin and skin appendages may be analyzed by Raman spectroscopy. This technique may be used in the future in a noninvasive analysis of structural changes in molecular compounds in the skin, hair, and nail associated with different dermatologic diseases.

Identification of a novel keratin epitope: evidence for association between non-helical sub-domains L12 during filament assembly

Keratin filaments in simple epithelial cells are heteropolymers of keratin 8 (K8) and keratin 18 (K18) polypeptides. The assembly of these polypeptides into intermediate filaments is a complex multi-stage phenomenon that involves several levels of associations. These molecular associations are not very well characterized. Monoclonal antibodies (MAbs) with defined specificities can be used to probe these associations and to isolate various intermediates in the assembly pathway. Here we describe the specificity of a MAb LE65 that has been widely used in keratin expression studies. We report that the MAb LE65 does not recognize individual keratin polypeptides but it instead reacts with a complex of K8 with K18. The MAb also did not react with complexes of K8 or K18 with other keratins. By allowing the antibody to react with complexes reconstituted from keratin fragments plus the complementary keratin, we have mapped the MAb LE65 epitope on the L12 sub-domains of K18, residues 214-231, and K8, residues 234-265, which must associate together to achieve antibody binding. These results suggest that the non-helical linkers, L12, of complementary keratins associate directly during filament assembly. This would explain why microinjection of MAb LE65 has been shown to disrupt keratin filaments. Furthermore, it may also help to explain the mechanism of filament disruption in some skin blistering syndromes induced by spontaneous mutations in the L12 region.

Protein chains in hair and epidermal keratin IF: structural features and spatial arrangements

Over the past decade the progress made in characterising the structural hierarchy of both the hard and the epidermal keratin intermediate filaments has exceeded all expectations. The origin of much of this progress can be traced back to the quantity of amino acid sequence data that became available in the early/mid 1980s, and their interpretation in terms of a heterodimeric molecular structure. Subdomains were subsequently identified in both the rod and terminal domains, and now the roles of most of these have been determined in principle, if not yet fully in detail. TEM and STEM, together with very revealing crosslinking analyses have also allowed details to be determined of the mechanism by which molecules assemble into oligomers and oligomers into IF. It remains for the three-dimensional packing of keratin molecules in the IF to be elucidated, but even here progress is being made. A particularly exciting development over the last two or three years has been the establishment of the link between keratinopathies and single point nucleotide mutations in keratin genes. Furthermore, the clustering of mutation sites in regions involved in a key structural mode of molecular aggregation has provided, for the first time, an understanding of keratin diseases at the molecular level.

Biochemical, structural, and transglutaminase substrate properties of human loricrin, the major epidermal cornified cell envelope protein

Loricrin is the major protein of the cornified cell envelope of terminally differentiated epidermal keratinocytes which functions as a physical barrier. In order to understand its properties and role in cornified cell envelope, we have expressed human loricrin from a full-length cDNA clone in bacteria and purified it to homogeneity. We have also isolated loricrin from newborn mouse epidermis. By circular dichroism and fluorescence spectroscopy, the in vivo mouse and bacterially expressed human loricrins possess no alpha or beta structure but have some organized structure in solution associated with their multiple tyrosines and can be reversibly denatured by either guanidine hydrochloride or temperature. The transglutaminase (TGase) 1, 2, and 3 enzymes expressed during epidermal differentiation utilized loricrin in vitro as a complete substrate, but the types of cross-linking were different. The TGase 3 reaction favored certain lysines and glutamines by forming mostly intrachain cross-links, whereas TGase 1 formed mostly large oligomeric complexes by interchain cross-links involving different lysines and glutamines. Together, the glutamines and lysines used in vitro are almost identical to those seen in vivo. The data support a hypothesis for the essential and complementary roles of both TGase 1 and TGase 3 in cross-linking of loricrin in vivo. Failure to cross-link loricrin by TGase 1 may explain the phenotype of lamellar ichthyosis, a disease caused by mutations in the TGase 1 gene.

