The coiled coil of in vitro assembled keratin filaments is a heterodimer of type I and II keratins: use of site-specific mutagenesis and recombinant protein expression

Transcriptional regulation of the differentiation-linked human K4 promoter is dependent upon esophageal-specific nuclear factors

The stratified squamous epithelium comprises actively proliferating basal cells that undergo a program of differentiation accompanied by morphological, biochemical, and genetic changes. The transcriptional regulatory signals and the genes that orchestrate this switch from proliferation to differentiation can be studied through the keratin gene family. Given the localization of keratin 4 (K4) to the early differentiated suprabasal compartment and having previously demonstrated that targeted disruption of this gene in murine embryonic stem cells results in impairment of the normal differentiation program in esophageal and corneal epithelial cells, we studied the transcriptional regulation of the human K4 promoter. A panel of K4 promoter deletions were found in transient transfection assays to be predominantly active in esophageal and corneal cell lines. A critical cis-regulatory element resides between -163 and -140 bp and contains an inverted CACACCT motif. A site-directed mutated version of this motif within the K4 promoter renders it inactive, whereas the wild-type version is active in a heterologous promoter system. It specifically binds esophageal-specific zinc-dependent transcriptional factors. Our studies demonstrate that regulation of the human K4 promoter is in part mediated through tissue-specific transcriptional factors.

A novel mutation in the L12 domain of keratin 5 in the Köbner variant of epidermolysis bullosa simplex

We have identified a novel mutation within the linker L12 region of keratin 5 (K5) in a family with the Köbner variant of epidermolysis bullosa simplex. The pattern of inheritance of the disorder in this family is consistent with an autosomal dominant mode of transmission. Affected individuals develop extensive and generalized blistering at birth or early infancy but in later years clinical manifestations are largely confined to palmoplantar surfaces. Direct sequencing of polymerase chain reaction products revealed a T to C transition within codon 323 of K5 in affected individuals, resulting in a valine to alanine substitution of the seventh residue within the L12 linker domain. This mutation was not observed in unaffected family members or in 100 K5 alleles of unrelated individuals with normal skin. The other critical regions of K5 and K14 were unremarkable in this family except for common polymorphisms that have been previously described. The valine at position 7 of the L12 domain is absolutely conserved in all type II keratins, and in other intermediate filament subunits as well, which suggests that this residue makes an important contribution to filament integrity. Secondary structure analysis revealed that alanine at this position markedly reduces both the hydrophobicity and the beta-sheet nature of the L12 domain. This is the first report of a mutation at this position in an intermediate filament subunit and reinforces the importance of this region to filament biology.

Expression of a wool intermediate filament keratin transgene in sheep fibre alters structure

Alteration of the protein composition of the wool fibre via transgenesis with sheep wool keratin and keratin associated protein (KAP) genes may lead to production of fibre types with improved processing and wearing qualities. Using this approach, we have demonstrated that high level cortical-specific expression of a wool type II intermediate filament (IF) keratin gene, K2.10, leads to marked alterations in both the microstructure and macrostructure of the wool fibres, which have higher lustre and reduced crimp. Analysis of mRNA found reduced levels of transcripts from endogenous cortical type I (p < 0.05) and type II (p < 0.01) keratin IF genes and from the KAP8 (p < 0.001) and KAP2 (p < 0.01) gene families. Examination of protein composition revealed an altered ratio in the keratin type II protein family of the wool fibre cortex. Whilst the over-expressed K2.10 transgene product constituted the majority of keratin type II IF protein, it appeared unable to form heterodimers with much of the expressed endogenous keratin type I IF. In comparison with non-transgenic sheep, fewer IF microfibrils were visible in the cortical cells of fibres from transgenics. The combined effect on fibre structure was disruption of the formation of orthocortical and paracortical cells in the fibre cortex, a factor which could account for the reduction in fibre crimp. No effects upon transcript or protein levels, or fibre microstructure or macrostructure were observed in transgenic sheep expressing the transgene at lower levels, indicating that subtle changes to the gene expression profile in sheep wool follicles can be tolerated. The data here also illustrate that control over endogenous transcript levels in the cortex results when factors acting on the endogenous keratin type I, keratin type II and KAP gene sequences are sequestered by the active K2.10 transgene locus. Moreover, interference to a transcriptional hierarchy shared by keratin and KAP genes may occur prior to establishment of the orthocortical and paracortical compartments of the follicle cortex, at the level of the chromatin.

Protein-protein interactions between keratin polypeptides expressed in the yeast two-hybrid system

Keratin filaments are obligatory heteropolymers of type I and type II keratin polypeptides. Specific type I/type II pairs are coexpressed in vivo. In contrast, all type I/type II pairs assemble into filaments in vitro, but the different pairs have different stabilities as demonstrated by treatment with increasing concentrations of urea. We have used the yeast two-hybrid system to analyse type I/type II interactions in a cellular context. We measured interactions between two different keratin pairs and we confirm the findings that K6+K17 form very stable heterodimers whereas K8+K18 interactions were weaker. The deletion of head domains did not reduce the strength of type I/type II interactions. Rather, the affinities were increased and the differences between the two pairs were retained in headless mutants. These findings argue against a major role of the head domains in directing heterodimer interactions and in defining heterodimer stabilities.

Primary sequence, secondary structure, gene structure, and assembly properties suggests that the lens-specific cytoskeletal protein filensin represents a novel class of intermediate filament protein

The ocular lens fiber cell assembles a novel cytoskeletal element, the Beaded Filament, from CP49 and filensin, two proteins expressed only in the differentiated lens fiber cell. We report the primary sequence, secondary structural analysis, gene structure and Yeast Two Hybrid interaction data for human filensin, and develop a consensus model of filensin from the human and previously reported bovine and chicken filensin sequences. This consensus model, combined with gene structure and Yeast Two Hybrid studies establish that filensin is a member of the Intermediate Filament family of proteins. Specifically, filensin exhibits (1) divergence at amino acid sequence motifs otherwise highly conserved among intermediate filament proteins, (2) a loss of 29 amino acids from the central rod domain which is unique among cytoplasmic intermediate filament proteins, (3) an absence of sequence identity with any existing class of intermediate filament protein, (4) a gene structure unique among intermediate filament family, (5) an inability to dimerize with representatives of Type I, II, and III intermediate filament proteins. Thus, at each level of analysis, we find that filensin is similar to the consensus model of intermediate filament proteins, supporting our conclusion that filensin's relatedness to the IF family is not the consequence of convergent evolution. However, filensin also shows unique or extreme distinctions from the consensus intermediate filament protein at each level of analysis, indicating that filensin constitutes a novel class of IF protein. Some of filensin's unique features are incompatible with current models of IF assembly. Analysis of filensin gene structure suggests that the 29 amino acid reduction in the central rod domain was not the result of a single splice site mutation, the mechanism suggested for the transition between nuclear lamins and cytoplasmic intermediate filament proteins.

