Do the ends justify the mean? Proline mutations at the ends of the keratin coiled-coil rod segment are more disruptive than internal mutations

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.

A conserved region in the tail domain of vimentin is involved in its assembly into intermediate filaments

Although the head and rod domains of intermediate filament (IF) proteins are known to play significant roles in filament assembly, the role of the tail domain in this function is unclear and the available information supports contradictory conclusions. We examined this question by comparing transfection of the same cDNA constructs, encoding vimentins with modified tail domains, into cell lines that do and do not contain endogenous IF proteins. By this approach, we were able to distinguish between the ability of a mutant IF protein to initiate assembly de novo, from that of incorporating into existing filament networks. Vimentins with modifications at or near a highly conserved tripeptide, arg-asp-gly (RDG), of the tail domain incorporated into existing IF networks in vimentin-expressing (vim+) cells, but were assembly-incompetent in cells that did not express IF proteins (vim-). The failure of the RDG mutant vimentins to assemble into filament arrays in vim- cells was reversible by re-introducing a wild-type vimentin cDNA, whereupon both wild-type and mutant vimentins coassembled into one and the same IF network. We conclude that the function of the tail domain of type III IF proteins, and possibly of keratins K8 and K18, in IF assembly is distinct from those of other domains; a region encompassing the RDG tripeptide appears to be important in the assembly process.

The rod domain of NF-L determines neurofilament architecture, whereas the end domains specify filament assembly and network formation

Neurofilaments, assembled from NF-L, NF-M, and NF-H subunits, are the most abundant structural elements in myelinated axons. Although all three subunits contain a central, alpha-helical rod domain thought to mediate filament assembly, only NF-L self-assembles into 10-nm filaments in vitro. To explore the roles of the central rod, the NH2-terminal head and the COOH-terminal tail domain in filament assembly, full-length, headless, tailless, and rod only fragments of mouse NF-L were expressed in bacteria, purified, and their structure and assembly properties examined by conventional and scanning transmission electron microscopy (TEM and STEM). These experiments revealed that in vitro assembly of NF-L into bona fide 10-nm filaments requires both end domains: whereas the NH2-terminal head domain promotes lateral association of protofilaments into protofibrils and ultimately 10-nm filaments, the COOH-terminal tail domain controls lateral assembly of protofilaments so that it terminates at the 10-nm filament level. Hence, the two end domains of NF-L have antagonistic effects on the lateral association of protofilaments into higher-order structures, with the effect of the COOH-terminal tail domain being dominant over that of the NH2-terminal head domain. Consideration of the 21-nm axial beading commonly observed with 10-nm filaments, the approximate 21-nm axial periodicity measured on paracrystals, and recent cross-linking data combine to support a molecular model for intermediate filament architecture in which the 44-46-nm long dimer rods overlap by 1-3-nm head-to-tail, whereas laterally they align antiparallel both unstaggered and approximately half-staggered.

Missing links: Weber-Cockayne keratin mutations implicate the L12 linker domain in effective cytoskeleton function

We have identified mutations in keratins K5 (Arg331Cys) and K14 (Val270Met) in two kinships affected by the dominantly-inherited skin blistering disease, Weber-Cockayne epidermolysis bullosa simplex (EBS-WC). Linkage analysis, DNA sequencing and clinical and ultrastructural analysis are combined to provide the first detailed description of classical EBS-WC. Both phenotypes show similar blistering on trauma, indicating that both mutations compromise the structural resilience of the basal keratinocytes by affecting the keratin cytoskeleton. The location of these mutations in the L12 linker, which bisects the alpha-helical rod region of intermediate filament proteins, identifies another keratin mutation cluster leading to hereditary skin fragility syndromes.