Lipids are covalently attached to rigid corneocyte protein envelopes existing predominantly as beta-sheets: a solid-state nuclear magnetic resonance study

C solid-state nuclear magnetic resonance at natural abundance was used to study isolated corneocyte envelopes from porcine stratum corneum. The presence of lipids covalently attached to the protein envelopes was detected by chemical shifts of methylene and methyl groups of the bound lipids. The corneocyte protein envelopes are rigid, as suggested by efficient 1H to 13C cross polarization and 13C spin-lattice relaxation studies. The chemical shift of the carbonyl carbons of the protein envelopes supports the prediction that the chemically bound lipid envelope is attached to proteins arranged predominantly in the beta-sheet conformation, allowing a dense palisade of ceramide molecules to form a water-impermeable external sheath.

Molecular analysis of elastic properties of the stratum corneum by solid-state 13C-nuclear magnetic resonance spectroscopy

To elucidate the precise molecular mechanisms underlying stratum corneum (SC) elasticity, we investigated the molecular dynamics of chemical residues within keratin fibers of human plantar SC under various conditions by cross polarization/magic angle spinning 13C-nuclear magnetic resonance. The intensities of nuclear magnetic resonance spectra responsible for amide carbonyl, C alpha methine, and side-chain aliphatic carbons in the intact SC decreased markedly with increasing water content of up to 30% in dry SC, and then remained constant at greater than 30%. Lipid extraction of intact SC with acetone/ether (1:1) did not induce any significant change in the nuclear magnetic resonance spectrum, whereas additional treatment with water, which released natural moisturizing factors (mainly amino acids), caused the SC to lose elasticity. The observed decrease in elasticity of the SC recovered after treatment with basic and neutral amino acids, but not after treatment with acidic amino acid. With the latter treatment, movement of amino acid molecules was significantly disturbed, suggesting a strong interaction with keratin fibers. Parallel studies of the complex elastic modulus of a pig SC sheet with a rheovibron also demonstrated that removal of natural moisturizing factor reduced the elasticity of the SC; this effect was also reversed by the application of basic and neutral amino acids, but not by the application of acidic amino acid. These findings suggest that structural keratin proteins, mainly consisting of 10-nm filaments, acquire their elasticity with the help of hydrated natural moisturizing factor via the reduction of intermolecular forces, probably through nonhelical regions between keratin fibers.

Structural features of keratin intermediate filaments

The first step in the assembly of a keratin intermediate filament (KIF) is the formation of a type I/type II heterodimer molecule in which two chains become aligned in parallel and close axial registration to form a flexible segmented alpha-helical coiled-coil rope 46 nm long. The segments of coiled-coil are interspersed by sequences that introduce irregularities of unknown structure. Here we have modeled two of these, the link L2 and the heptad discontinuity located near the middle of segment 2B. In a model for L2, the orientation of the coiled-coil structure is turned through about 180 degrees over the eight residue stretch constituting this link segment. In contrast, the heptad discontinuity in segment 2B would seem to result in only minimal distortion of the coiled-coil rope, contrary to previous expectations. Little is known about how the neighboring molecules are aligned and packed within the assembled KIF. Crosslinking experiments with KIF have determined that two neighboring molecules are aligned anti-parallel and axially in three ways, and predict that similarly-directed molecules could be overlapped by about 1 nm. The two-dimensional surface lattice resulting from these data predicts an axial periodicity of 22.6 nm, which in fact is visible by electron microscopy of shadowed KIF. Interestingly, most of the amino acid substitutions resulting from mutations in the keratin genes found in genodermatoses are clustered in this molecular overlap region. Although we do not yet know how the rows of antiparallel molecules fold in three dimensions to form an intact KIF, certain of the observed crosslinks could also occur between nearest neighbor parallel molecules across a four-molecule strand; that is, KIF may be built from bundles or protofibrils. These insights on molecular structure and molecular packing provide new constraints on models for KIF structure.