Lessons from keratin 18 knockout mice: formation of novel keratin filaments, secondary loss of keratin 7 and accumulation of liver-specific keratin 8-positive aggregates

Here, we report on the analysis of keratin 18 null mice. Unlike the ablation of K8, which together with K18 is expressed in embryonic and simple adult epithelia, K18 null mice are viable, fertile, and show a normal lifespan. In young K18 null mice, hepatocytes were completely devoid of keratin filaments. Nevertheless, typical desmosomes were formed and maintained. Old K18 null mice, however, developed a distinctive liver pathology with abnormal hepatocytes containing K8-positive aggregates. These stained positively for ubiquitin and MM120-1 and were identified as Mallory bodies, one hallmark of human alcoholic hepatitis. This is the first demonstration that the ablation of one keratin leads to the accumulation of its single partner. Another striking finding was the absence or drastic down regulation of K7 in several tissues despite its ongoing transcription. Moreover, K18 null mice revealed new insights in the filament-forming capacity of the tail-less K19 in vivo. Due to the unexpected secondary loss of K7, only K8/19 are expressed in the uterine epithelium of K18 null mice. Immunoelectron microscopy of this tissue demonstrated the presence of typical K8/19 IF, thus highlighting in vivo that K19 is a fully competent partner for K8.

A proline residue in the alpha-helical rod domain of type I keratin 16 destabilizes keratin heterotetramers

The type I keratins 14 (K14) and 16 (K16) are distinct in their assembly properties and their expression pattern despite a high degree of sequence identity. Understanding K16 function and regulation is of interest, given its strong induction in keratinocytes located at the wound edge after injury to stratified epithelia. We reported previously that, compared with K14, K16 forms unstable heterotetramers with either K5 or K6 as the type II keratin pairing partner (Paladini, R. D., Takahashi, K., Bravo, N. S., and Coulombe, P. A. (1996) J. Cell Biol. 132, 381-397). We show here that yet another related type I keratin, K17, forms stable heterotetramers with a variety of type II keratins, further accentuating the unique nature of K16. Analysis of chimeric K14-K16 proteins in a heterotetramer formation assay indicated that the instability determinant resides in a 220-amino acid segment within the alpha-helical rod domain of K16. Site-directed mutagenesis revealed that Pro188, an amino acid residue located in subdomain 1B of the rod, accounts quantitatively for the instability of K16-containing heterotetramers under denaturing conditions. In vitro polymerization studies suggest that the presence of Pro188 correlates with a reduction in assembly efficiency. In addition to their implications for the stable conformation of the keratin heterotetramers, these findings suggest that the tetramer-forming properties of K16 may influence its partitioning between the soluble and polymer pools, and hence contribute to its regulation in epithelial cells under resting and wound repair conditions.

Heterodimeric associations between neuronal intermediate filament proteins

Formation of protein dimers involving alpha-internexin, peripherin, and the neurofilament (NF) proteins NFH, NFM, and NFL was investigated by partial renaturation of various combinations of individually purified subunits in buffered 2 M urea. Oligomers that were formed were resolved by "blue" native electrophoresis (Schägger, H., Cramer, W. A., and von Jagow, G. (1994) Anal. Biochem. 217, 220-230) modified to include urea in the polyacrylamide gels. Combining this method with Western blot analysis, disulfide cross-linking, and SDS-polyacrylamide gel electrophoresis in the second dimension showed that NFL readily forms significant amounts of heterodimer with NFH, NFM, alpha-internexin, or peripherin in the presence of 2 M urea. alpha-Internexin and peripherin also formed heterodimers with NFH or NFM under these conditions. The modified version of blue native gel electrophoresis described here may be useful in monitoring the impact of post-translational modifications and mutations on the dimerization of intermediate filament proteins.

Isolation and chromosomal localization of a cornea-specific human keratin 12 gene and detection of four mutations in Meesmann corneal epithelial dystrophy

Keratin 12 (K12) is an intermediate-filament protein expressed specifically in corneal epithelium. Recently, we isolated K12 cDNA from a human corneal epithelial cDNA library and determined its full sequence. Herein, we present the exon-intron boundary structure and chromosomal localization of human K12. In addition, we report four K12 mutations in Meesmann corneal epithelial dystrophy (MCD), an autosomal dominant disorder characterized by intraepithelial microcysts and corneal epithelial fragility in which mutations in keratin 3 (K3) and K12 have recently been implicated. In the human K12 gene, we identified seven introns, defining eight individual exons that cover the coding sequence. Together the exons and introns span approximately 6 kb of genomic DNA. Using FISH, we found that the K12 gene mapped to 17q12, where a type I keratin cluster exists. In this study, four new K12 mutations (Arg135Gly, Arg135Ile, Tyr429Asp, and Leu140Arg) were identified in three unrelated MCD pedigrees and in one individual with MCD. All mutations were either in the highly conserved alpha-helix-initiation motif of rod domain 1A or in the alpha-helix-termination motif of rod domain 2B. These sites are essential for keratin filament assembly, suggesting that the mutations described above may be causative for MCD. Of particular interest, one of these mutations (Tyr429Asp), detected in both affected individuals in one of our pedigrees, is the first mutation to be identified within the alpha-helix-termination motif in type I keratin.

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.