CRF and related peptides as anti-inflammatory agonists

The permeability of endothelial surfaces increases in response to injury. We have shown that vascular leakage in experimental models of tissue injury can be inhibited by CRF and by a novel class of peptides that we call mystixins. Binding sites for iodinated-Tyro-CRF have been revealed in mucous membranes, and immunoreactive CRF-like materials have been found in inflamed tissues. Perhaps the breakdown of cytoskeletal intermediate filaments after insult generates or exposes peptide domains similar to mystixins. Endogenous CRF-like or mystixin-like peptides, if activated or released locally in injured tissues, may function as agonists to counteract the immediate inflammatory response. If this is so, the peripheral actions of these peptides add a new dimension to the idea that CRF and related substances organize and regulate an organism's response to stress.

Suction-induced basal cell cytolysis in the Weber-Cockayne variant of epidermolysis bullosa simplex

In the Weber-Cockayne form of epidermolysis bullosa simplex (EBS-WC), trauma induces blisters which are confined to the palms and soles. Histologically, basal cell cytolysis is seen. We studied 6 patients with EBS-WC to determine the ultrastructural level at which artificially-induced suction blisters form. Blisters were raised by application of a suction blister cup to uninvolved forearm skin, the cup being connected to a negative pressure of 200 mm of mercury. The blisters were biopsied and examined by light and electron microscopy. On light microscopy, all biopsies showed marked vacuolization of keratinocytes in the lower two-thirds of the epidermis, and in all but one there was a cleavage plane through the basal keratinocytes. These findings were confirmed by electron microscopy in 4 patients. The separation through the basal cells is in contrast to the situation in normal individuals in whom cleavage occurs below the level of the basal cells, within the lamina lucida. Thus, even apparently normal skin from non-acral sites has the same structural abnormality as the affected acral sites in EBS-WC.

Assembly of intermediate filaments

The assembly of intermediate filaments is a fundamental property of the central rod domain of the individual subunit proteins. This rod domain, with its high propensity for alpha-helix formation, is the common and identifying feature of this family of proteins. Assembly occurs in vitro in the absence of other proteins or exogenous sources of energy; in vivo, it appears as if other factors, as yet poorly understood, modulate the assembly of intermediate filaments. Parallel, in-register dimers form via coiled-coil interactions of the rod domain. Tetramers may form from staggered arrays of parallel or antiparallel arrangements of dimers. Higher-order polymerization, which occurs spontaneously if the ionic strength of a mixture of dimers and tetramers is raised, proceeds rapidly through poorly described intermediates to the final 10 nm filament. This process is dependent on and modulated by the non-alpha-helical end domains, as well as those amino acids present at the very beginning and end of the rod domain. The interactions governing tetramer formation are most probably the same ones that are responsible for the lateral and longitudinal associations within intermediate filaments.

Genetic skin diseases caused by mutations in keratin intermediate filaments

Keratin intermediate filaments are the major differentiation products of epithelial cells such as the epidermis. The filaments are highly dynamic entities involved in the maintenance of the structural integrity of both the individual cells and the entire tissue. Recent biochemical studies suggest that the keratin proteins overlap each other in several key locations when packed together in filaments. Interestingly, mutations that introduce inappropriate amino acid substitutions in at least some of these overlap regions cause defective keratin filaments that result in at least three classes of autosomal dominant skin disease.

The genetic basis of Weber-Cockayne epidermolysis bullosa simplex

Epidermolysis bullosa simplex (EBS) is a group of autosomal dominant skin diseases characterized by blistering, due to mechanical-stress-induced degeneration of basal epidermal cells. Recently, it was discovered that the more severe types, Dowling-Meara and Koebner, are genetic disorders of the basal epidermal keratins, keratin 5 (K5) and keratin 14 (K14). Here, we show that the mildest type of EBS, Weber-Cockayne, is also a disorder of these keratins. Affected members of two unrelated families with Weber-Cockayne EBS had a T-->G point mutation in the second base position of codon 161 of one of two K5 alleles, leading to an Ile-->Ser mutation. This mutation was not present in unaffected members or in 156 alleles from normal individuals. Linkage analyses mapped the defect to the type II keratin gene cluster on chromosome 12q11-q13 (peak logarithm of odds score at theta = 0 of 3.0), providing strong additional evidence that this mutation is responsible for the Weber-Cockayne EBS phenotype. Conserved among type II keratins, Ile-161 is in the nonhelical head domain of K5, a region previously shown to be important for 10-nm filament assembly. The mutation generates a potential substrate site for protein kinase C, which could influence intermediate filament architecture, perhaps leading to the intrafilament association seen ultrastructurally in patients with the mutation.