Vimentin's tail interacts with actin-containing structures in vivo

The tail domain of the intermediate filament (IF) protein vimentin is unnecessary for IF assembly in vitro. To study the role of vimentin's tail in vivo, we constructed a plasmid that directs the synthesis of a ‘myc-tagged’ version of the Xenopus vimentin-1 tail domain in bacteria. This polypeptide, mycVimTail, was purified to near homogeneity and injected into cultured Xenopus A6 cells. In these cells the tail polypeptide co-localized with actin even in the presence of cytochalasin. Two myc-tagged control polypeptides argue for the specificity of this interaction. First, a similarly myc-tagged lamin tail domain localizes to the nucleus, indicating that the presence of the myc tag did not itself confer the ability to co-localize with actin (Hennekes and Nigg (1994) J. Cell Sci. 107, 1019–1029). Second, a myc-tagged polypeptide with a molecular mass and net charge at physiological pH (i.e. -4) similar to that of the mycVimTail polypeptide, failed to show any tendency to associate with actin-containing structures, indicating that the interaction between mycVimTail and actin-containing structures was not due to a simple ionic association. Franke (1987; Cell Biol. Int. Rep. 11, 831) noted a similarity in the primary sequence between the tail of the type I keratin DG81A and vimentin. To test whether the DG81A tail interacted with actin-containing structures, we constructed and purified myc-tagged DG81A tail polypeptides. Unexpectedly, these keratin tail polypeptides were largely insoluble under physiological conditions and formed aggregates at the site of injection. While this insolubility made it difficult to determine if they associated with actin-containing structures, it does provide direct evidence that the tails of vimentin and DG81A differ dramatically in their physical properties. Our data suggest that vimentin's tail domain has a highly extended structure, binds to actin-containing structures and may mediate the interaction between vimentin filaments and microfilaments involved in the control of vimentin filament organization (Hollenbeck et al. (1989) J. Cell Sci. 92, 621; Tint et al. (1991) J. Cell Sci. 98, 375).

Characterization of multiple oligomeric vimentin intermediate filament units by transient electric birefringence measurements

In this work we have studied the structure of soluble vimentin units from which intermediate filaments (IFs) are built. Several oligomeric forms have been presented in the literature as IF "building blocks", but there is still no agreement on this matter. By comparing our data with various models as proposed in the literature we can favour certain models and reject others. Transient electric birefringence (TEB) measurements were performed from which information is obtained concerning electric and hydrodynamic properties of the particles under investigation. TEB decay analysis at pH 6.8 after 70 microseconds pulses (at 20 degrees C in aqueous solution) yielded three decay times: 1.1(+/- 0.3) microseconds, 4.0(+/- 1.0) microseconds and 20.0(+/- 5.0) microseconds, with amplitudes of 45% to 60%, 30% to 45%, and less than 10%, respectively. At pH 8.5 after 70 microseconds pulses, more than 90% of the TEB signal with the second decay time is obtained, while the remainder had a decay time of 15.0(+/- 4.0) microseconds. Only when the pulse duration was decreased, the fast decay time around 1 microsecond was observed, suggesting that only a minor fraction of the particles at this pH value causes such a short decay time. At both pH values, the TEB measurements indicated that, at least in part, the molecules are oriented by a permanent dipole moment. It will be shown that the shortest decay time originates from bent or flexible dimers, and the second decay time from particles with a length of 54 to 65 nm containing, at least in part, a relatively large overall dipole moment. The longest decay time is probably due to larger aggregates. These results are consistent with a model in which single dimers, antiparallel staggered tetramers and hexamers coexist. Alternatively, but less likely on the basis of literature data, a model of parallel in-register tetramers with a considerable length contribution of the head groups would fit our research.

Keratin and keratinization

A flood of new knowledge and discoveries in the basic science of keratins and keratinization has appeared in the past several years. This review summarizes this recent information with a focus on the epithelial keratin polypeptides, keratin intermediate filaments, keratohyaline granule proteins, cell envelope formation and cell envelope proteins, "soft" keratinization, true disorders of keratinization (i.e., epidermolysis bullosa simplex and epidermolytic hyperkeratosis), and disease and drug effects on keratinization.