Keratin intermediate filament dynamics in cell heterokaryons reveals diverse behaviour of different keratins

To study the dynamics of keratin intermediate filaments, we fused two different types of epithelial cells (PtK2 and BMGE+H) and studied how the keratins from the parental cells recombine and copolymerize to form the heterokaryon cytoskeleton. The behaviour of the keratins during this process was followed by immunofluorescence using specific antibodies. After fusion, the parental cytoskeletons undergo a depolymerization process most apparent in the region adjacent to the fusion area. The depolymerized subunits spread throughout the heterokaryon and copolymerize into a new hybrid cytoskeleton. The complete process is very rapid, occurring in 3–4 hours, thus demonstrating the highly dynamic nature of the keratin cytoskeleton. Although newly synthesised subunits contribute to the formation of the hybrid cytoskeleton, the process takes place with similar kinetics in the absence of protein synthesis, showing the dynamic nature of the keratins from pre-existing cytoskeletons. During this process, specific keratins behave differently. Keratins K8, K18, K5 and K10 are mobilised from the parental cytoskeletons and reassemble rapidly into the hybrid cytoskeleton (3–6 hours), whereas K14 requires a substantially longer period (9–24 hours). Thus, different keratins, even when they form part of the same heterodimeric/tetrameric complexes, as is the case for K5 and K14, exhibit different dynamics. This suggests that individual polypeptides or homopolymeric complexes rather than exclusively heterodimeric/ tetrameric subunits, as is currently thought, can also take part in keratin intermediate filament assembly and dynamics. Biochemical analysis performed in the absence of protein synthesis revealed greater amounts of K5 than of K14 in the soluble pool of BMGE+H cells. Crosslinking and immunoprecipitation experiments indicated an excess of monomeric K5, as well as of K5/K14 heterodimers and K5 homodimers in the soluble pool. These results are in agreement with the different dynamic behaviour of these keratins observed in immunofluorescence. On the contrary, the phosphorylation levels of K5 and K14 are similar in both the soluble pool and the polymerized fraction, suggesting that phosphorylation does not play an important role in the different dynamics displayed by these two proteins. In summary, our results demonstrate that, following fusion, the keratin intermediate filament network reshapes rather rapidly and that keratins are highly dynamic proteins, although this mobility depends on each particular polypeptide.

Keith R. Porter Lecture, 1996. Of mice and men: genetic disorders of the cytoskeleton

Since the time when I was a postdoctoral fellow under the supervision of Dr. Howard Green, then at the Massachusetts Institute of Technology, I have been interested in understanding the molecular mechanisms underlying growth, differentiation, and development in the mammalian ectoderm. The ectoderm gives rise to epidermal keratinocytes and to neurons, which are the only two cell types of the body that devote most of their protein-synthesizing machinery to developing an elaborate cytoskeletal architecture composed of 10-nm intermediate filaments (IFs). Our interest is in understanding the architecture of the cytoskeleton in keratinocytes and in neurons, and in elucidating how perturbations in this architecture can lead to degenerative diseases of the skin and the nervous system. I will concentrate on the intermediate filament network of the skin and its associated genetic disorders, since this has been a long-standing interest of my laboratory at the University of Chicago.

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.

The cytoskeleton and disease: genetic disorders of intermediate filaments

Specialized cytoskeletons play many fascinating roles, including mechanical integrity and wound-healing in epidermal cells, cell polarity in simple epithelia, contraction in muscle cells, hearing and balance in the inner ear cells, axonal transport in neurons, and neuromuscular junction formation between muscle cells and motor neurons. These varied functions are dependent upon cytoplasmic networks of actin microfilaments (6 nm), intermediate filaments (10 nm) and microtubules (23 nm), and their many associated proteins. In this chapter, I review what is known about the cytoskeletons of intermediate filaments and their associated proteins. I focus largely on epidermal cells, which devote most of their protein-synthesizing machinery to producing an extensive intermediate filament network composed of keratin. Recent studies have shown that many of the devastating human disorders that arise from degeneration of this cell type have as their underlying basis either defects in the genes encoding keratins or abnormalities in keratin IF networks. I discuss what we know about the functions of IFs, and how the link to genetic disease has enhanced this understanding.

Cloning and characterization of a new armadillo family member, p0071, associated with the junctional plaque: evidence for a subfamily of closely related proteins

Cell contacts of the adherens type are organized around transmembrane proteins of the cadherin family. Whereas the extracellular domains mediate homophilic interactions between cadherins of neighbouring cells the cytoplasmic domains organize a set of proteins into the junctional plaque. Among these junctional plaque proteins are members of the armadillo gene family, beta-catenin, plakoglobin (gamma-catenin), B6P/plakophilin and p120. These proteins are assumed to play a key role in cell cell signalling through intercellular junctions. Here we report cloning of a cDNA encoding a new armadillo family member, p0071, closely related to p120 and B6P/plakophilin and more distantly related to armadillo, plakoglobin, beta-catenin and other members of the gene family. The deduced amino acid sequence encodes a basic protein of 1,211 amino acids with a central armadillo repeat region which is conserved in sequence and organization of its ten individual motifs between p120, B6P/plakophilin and p0071. In contrast the end domains of the three proteins are variable in size and sequence. The RNA coding for p0071 is expressed in all tissues examined. Using antibodies generated against the armadillo repeat region of the protein we show that p0071 is localized at cell-cell borders and is expressed in the desmosomal plaque of some cultured epithelial cells. The protein seems to be an accessory component of the desmosomal plaque as well as of other adhesion plaques and might be involved in regulating junctional plaque organization and cadherin function. Our data provide evidence for a subfamily of armadillo related proteins that share not only structural features but also have in common their localisation in the junctional plaque. We therefore suggest that family members exert similar functions and might be involved in cell signalling through cell contacts.

Neurofilament triplet protein interactions: evidence for the preferred formation of NF-L-containing dimers and a putative function for the end domains

In this report we examine the molecular interactions that lead to formation of neurofilaments, the intermediate filaments in neurons. Using the yeast two-hybrid system, we found that the rod domains of all three NF triplet proteins interacted strongly with one another and with rod domains of the Type III IF proteins, vimentin and desmin. A slight preference toward NF-L-containing dimers was observed over ones not containing NF-L. Interactions among the full length NF triplet proteins exhibited more specificity. Full length NF-L had only a relatively weak interaction with another full length NF-L molecule, but reacted more robustly with full length NF-M or NF-H lacking only part of the head domain. No homologous or heterologous dimerization of NF-M and NF-H was detectable. These results support the hypothesis that neurofilaments are obligate heteropolymers and that heterodimeric subunits are the preferred building blocks. They further suggest that the mechanism that specifies heterodimeric interaction among the NF triplet proteins resides in the end domains.