The roles of the rod end and the tail in vimentin IF assembly and IF network formation

Using mutagenesis, we investigated the importance of two vimentin domains: (a) a highly conserved segment near the carboxy end of the alpha-helical rod, and (b) the tail, with which the rod end is known to interact. As judged by in vitro filament assembly and expression in transiently transfected cells lacking an endogenous vimentin network, the rod-tail interaction is not essential for 10 nm filament structure in vitro or for formation of fibrous arrays in culture. However, when mutated, amino acid residues within the rod and the tail segments can cause perturbations in IF assembly and in IF network formation. Finally, our studies show that the vimentin tail seems to play a role both in thermodynamically stabilizing IF structure in vitro and in establishing proper IF networks in vivo.

Dynamics of keratin assembly: exogenous type I keratin rapidly associates with type II keratin in vivo

Keratin intermediate filaments (IF) are obligate heteropolymers containing equal amounts of type I and type II keratin. We have previously shown that microinjected biotinylated type I keratin is rapidly incorporated into endogenous bundles of keratin IF (tonofilaments) of PtK2 cells. In this study we show that the earliest steps in the assembly of keratin subunits into tonofilaments involve the extremely rapid formation of discrete aggregates of microinjected keratin. These are seen as fluorescent spots containing both type I and type II keratins within 1 min post-injection as determined by double label immunofluorescence. These observations suggest that endogenous type II keratin subunits can be rapidly mobilized from their endogenous state to form complexes with the injected type I protein. Furthermore, confocal microscopy and immunogold electron microscopy suggest that the type I-type II keratin spots from in close association with the endogenous keratin IF network. When the biotinylated protein is injected at concentrations of 0.3-0.5 mg/ml, the organization of the endogenous network of tonofilaments remains undisturbed during incorporation into tonofilaments. However, microinjection of 1.5-2.0 mg/ml of biotinylated type I results in significant alterations in the organization and assembly state of the endogenous keratin IF network soon after microinjection. The results of this study are consistent with the existence of a state of equilibrium between keratin subunits and polymerized keratin IF in epithelial cells, and provide further proof that IF are dynamic elements of the cytoskeleton of mammalian cells.

Disease severity correlates with position of keratin point mutations in patients with epidermolysis bullosa simplex

Keratins are the major structural proteins of the epidermis. Recently, it was discovered that point mutations in the epidermal keratins can lead to the blistering skin diseases epidermolysis bullosa simplex (EBS) and epidermolytic hyperkeratosis (EH), involving epidermal cell fragility and rupture upon mechanical stress. In this study, we demonstrate a correlation between disease severity, location of point mutations within the keratin polypeptides, and degree to which these mutations perturb keratin filament structure. Interestingly, of the 11 EBS or EH mutations thus far identified, 6 affect a single highly evolutionarily conserved arginine residue, which, when mutated, markedly perturbs keratin filament structure and keratin network formation. This site also appears to be a hot spot for mutation by CpG methylation and deamination. In the four epidermal keratins, there are several other CpG dinucleotides that exist at codons within the highly conserved ends of the keratin rod. To elucidate why mutations at these sites have not been detected in severe cases of EBS, we engineered 7 of these C-->T transitions in K14 and tested their ability to perturb keratin network formation and keratin filament assembly in vitro. The effects of these mutants on keratin filament network formation were significantly less severe than the EBS/EH arginine mutation, suggesting that the high incidence of mutations of the residue in EBS and EH patients is a result of both a special sensitivity of filament structure to perturbations in this residue and its susceptibility to mutagenesis.