Overexpression of human loricrin in transgenic mice produces a normal phenotype

The cornified cell envelope (CE) of terminally differentiating stratified squamous epithelial cells is a complex multiprotein assembly about 15 nm thick of which loricrin is a major component. We have produced transgenic mice bearing the human loricrin transgene in order to study the role of loricrin in CE assembly, structure, and function. By analyses of RNA and protein, we show that the human transgene is expressed in mouse epithelial tissues in an appropriate developmental manner but at an overall level about twice that of endogenous mouse loricrin. Thus the 20-kbp construct used contains all necessary regulatory elements. By immunogold electron microscopy, all of the expressed protein is incorporated into the CE. That no alternations were noted indicates that overproduction of the loricrin component of the CE does not affect the flexible structure or function of the epithelial tissues. Furthermore, these data imply that loricrin may be the last protein to be deposited onto, and thus lines, the intracellular surface of the CE, where it may be accessible to interact with the subjacent keratin intermediate-filament network.

Structure, function, and dynamics of keratin intermediate filaments

The recent widespread application of modern methods of structural biology, molecular biology, and molecular genetics has provided a wealth of new information on the structure and function of the KIF of the epidermis. One of the more surprising aspects of this work has been the realization of the dynamic behavior of the KIF in living cells. Perhaps one of the more exciting aspects has been the discovery and understanding of how simple, single-nucleotide-point mutations in the keratin proteins can cause defects in the KIF that in turn cause serious pathology in the epidermis. The serendipitous and coincident nature of these studies shows us how an integrated, multifaceted approach will be necessary to solve further fundamental questions and to devise useful therapeutic approaches for the management of diseases of cornification. I fully expect that these issues will advance rapidly in the near future.

The two size alleles of human keratin 1 are due to a deletion in the glycine-rich carboxyl-terminal V2 subdomain

Two size variants of the type II human keratin 1 protein chain, termed 1a and 1b, have been described previously. Using amplification of genomic DNA by the polymerase chain reaction and sequence analysis we show here that the difference between these two alleles is due to a deletion of 21 bp in sequences encoding the V2 subdomain. This deletion corresponds to an entire glycine loop of seven amino acids. Pedigree analysis showed that the alleles are inherited as normal Mendelian traits. No additional alleles were detected in a survey of 88 alleles from 44 unrelated individuals, and the allelic frequency of 1a and 1b was 0.61 and 0.39. To determine the molecular basis of inherited dermatoses it is preferable to perform genetic linkage studies utilizing candidate genes directly as polymorphic markers. The PCR-based keratin 1 alleles characterized here, together with previously described PCR-based size variants in the keratin 10 gene, provide useful markers for the keratin clusters on chromosome 12 and 17, respectively.

A leucine----proline mutation in the H1 subdomain of keratin 1 causes epidermolytic hyperkeratosis

Epidermolytic hyperkeratosis is an autosomal dominant disorder affecting the structural integrity of the suprabasal layers of human epidermis. We have recently documented in one family linkage of the disease phenotype to the cluster of type II keratins. We have now identified a leucine----proline amino acid substitution in the conserved H1 subdomain of keratin 1 that is present only in affected family members. Using a quantitative assay and electron microscopy with synthetic peptides, we show that, whereas the wild-type H1 peptide rapidly disassembles preformed keratin filaments in vitro, the mutant peptide does this far less efficiently. Therefore the mutation in keratin 1 is likely to cause defective keratin filaments and hence a defective cytoskeleton in the epidermal cells in vivo.