A discourse on human hair fibers and reflections on the conservation of drug molecules

A gross discourse on human hair fibers and their formation is presented stressing the various interdisciplinary aspects, such as the morphological, biological, structural and biochemical data considered to be important in the field of hair analysis. An attempt is made to explain the incorporation of drug molecules during hair fiber formation by using the classical concepts of drug absorption based on lipoid theory and the pH-partition hypothesis as well as a modern biological approach on the permeability of cell membranes. In addition to the physiochemical considerations of the transport properties of a particular drug molecule such as a) the lipophilicity, which determines permeability through the membrane, b) the pKa value, c) the plasma protein binding and d) the molecular size and shape of the drug molecule, drug absorption is thought to be limited by the surface area and the residence time in the hair bulb. The thermodynamic approach according to the Kedem-Katchalsky equations seems even more satisfying. When the principles of biological transport across cell membranes are applied to the cell populations present in the hair root, a hypothesis of extracellular and intracellular drug localizations results. It is speculated that the cell membrane complex (CMC) and the melanin granules present the main sources of incorporated drug molecules within the keratinized hair fibers.

Gene targeting at the mouse cytokeratin 10 locus: severe skin fragility and changes of cytokeratin expression in the epidermis

Bullous congenital ichthyosiform erythroderma (BCIE) is a dominantly inherited blistering skin disorder caused by point mutations in the suprabasal cytokeratins 1 or 10. Targeting the murine cytokeratin 10 gene in ES cells resulted in mice with different phenotypes in the homozygotes and heterozygotes; both of which exhibit similarities to specific clinical characteristics of BCIE. Homozygotes suffered from severe skin fragility and died shortly after birth. Heterozygotes were apparently unaffected at birth, but developed hyperkeratosis with age. In both genotypes, aggregation of cytokeratin intermediate filaments, changes in cytokeratin expression, and alterations in the program of epidermal differentiation were observed. In addition we demonstrate, for the first time, the existence of the murine equivalent of human cytokeratin 16.

Onset of re-epithelialization after skin injury correlates with a reorganization of keratin filaments in wound edge keratinocytes: defining a potential role for keratin 16

Injury to stratified epithelia causes a strong induction of keratins 6 (K6) and 16 (K16) in post-mitotic keratinocytes located at the wound edge. We show that induction of K6 and K16 occurs within 6 h after injury to human epidermis. Their subsequent accumulation in keratinocytes correlates with the profound reorganization of keratin filaments from a pan-cytoplasmic distribution to one in which filaments are aggregated in a juxtanuclear location, opposite to the direction of cell migration. This filament reorganization coincides with additional cytoarchitectural changes and the onset of re-epithelialization after 18 h post-injury. By following the assembly of K6 and K16 in vitro and in cultured cells, we find that relative to K5 and K14, a well-characterized keratin pair that is constitutively expressed in epidermis, K6 and K16 polymerize into short 10-nm filaments that accumulate near the nucleus, a property arising from K16. Forced expression of human K16 in skin keratinocytes of transgenic mice causes a retraction of keratin filaments from the cell periphery, often in a polarized fashion. These results imply that K16 may not have a primary structural function akin to epidermal keratins. Rather, they suggest that in the context of epidermal wound healing, the function of K16 could be to promote a reorganization of the cytoplasmic array of keratin filaments, an event that precedes the onset of keratinocyte migration into the wound site.

A nontetrameric species is the major soluble form of keratin in Xenopus oocytes and rabbit reticulocyte lysates

Inside the interphase cell, approximately 5% of the total intermediate filament protein exists in a soluble form. Past studies using velocity gradient sedimentation (VGS) indicate that soluble intermediate filament protein exists as an approximately 7 S tetrameric species. While studying intermediate filament assembly dynamics in the Xenopus oocyte, we used both VGS and size-exclusion chromatography (SEC) to analyze the soluble form of keratin. Previous studies (Coulombe, P. A., and E. Fuchs. 1990. J. Cell Biol. 111:153) report that tetrameric keratins migrate on SEC with an apparent molecular weight of approximately 150,000; the major soluble form of keratin in the oocyte, in contrast, migrates with an apparent molecular weight of approximately 750,000. During oocyte maturation, the keratin system disassembles into a soluble form (Klymkowsky, M. W., L. A. Maynell, and C. Nislow. 1991. J. Cell Biol. 114:787) and the amount of the 750-kD keratin complex increases dramatically. Immunoprecipitation analysis of soluble keratin from matured oocytes revealed the presence of type I and type II keratins, but no other stoichiometrically associated polypeptides, suggesting that the 750-kD keratin complex is composed solely of keratin. To further study the formation of the 750-kD keratin complex, we used rabbit reticulocyte lysates (RRL). The 750-kD keratin complex was formed in RRLs contranslating type I and type II Xenopus keratins, but not when lysates translated type I or type II keratin RNAs alone. The 750-kD keratin complex could be formed posttranslationally in an ATP-independent manner when type I and type II keratin translation reactions were mixed. Under conditions of prolonged incubation, such as occur during VGS analysis, the 750-kD keratin complex disassembled into a 7 S (by VGS), 150-kD (by SEC) form. In urea denaturation studies, the 7 S/150-kD form could be further disassembled into an 80-kD species that consists of cofractionating dimeric and monomeric keratin. Based on these results, the 750-kD species appears to be a supratetrameric complex of keratins and is the major, soluble form of keratin in both prophase and M-phase oocytes, and RRL reactions.

Purification, in vitro reassembly, and preliminary sequence analysis of epiplasmins, the major constituent of the membrane skeleton of Paramecium

The epiplasmic layer, a continuous rigid granulo-fibrillar sheet directly subtending the surface membranes of Paramecium, is one of the outermost of the various cytoskeletal networks that compose it cortex. We have previously shown that the epiplasm consists of a set of 30 to 50 protein bands on SDS-PAGE in the range 50 to 33 kDa, the epiplasmins. We report a purification procedure for the set of epiplasmic proteins, a description of their physicochemical and reassembly properties, and a preliminary characterization of their sequence. The conditions for solubilization of the epiplasm and for in vitro reassembly of its purified constituents ar described. Reassembly of the entire set of proteins and of some (but not all) subsets are shown to yield filamentous aggregates. Microsequences of two purified bands of epiplasmins reveal a striking amino acid sequence consisting of heptad repeats of only three main amino acids, P, V, and Q. These repeats were confirmed by DNA sequencing of polymerase chain reaction products. The motif is QPVQ-h, in which h is a hydrophobic residue. This may constitute the core of the epiplasmin sequence and, in view of the tendency of such a sequence to form a coiled-coil, may account for the remarkable self-aggregation properties of epiplasmins.