A missense mutation in the rod domain of keratin 14 associated with recessive epidermolysis bullosa simplex

Epidermolysis bullosa simplex (EBS) is a group of epidermal blistering diseases almost invariably transmitted as a dominant trait, which has recently been shown to arise from mutations in keratins 14 and 5 (K14 and K5). We describe a family with recessive EBS in which the disease is tightly linked to the substitution of the highly conserved glutamic acid-144 to alanine in the first helical segment of the rod domain of keratin 14. In contrast, linkage with keratin 5 was excluded. The loss of an ionic interaction with keratin 5 is likely to affect K14-K5 heterodimer formation. Our data suggest that this mutation underlies EBS in our family, and that mutations in keratin genes may impair the mechanical integrity of basal keratinocytes in a recessive as well as dominant fashion.

Intermediate filament structure and assembly

Intermediate filaments are constructed from two-chain alpha-helical coiled-coil molecules arranged on an imperfect helical lattice. Filament structure and assembly can be influenced at several different structural levels, including molecular structure, oligomer formation and filament nucleation and elongation. Consequently, it can sometimes be difficult to interpret mutagenesis data unequivocally, although regions near the amino and carboxyl termini of the rod domain of the molecule are known to be important for the production of native filaments. Imperfections in molecular packing may be important in filament assembly and dynamics.

The cellular and molecular biology of keratins: beginning a new era

The past year has been extremely fruitful for research on intermediate filaments in general, and keratins in particular. Unprecedented progress has been made in our understanding of the structural requirements for keratin filament assembly and network formation, the dynamism characterizing keratin filaments, their function, and implication in human genetic disorders primarily affecting the skin. These exciting findings have several implications for future research.

Confirmation and an unusual quality of the differentiated keratin peptide (K1) in cultured human squamous cell carcinomas

Recently K1 keratin peptide (K1, 68 kDa) was found to be present in two kinds of cultured human squamous cell carcinomas (HSCs) using a low-salt aqueous solution, rather than the high-salt solution containing Triton X-100 employed by many researchers up until now. To confirm whether this phenomenon is universal in cultured HSCs we analyzed K1 peptide in four other kinds of HSCs using the same procedures. Moreover, the K1 peptide detected was a little unusual with respect to solubility versus urea concentration. Epidermal K1 peptide is usually solubilized by 6-8 M urea and reductant; however, the K1 peptide in cultured HSCs was about 80-90% extracted by 1-2 M urea in a stepwise extraction procedure. This finding may have important implications regarding evaluation of keratin extracted from normal epidermal and cultured keratinocytes.

Mitotic regulation of protein phosphatases by the fission yeast sds22 protein

BACKGROUND

Cell cycle progression requires the activity of protein kinases and phosphatases at critical points in the cell cycle in all eukaryotes. We have previously reported that the dis2(+) and sds2(+) genes of fission yeast encode redundant catalytic subunits of a type 1-like protein phosphatase. The sds22(+) gene was shown to be essential for cell viability and to interact genetically with dis2(+) and sds21(+).

RESULTS

Here we show by immunoprecipitation that the sds22 protein physically interacts with the dis2 and sds21 proteins, and that sds22-associated phosphatase activity has altered substrate specificity, The loss of sds22 function by a temperature sensitive mutation leads to cell cycle arrest at mid-mitosis, at which point cdc2-dependent histone Hl kinase activity is high while sds22-dependent H1 phosphatase activity is low. To examine the unusual properties of sds22 protein structure, we analyzed a collection of sds22 deletion and point mutants by a variety of functional criteria.

CONCLUSION

We propose that sds22 is a regulatory subunit of the dis2/sds21 phosphatase catalytic subunits and that sds22-bound phosphatase carries a key phosphatase activity essential for the progression from metaphase to anaphase. Mutational analysis indicates that dis2/sds21 interacts with the central repetitive domain of sds22, while the C-terminal and central regions of sds22 may be involved in subcellular targeting and the N-terminus is important for stability.