Steady state dynamics of intermediate filament networks

We have conducted experiments to examine the dynamic exchange between subunit and polymer of vimentin intermediate filaments (IF) at steady state through the use of xrhodamine-labeled vimentin in fluorescence recovery after photobleaching (FRAP) analysis. The xrhodamine-vimentin incorporated into the endogenous vimentin IF network after microinjection into fibroblasts and could be visualized with a cooled charge-coupled device (CCD) camera and digital imaging fluorescence microscopy. Bar shaped regions were bleached in the fluorescent IF network using a beam from an argon ion laser and the cells were monitored at various times after bleaching to assess recovery of fluorescence in the bleached zones. We determined that bleached vimentin fibers can recover their fluorescence over relatively short time periods. Vimentin fibers in living cells also can exhibit significant movements, but the recovery of fluorescence was not dependent upon movement of fibers. Fluorescence recovery within individual fibers did not exhibit any marked polarity and was most consistent with a steady state exchange of vimentin subunits along the lengths of IF.

Extensive size polymorphism of the human keratin 10 chain resides in the C-terminal V2 subdomain due to variable numbers and sizes of glycine loops

Existing data suggest that the human keratin 10 intermediate filament protein is polymorphic in amino acid sequence and in size. To precisely define the nature of the polymorphism, we have used PCR amplification and sequence analyses on DNA from several individuals including five with documented size variations of the keratin 10 protein. We found no variation in the N-terminal or rod domain sequences. However, we observed many variations in the V2 subdomain near the C terminus in glycine-rich sequences with a variation of as much as 114 base pairs (38 amino acids), but all individuals had either one or two variants. Our results show that (i) the keratin 10 system is far more polymorphic than previously realized, (ii) the polymorphism is restricted to insertions and deletions of the glycine-rich quasipeptide repeats that form the glycine-loop motif in the C-terminal domain, (iii) the polymorphism can be accounted for by simple allelic variations that segregate by normal Mendelian mechanisms, and (iv) the differently sized PCR products most likely represent different alleles of a single-copy gene per haploid genome.

Intermediate filament structure

In the past year, several new developments concerning the structure of intermediate filament proteins and their assembly into intact intermediate filaments have been made: the coiled-coil structure of a rod domain has been elucidated; the basis of the chain interaction and its role in intermediate filament assembly has been specified; the organization of nearest-neighbour molecules in keratin intermediate filaments has been determined; and the glycine loop structures of the terminal domains of epidermal keratin chains have been defined. In addition, mutations in intermediate filament chains that promote pathology have been reported for the first time.

Organization of coiled-coil molecules in native mouse keratin 1/keratin 10 intermediate filaments: evidence for alternating rows of antiparallel in-register and antiparallel staggered molecules

There is considerable diversity of opinion in the literature concerning the organization of two-chain coiled-coil molecules in intermediate filaments. I have reexplored this issue using the limited proteolysis paradigm with native mouse epidermal keratin intermediate filaments (KIF), consisting of keratins 1 and 10. KIF were harvested as cytoskeletal pellets, dissociated into subfilamentous forms at pH 9.8, 9.0, or 2.6, and were subjected to limited proteolytic digestion to recover alpha-helix-enriched particles that derived from the rod domains of the constituent chains, using conditions that do not promote reorganization of the constituent protein chains or coiled-coil molecules. The multichain particles were subjected to physicochemical analyses, amino acid sequencing, and electron microscopy in order to determine their composition, structure, and organization within the intact KIF. The results predict two principal modes of alignment: neighboring molecules may be aligned in register and antiparallel or staggered and antiparallel. From known structural constraints, this permits construction of a two-dimensional surface lattice for KIF which consists of alternating antiparallel rows of in-register and staggered molecules. These data establish the level of hierarchy at which the well-known antiparallelity and staggered features of KIF are introduced. This model supports the proposals of KIF structure based on theoretical considerations of ionic interactions scores (Crewther et al., 1983). When the KIF are dissociated at extremes of pH, this structural model allows for disruption along alternate axes; the in-register antiparallel alignment is seen only when KIF are dissociated at high pH values; below pH 9, only the staggered antiparallel alignment is seen. The process of molecule realignment especially in concentrated urea solutions indicates that the staggered antiparallel alignment is the more thermodynamically stable form in solution.