Transcription factor regulation of epidermal keratinocyte gene expression

The epidermis is a tissue that undergoes a very complex and tightly controlled differentiation program. The elaboration of this program is generally flawless, resulting in the production of an effective protective barrier for the organism. Many of the genes expressed during keratinocyte differentiation are expressed in a coordinate manner; this suggests that common regulatory models may emerge. The simplest model envisions a 'common regulatory element' that is possessed by all genes that are regulated together (e.g., involucrin and transglutaminase type 1). Studies to date, however, have not identified any such elements and, if anything, the available studies suggest that appropriate expression of each gene is achieved using sometime subtly and sometime grossly different mechanisms. Recent studies indicate that a variety of transcription factors (AP1, AP2, POU domain. Sp1, STAT factors) are expressed in the epidermis and, in many cases, multiple members of several families are present (e.g., AP1 and POU domain factors). The simultaneous expression of multiple members of a single transcription factor family provides numerous opportunities for complex regulation. Some studies suggest that specific members of these families interact with specific keratinocyte genes. The best studied of these families in epidermis is the AP1 family of factors. All of the known AP1 factors are expressed in epidermis [52] and each is expressed in a specific spatial pattern that suggests the potential to regulate multiple genes. It will be important to determine the role of each of these members in regulating keratinocyte gene expression. Finally, information is beginning to emerge regarding signal transduction in keratinocytes. Some of the early events in signal transduction have been identified (e.g., PLC and PKC activation, etc.) and some of the molecular targets of these pathways (e.g., AP1 transcription factors) are beginning to be identified. Eventually we can expect to elucidation of all of the steps between the interaction of the stimulating agent with its receptor and the activation of target gene expression.

Molecular modeling indicates that homodimers form the basis for intermediate filament assembly from human and mouse epidermal keratins

Mammalian epidermal keratin molecules adopt rod-shaped conformations that aggregate to form cytoplasmic intermediate filaments. To investigate these keratin conformations and the basis for their patterns of molecular association, graphical methods were developed to relate known amino acid sequences to probable spacial configurations. The results support the predominantly alpha-helical conformation of keratin chains, interrupted by short non-alpha-helical linkages. However, it was found that many of the linkages have amino acid sequences typical of beta-strand conformations. Space-filling atomic models revealed that the beta-strand sequences would permit the formation of 2-chain and 4-chain cylindrical beta-helices, fully shielding the hydrophobic amino acid chains that alternate with hydrophilic residues in these sequences. Because of the locations of the beta-helical regions in human and mouse stratum corneum keratin chains, only homodimers of the keratins could interact efficiently to form 2-chain and 4-chain beta-helices. Tetramers having the directions and degrees of overlap of constituent dimers that have been identified by previous investigators are also predicted from the interactions of beta-helical motifs. Heterotetramers formed from dissimilar homodimers could combine, through additional beta-helical structures, to form higher oligomers having the dimensions seen in electron microscopic studies. Previous results from chemical crosslinking studies can be interpreted to support the concept of homodimers rather than heterodimers as the basis for keratin filament assembly.

Epitope mapping of monoclonal antibodies to keratin 19 using keratin fragments, synthetic peptides and phage peptide libraries

To generate tools for monitoring processing and folding in keratin intermediate filaments, a group of monoclonal antibodies reacting with the intermediate filament protein keratin 19 were studied using different approaches to define the structure and localization of their epitopes. The binding pattern to bacterially expressed human keratin 19 fragments allowed the definition of minimal amino acid sequences required for antibody binding. The screening of overlapping 15-residue peptides confirmed and further specified the epitope locations for a subset of the tested antibodies. In addition, the epitope of an antibody with apparent species-restricted specificity (LE64) was revealed by isolating and characterizing a full-length keratin 19 clone from a PtK2 cDNA library. Taken together with species cross-reactivity of individual antibodies and sequence information obtained by probing a phage display library, specific amino acid residues could be highlighted as likely to be involved in the antibody binding.

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.

Keratin 16 and keratin 17 mutations cause pachyonychia congenita

Pachyonychia congenita (PC) is a group of autosomal dominant disorders characterized by dystrophic nails and other ectodermal aberrations. A gene for Jackson-Lawler PC was recently mapped to the type I keratin cluster on 17q. Here, we show that a heterozygous missense mutation in the helix initiation motif of K17 (Asn92Asp) co-segregates with the disease in this kindred. We also show that Jadassohn-Lewandowsky PC is caused by a heterozygous missense mutation in the helix initiation peptide of K16 (Leu130Pro). The known expression patterns of these keratins in epidermal structures correlates with the specific abnormalities observed in each form of PC.

The importance of intramolecular ion pairing in intermediate filaments

Nuclear and cytoskeletal networks of 10-nm intermediate filaments (IFs) are probably ubiquitous in multicellular eukaryotes. They likely play a role in maintaining the mechanical integrity of a cell. With the exception of the nuclear lamins, IF proteins can form IFs in vitro in the absence of cofactors or associated proteins. Below we present data suggesting that the large alpha-helical "rod" domains of IF proteins are stabilized by large numbers (up to 50) of intra-helical ion pairs formed by residues of opposite charge situated four residues apart. These many ion pairs, sometimes involving up to 30% of the residues within a coiled-coil IF segment, can potentially contribute as much as 10-25 kcal/mol (1 kcal = 4.18 kJ) to the stability of a single alpha-helical rod. Such stabilization is likely to play a major role in the chemical and physical stability of IF networks in vitro and in vivo. An investigation of other coiled-coil proteins shows that selection for intrahelical ion pairing is not simply a property intrinsic to coiled-coil proteins. Rather, there is a correlation between the degree to which there is selection for intrahelical ion pairs and the extent to which a coiled-coil protein participates in highly ordered multimolecular interactions--e.g., as in IFs and myosin thick filaments. The propensity of putative ion pairs in some IF proteins--e.g., epidermal keratins--suggests that an underlying structural stability at the level of the monomer may play an important role in the extraordinary stability of dimers and higher ordered structures in cytoplasmic IFs.

Protein and mRNA expression of simple epithelial keratins in normal, dysplastic, and malignant oral epithelia

Simple epithelial keratins K7, K8, and K18 are present in no more than trace amounts in normal stratified squamous epithelial but have been reported in squamous cell carcinomas. With the aim of determining the level at which keratin synthesis is regulated in vivo, we have compared the expression of mRNA by in situ hybridization and protein by immunohistochemistry for K7, K8, and K18 in a series of normal, dysplastic, and malignant oral epithelia. In normal epithelia mRNAs for K7, K8, and K18 were present in basal and lower spinous cells but adjacent sections were generally negative for the respective proteins. In severe dysplasia there was irregular suprabasal extension of K8 and K18 mRNAs in all cases and their proteins were expressed in more than half of the cases. The carcinomas expressed K8 and K18 mRNAs homogeneously and were strongly reactive for these keratin proteins but K7 expression appeared reduced in malignancy. These results are consistent with the post-transcriptional regulation of K7, K8, and K18 expression in normal epithelia and the presence of their proteins in dysplastic and malignant epithelia suggests the release of these epithelial cells from a post-transcriptional block on K8 and K18 translation. Alternatively, rapid degradation of K8 and K18 protein might be occurring in normal epithelia but be suppressed in dysplasia and malignancy.

A functional "knockout" of human keratin 14

The importance of keratins and other intermediate filaments in the maintenance of tissue structure is emphasized by the discovery that many hereditary skin-blistering diseases are caused by mutations in keratin genes. Here, we describe a situation in which keratin 14 (K14) is missing altogether in the epidermis: A homozygous 2-nucleotide deletion in exon I of the K14 gene causes premature termination of the mRNA transcripts upstream from the start of the rod domain and results in a K14 null phenotype. In this individual no keratin intermediate filaments are visible in basal epidermal cells, although filaments are present in the upper layers of the epidermis. No compensating keratin expression is detected in vivo, and K14 mRNA is down-regulated. The individual, diagnosed as Köbner (generalized) EBS, suffers from severe widespread keratinocyte fragility and blistering at many body sites, but although the phenotype is severe, it is not lethal. This K14-/- phenotype confirms that only one K14 gene is expressed in human epidermis and provides an important model system for examining the interdependence of different keratin filament systems and their associated structures in the skin.

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.

A human keratin 14 "knockout": the absence of K14 leads to severe epidermolysis bullosa simplex and a function for an intermediate filament protein

Since their discovery, the function of intermediate filaments (IFs) has remained obscure. In skin, epidermal cells have extensive cytoskeletal architectures of IFs, composed of type I and type II keratin heterodimers. Clues to possible functions of these proteins have come from recent studies showing that several autosomal-dominant, blistering skin disorders are caused by defects in genes that encode epidermal keratins. These diseases all exhibit cell degeneration and keratin network perturbations in cells that express the particular mutant keratin gene. However, it is not clear from these studies whether cytolysis arises from the presence of large insoluble keratin aggregates that compromise cellular physiology or from the absence of an extensive keratin filament network, which jeopardizes mechanical integrity. We report here the analysis of an extremely rare case of severe recessive epidermolysis bullosa simplex (EBS), where the patient lacks a discernible keratin filament network in basal epidermal cells. Genetic analyses revealed a homozygous point mutation that yielded a premature termination codon in the major basal type I keratin gene and caused complete ablation of K14. The consanguineous parents were normal, each harboring one copy of the null K14 mutation. Analysis of cultured keratinocytes enabled us to document that the loss of K14 is not compensated for by the up-regulation of any other type I keratin. When taken together with the in vivo studies showing the presence of cell fragility generated from the lack of an extensive basal keratin network, these findings provide the first clear demonstration of loss of function associated with the absence of an IF protein in vivo.

Increased expression of keratin 16 causes anomalies in cytoarchitecture and keratinization in transgenic mouse skin

Injury to epidermis and other stratified epithelia triggers profound but transient changes in the pattern of keratin expression. In postmitotic cells located at the wound edge, a strong induction of K6, K16, and K17 synthesis occurs at the expense of the keratins produced under the normal situation. The functional significance of these alterations in keratin expression is not known. Here, we report that overexpression of a wild-type human K16 gene in a tissue-specific fashion in transgenic mice causes aberrant keratinization of the hair follicle outer root sheath and proximal epidermis, and it leads to hyperproliferation and increased thickness of the living layers (acanthosis), as well as cornified layers (hyperkeratosis). The pathogenesis of lesions in transgenic mouse skin begins with a reorganization of keratin filaments in postmitotic keratinocytes, and it progresses in a transgene level-dependent fashion to include disruption of keratinocyte cytoarchitecture and structural alterations in desmosomes at the cell surface. No evidence of cell lysis could be found at the ultrastructural level. These results demonstrate that the disruption of the normal keratin profile caused by increased K16 expression interferes with the program of terminal differentiation in outer root sheath and epidermis. They further suggest that when present at sufficiently high intracellular levels, K16, along with K6 and K17, appear capable of inducing a reorganization of keratin filaments in the cytoplasm of skin epithelial cells.

Coiled-coil stutter and link segments in keratin and other intermediate filament molecules: a computer modeling study

Structural discontinuities have previously been identified in four regions of the coiled-coil rod domain structure present in intermediate filament (IF) protein molecules. These include a point at which a phase shift occurs in the heptad periodicity characteristic of the sequence of polar and apolar residues in alpha-helical coiled-coils, and three links that lack a heptad substructure. We have studied these regions by computer-based molecular modeling and comparative sequence analysis and conclude that the phasing discontinuity can be accommodated without significant distortion of the overall double-helical chain conformation; the L2 link has a similar conformation in all different types of IF molecules, a favorable conformation being one in which the two strands wrap tightly around each other; the L12 links vary in length between different IF types but contain important sequence similarities suggestive of a partial beta structure; the L1 links show larger variations in length, a lower degree of similarity, and probably diverse structures. Variations in the overall charges of the different links suggest that ionic interactions may play a significant role in filament assembly. The results also have general significance for other alpha-fibrous proteins in which either the characteristic heptad phasing undergoes a discontinuity or where a short non-coiled-coil sequence occurs within a coiled-coil rod domain structure.