Glycine loops in proteins: their occurrence in certain intermediate filament chains, loricrins and single-stranded RNA binding proteins

Quasi-repetitive, glycine-rich peptide sequences are widespread in at least three distinct families of proteins: the keratins and other intermediate filament proteins, including nuclear lamins; loricrins, which are major envelope components of terminally differentiated epithelial cells; and single-stranded RNA binding proteins. We propose that such sequences comprise a new structural motif termed the 'glycine loop'. The defining characteristics of glycine loop sequences are: (1) they have the form x(y)n, where x is usually an aromatic or occasionally a long-chain aliphatic residue; y is usually glycine but may include polar residues such as serine, asparagine, arginine, cysteine, and rarely other residues; and the value of n is highly variable, ranging from 1 to 35 in examples identified to date. (2) Glycine-loop-containing domains are thought to form when at least two and to date, as many as 18, such quasi-repeats are configured in tandem, so that the entire domain in a protein may be 50-150 residues long. (3) The average value of n, the pattern of residues found in the x position and the non-glycine substitutions in the y position appear to be characteristic of a given glycine loop containing domain, whereas the actual number of repeats is less constrained. (4) Glycine loop sequences display a high degree of evolutionary sequence variability and even allelic variations among different individuals of the same vertebrate species. (5) Glycine loop sequences are expected to be highly flexible, but possess little other regular secondary structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of kinase activation in the receptor for colony-stimulating factor 1

Receptor tyrosine kinases remain dormant until activated by ligand binding to the extracellular domain. Two mechanisms have been proposed for kinase activation: (i) ligand binding to the external domain of a receptor monomer may induce a conformational change that is transmitted across the cell membrane (intramolecular model) or (ii) the ligand may facilitate oligomerization, thereby allowing interactions between the juxtaposed kinase domains (intermolecular model). The receptor for colony-stimulating factor 1 was used to test these models. Large insertions at the junction between the external and transmembrane domains of the receptor, introduced by site-directed mutagenesis of the cDNA, were positioned to isolate the external domain and prevent transmembrane conformational propagation while allowing for receptor oligomerization. Such mutant receptors were expressed on the cell surface, bound ligand with high affinity, exhibited ligand-stimulated autophosphorylation, and signaled mitogenesis and cellular proliferation in the presence of ligand. A second experimental strategy directly tested the intermolecular model of ligand activation. A hybrid receptor composed of the external domain of human glycophorin A and the transmembrane and cytoplasmic domains of the colony-stimulating factor 1 receptor exhibited anti-glycophorin antibody-induced kinase activity that supported mitogenesis. Our data strongly support a mechanism of receptor activation based on ligand-induced receptor oligomerization.

Phosphorylation modulates keratin structure

Bovine hoof keratin was shown to be a substrate for cAMP-dependent protein kinase using [gamma-32P]ATP. Natural-abundance cross-polarization (CP) MAS 13C NMR was used to examine the effect of phosphorylation on keratin structure. When short contact times were used, phosphorylation was shown to increase the number of residues in the motionally restricted portions of the protein; i.e., a portion(s) of the protein became more rigid upon phosphorylation. Circular dichroism (CD) spectra showed a spectral shape characteristic of alpha helix for this keratin. Phosphorylation of the keratin by cAMP-dependent protein kinase resulted in a CD spectrum with the same shape but of greater apparent intensity. This may have been the result of an increase in the alpha-helical content of the protein. These data showed that the structure of keratin changed significantly upon phosphorylation by cAMP-dependent protein kinase. The region of the keratin molecule most likely to be altering its structure was the end of the molecule, which was involved in the formation of, and intracellular attachment of, intermediate filaments. Therefore, these data suggested that cAMP-dependent phosphorylation may produce significant changes in the intracellular organization of intermediate filaments. When the keratin was phosphorylated using cold ATP, magic-angle spinning (MAS) 31P nuclear magnetic resonance (NMR) revealed two resonances arising from the phosphorylation sites on the keratin. The more shielded resonance was shown to arise from cAMP-dependent protein kinase phosphorylation. Static 31P NMR measurements suggested that at least two classes of cAMP-dependent sites existed with the same isotropic 31P chemical shift; one was considerably motionally restricted with respect to the other.