Band 6 protein, a major constituent of desmosomes from stratified epithelia, is a novel member of the armadillo multigene family

Desmosomes are intercellular adhering junctions characteristic of epithelial cells. Several constitutive proteins--desmoplakin, plakoglobin and the transmembrane glycoproteins desmoglein and desmocollin--have been identified as fundamental constituents of desmosomes in all tissues. A number of additional and cell type-specific constituents also contribute to desmosomal plaque formation. Among these proteins is the band 6 polypeptide (B6P). This positively charged, non-glycosylated protein is a major constituent of the plaque in stratified and complex glandular epithelia. Using an overlay assay we show that purified keratins bind in vitro to B6P. Thus B6P may play a role in ordering intermediate filament networks of adjacent epithelial cells. To characterize the structure of B6P in the desmosome we have isolated cDNA clones representing the entire coding sequence. The predicted amino acid sequence of human B6P shows strong sequence homology with a murine p120 protein, which is a substrate of protein tyrosine kinase receptors and of p60v-src. P120 and B6P show amino-terminal domains differing distinctly in length and sequence. These are followed in both proteins by 460 residues that display a series of imperfect repeats corresponding to the repeats in the cadherin binding proteins armadillo, plakoglobin and beta-catenin. Over this repeat region B6P and p120 share 33% sequence identity (54% similarity). These sequence characteristics define B6P as a novel member of the armadillo multigene family and raise the question of whether the structural proteins B6P, plakoglobin, beta-catenin and armadillo share some function. Since armadillo, plakoglobin, beta-catenin and p120 seem involved in signal transduction this may also hold for B6P. The amino-terminal region of B6P (residues 1 to 263) shows no significant homology to any known protein sequence. It may therefore be involved in unique functions of B6P.

Genetic disorders of keratin: are scarring alopecias a sub-set?

Recent advances have challenged the prevailing view that keratins are merely passive bystanders of keratinocyte biology. With the exciting discovery that three autosomal dominant genetic skin disorders, epidermolysis bullosa simplex (EBS), epidermolytic hyperkeratosis (EHK) and palmoplantar keratoderma (PPK), are in fact disorders of keratins comes the realization that the integrity of the keratin filament network is crucial to the structural integrity of the skin. Since it has been recently established that mutations in keratins K5/K14, K1/K10 and K9 are causative for these keratinocyte disorders, it is very likely that mutations in K6 or in its obligate partner, K16 will result in disease. In order to test this we have produced transgenic mice that express a mutant K6 gene. These mice develop a progressive scarring alopecia at about 6 months of age. Later, the denuded areas developed a keratosis which was prone to infection. Ultrastructural analysis suggests that hair loss is due to the destruction of the outer root sheath. We believe that these mice are models of another keratin disorder.

Identification of two N-terminal non-alpha-helical domain motifs important in the assembly of glial fibrillary acidic protein

The non-alpha-helical N-terminal domain of intermediate filament proteins plays a key role in filament assembly. Previous studies have identified a nonapeptide motif, SSYRRIFGG, in the non-alpha-helical N-terminal domain of vimentin that is required for assembly. This motif is also found in desmin, peripherin and the type IV intermediate filament proteins. GFAP is the only type III intermediate filament protein in which this motif is not readily identified. This study has identified two motifs in the non-alpha-helical N-terminal domain of mouse GFAP that play important roles in GFAP assembly. One motif is located at the very N terminus and has the consensus sequence, MERRRITS-ARRSY. It has some characteristics in common with the vimentin nonapeptide motif, SSYRRIFGG, including its location in the non-alpha-helical N-terminal domain and a concentration of arginine residues. Unlike the vimentin motif in which even conserved sequence changes affect filament assembly, the GFAP consensus sequence, MERRRITS-ARRSY, can be replaced by a completely unrelated sequence; namely, the heptapeptide, MVRANKR, derived from the lambda cII protein. When fused to GFAP sequences with sequential deletions of the N-terminal domain, the lambda cII heptapeptide was used to help identify a second motif, termed the RP-box, which is located just upstream of the GFAP alpha-helical rod domain. This RP-box affected the efficiency of filament assembly as well as protein-protein interactions in the filament, as shown by sedimentation assays and electron microscopy. These results are supported by previous data, which showed that the dramatic reorganization of GFAP within cells was due to phosphorylation-dephosphorylation of a site located in this RP-box. The results in this study suggest the RP-box motif to be a key modulator in the mechanism of GFAP assembly, and support a role for this motif in both the nucleation and elongation phases of filament assembly. The RP-box motif in GFAP has the consensus sequence, RLSL-RM-PP. Sequences similar to the GFAP RP-box motif are also to be found in vimentin, desmin and peripherin. Like GFAP, these include phosphorylation and proteolysis sites and are adjacent to the start of the central alpha-helical rod domain, suggesting that this motif of general importance to type III intermediate filament protein assembly.

Function of type I and type II keratin head domains: their role in dimer, tetramer and filament formation

To examine the role of the keratin head region and its subdomains in filament assembly we constructed several deletion mutants of type I and type II keratins and analysed their in vitro IF forming capacity. The delta K8 (1–74) and delta K18 (1–56), mutants formed only soluble oligomers, predominantly tetramers with their heterotypic partners. K8 mutants that retained either the entire (delta K8 (1–64)) or nearly the entire (delta K8 (1–66)) H1 subdomain formed some short and irregular IF-like structures with K18. However, filaments never reached the normal length and more protofilamentous material was observed. Analysis of the soluble complexes in 2 M guanidine-HCl indicated that tetramer formation was impaired in the truncated molecules. The length of the deletion correlated with the degree of tetramer destabilization. These results suggest that the head domain--specifically the H1 subdomain of type II keratins-plays a direct role in IF assembly. Its functions include a stabilization of the tetramer molecule, suggesting a role in directing the alignment of dimers as well as in elongation. We also analysed whether both head domains are required or if either type I or type II head domains alone are sufficient for IF formation. Hybrid molecules carrying their partner keratins head domains (K18 (8 head) and K8 (18 head)) were combined with their wild-type partners and tested for IF-forming ability. Both combinations formed filaments distinct from normal IF. The effect of the ‘replaced’ head domains was not compensated when both hybrid molecules were combined. Taken together, the results indicate that complete removal of the head domains of either K8 or K18 arrested IF assembly at the state of soluble oligomers. Replacement of the head domains by head domains of the complementary partner partly compensated for the effect. However, regular IF formation could not take place when either the head domain was missing or it was replaced by the partner's keratin head.

Keratin 9 gene mutational heterogeneity in patients with epidermolytic palmoplantar keratoderma

Mutations in the human keratin 9 gene have recently been shown to be involved in the etiology of palmoplantar keratoderma (PPK). We have investigated eleven unrelated German kindreds with the epidermolytic variant of PPK (EPPK) for mutations in the keratin 9 gene. We have identified two novel mutations, M156V and Q171P, both in the coil 1A segment of keratin 9. Mutation M156V was detected in two unrelated patients with EPPK, and mutation Q171P was shown to cosegregate with the disease in a large four-generation family. These findings confirm the functional importance of coil 1A integrity for heterodimerisation in keratins and for intermediate filament assembly. Our results provide further evidence for mutational heterogeneity in EPPK, and for the involvement of keratins in diseases of hyperkeratinisation and epidermolysis.

Keratin diseases

The recent discovery that epidermal fragility syndromes can be caused by mutations in the genes for keratin intermediate filaments has been a turning point for research into these structural proteins. Clustering of pathogenic mutations implies differential structural sensitivity along the keratin molecule, and implications for filament function require a new look at culture assay systems, plus a reassessment of structural defects in epithelial and other tissues.

Abnormal keratin 1 and 10 cytoskeleton in cultured keratinocytes from epidermolytic hyperkeratosis caused by keratin 10 mutations

Epidermolytic hyperkeratosis is caused by mutations of the differentiation-specific keratins K1 and K10. These mutations produce a weakened cytoskeleton that is prone to collapse resulting in cell fragility and lysis. In this study we have analyzed cultured keratinocytes from EHK patients bearing 10R-to-H and 15L-to-S mutations within the 1A segment of the K10 rod domain. Keratinocytes were grown submerged in serum-free medium and induced to differentiate by growing to confluence and increasing the Ca++ concentration in the medium. Cultures were either harvested for mRNA sequence analysis or subjected to immunofluorescence microscopy. Differentiating keratinocytes from these patients were found to express these K10 mutations in their mRNA. Moreover, these cells could be distinguished from normal keratinocytes by their aberrant morphology. EHK keratinocytes frequently exhibited a collapsed perinuclear network of K1/K10 filaments and sometimes peripheral granules of K1 and K10 aggregates, reminiscent of the cells of the suprabasal layers in these patients. This report documents the expression of mutant keratin 10 in cultured EHK keratinocytes.

Genetic mutations in the K1 and K10 genes of patients with epidermolytic hyperkeratosis. Correlation between location and disease severity

Epidermolytic hyperkeratosis (EH) is a skin disease caused by mutations in the genes encoding K1 and K10, the differentiation-specific keratins of epidermis. To explore the heterogeneity of mutations and to assess whether a correlation exists between disease severity and the extent to which a mutation interferes with keratin network formation, we determined the genetic bases of four severe incidences of EH and one unusually mild case. Two severe cases have the same mutation, K10-R156:C, at a conserved arginine that we previously showed was mutated to a histidine in two unrelated EH families. An additional severe case has a mutation six residues away, still within the amino end of the alpha-helical rod domain of K10. The other severe case has a mutation in the conserved carboxy end of the K1 rod. In contrast, affected members of the atypically mild family have a mutation just proximal to the conserved carboxy end of the K10 rod. By genetic engineering and gene transfection, we demonstrate that each mutation is functionally responsible for the keratin filament aberrations that are typical of keratinocytes cultured from these patients. Moreover, we show that the mild EH mutation less severely affects filament network formation. Taken together, our studies strengthen the link between filament perturbations, cell fragility, and degeneration.

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.

Localization of the Kar3 kinesin heavy chain-related protein requires the Cik1 interacting protein

The Kar3 protein (Kar3p), a protein related to kinesin heavy chain, and the Cik1 protein (Cik1p) appear to participate in the same cellular processes in S. cerevisiae. Phenotypic analysis of mutants indicates that both CIK1 and KAR3 participate in spindle formation and karyogamy. In addition, the expression of both genes is induced by pheromone treatment. In vegetatively growing cells, both Cik1::beta-gal and Kar3::beta-gal fusions localize to the spindle pole body (SPB), and after pheromone treatment both fusion proteins localize to the spindle pole body and cytoplasmic microtubules. The dependence of Cik1p and Kar3p localization upon one another was investigated by indirect immunofluorescence of fusion proteins in pheromone-treated cells. The Cik1p::beta-gal fusion does not localize to the SPB or microtubules in a kar3 delta strain, and the Kar3p::beta-gal fusion protein does not localize to microtubule-associated structures in a cik1 delta strain. Thus, these proteins appear to be interdependent for localization to the SPB and microtubules. Analysis by both the two-hybrid system and co-immunoprecipitation experiments indicates that Cik1p and kar3p interact, suggesting that they are part of the same protein complex. These data indicate that interaction between a putative kinesin heavy chain-related protein and another protein can determine the localization of motor activity and thereby affect the functional specificity of the motor complex.

A monoclonal antibody epitope on keratin 8 identified using a phage peptide library

A bacteriophage random hexapeptide library was used to define the epitope of a monoclonal anti-keratin antibody. Phage selected by the keratin 8-specific antibody LE41 displayed highly related sequences on their pIII coat protein. The consensus sequence S(X)LNP allowed the precise localization of an LE41 epitope (SLLSP) within the head domain (H1 subdomain) of human keratin 8, known to be important for correct filament polymerisation. By sequencing the immunizing antigen, keratin 8 from Potorous tridactylis, it was shown that the natural epitope of LE41 is the pentapeptide SLLNP, which confirmed predictions from the phage library results. An SLL(X)P motif is found in the H1 region of all type II keratins (keratins 1 to 8) in different species, but mutational analysis revealed that LE41 can only bind to keratin 8 when Asn (N) or Ser (S) is found in the (X) position. Thus the monoclonal antibody LE41 retains its specificity for keratin 8, dependent on a single amino acid residue, even though it recognizes an epitope within the highly conserved H1 subdomain of the head region. Six other monoclonal antibodies tested on the phage library failed to select motifs.

Mutations in the rod 1A domain of keratins 1 and 10 in bullous congenital ichthyosiform erythroderma (BCIE)

Bullous congenital ichthyosiform erythroderma is a human hereditary skin disorder in which suprabasal keratinocytes rupture. Recent reports have implicated keratins K1 and K10 in this disease. Here we describe four diverse keratin mutations that are all significantly associated with this disease. Two of these are in the helix 1A subdomain of the type II keratin 1, giving a serine-to-proline substitution in codon 185 and an asparagine-to-serine substitution in codon 187. In the analogous region of type I keratin 10, an arginine-to-proline and an arginine-to-serine transition in codon 156 have been identified. All four mutations create restriction fragment length polymorphisms that were used exclude the mutations from 120 normal chromosomes. Insertional polymorphism (in the V2 subdomains of the non-helical tails of K1 and K10) was excluded as the cause of the phenotypic heterogeneity observed within one family.

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.