Identification of a leucine-to-proline mutation in the keratin 5 gene in a family with the generalized Köbner type of epidermolysis bullosa simplex

We have previously reported linkage of a large Finnish family with the generalized (Köbner) type of epidermolysis bullosa simplex to chromosome 12q in the region containing the type II keratin gene cluster (Ryynänen et al., Am J Human Genet 49:978-984, 1991). In this study, we examined the possibility that keratin 5, the type II keratin expressed in the basal keratinocytes, harbors the mutation in this family. Nucleotide sequencing revealed a T-to-C transition within exon 7 of the keratin 5 gene in the affected individuals of the family, while the unaffected individuals showed no evidence of C. The presence of the T-to-C transition in the affected individuals was confirmed by restriction enzyme digestion analysis with NciI endonuclease, as well as with PCR amplification of specific alleles (PASA) analysis. The PASA analysis also indicated that the mutated allele was not found among the 100 alleles tested within the general Finnish population indicating that the mutated allele is not a common polymorphism. Furthermore, the mutated allele was not present in nine individuals representing three different EBS families of Finnish origin. The T-to-C transition at the nucleotide level resulted in substitution of a leucine by a proline at the amino acid level, and the substitution affected a leucine residue which was invariant among eight different human keratins in a highly conserved segment at the carboxy-terminal region of the keratin 5 polypeptide.(ABSTRACT TRUNCATED AT 250 WORDS)

The roles of K5 and K14 head, tail, and R/K L L E G E domains in keratin filament assembly in vitro

Type I and type II keratins form obligatory heterodimers, which self-assemble into 10-nm intermediate filaments (IFs). Like all IF proteins, they have a central alpha-helical rod domain, flanked by nonhelical head and tail domains. The IF rod is more highly conserved than head and tail, and within the rod, the carboxy R/K L L E G E sequence is more highly conserved than most other regions. Mutagenesis studies have shed some light on the roles of the head, tail, and R/K L L E G E sequence in 10-nm filament structure. However, interpretations have often been complicated in part because many of these studies have focused on transfected cells, where filament structure cannot be evaluated. Of the few in vitro assembly studies thus far conducted, comparison of keratin mutants with other IF mutants have often been difficult, due to the obligatory heteropolymeric nature of keratin IFs. In this report, we describe in vitro filament assembly studies on headless, tailless, headless/tailless, and R/K L L E G E truncated mutants of keratin 5 and its partner keratin 14. Using varying conditions of ionic strength and pH, we examine effects of analogous K5 and K14 mutations on the stability of 10-nm filament structure. Using EM, we examine effects of mutations on the ability of subunits/protofibrils to (a) elongate and (b) laterally associate. Our results demonstrate that (a) tails of K5 and K14 are required for filament stabilization; (b) the head of K5, but not of K14, is required for filament elongation and lateral alignments; and (c) the R/K L L E G E domains are required for lateral alignments, but not for filament elongation.

The genetic basis of epidermolytic hyperkeratosis: a disorder of differentiation-specific epidermal keratin genes

Epidermolytic hyperkeratosis (EH) is a skin disease characterized by keratin filament clumping and degeneration in terminally differentiating epidermal cells. We have discovered that the genetic basis for EH resides in mutations in differentiation-specific keratins. Two of six distinct incidences of EH had a keratin 10 (K10) point mutation in a highly conserved arginine. Remarkably, this same residue is mutated in the basal epidermal K14 in three incidences of another skin disease, epidermolysis bullosa simplex (EBS). By genetic engineering, gene transfection, and 10 nm filament assembly, we show that this mutation is functionally responsible for the keratin filament clumping that occurs in basal (EBS) or suprabasal (EH) cells. These studies strengthen the link between filament perturbations, cell fragility, and degeneration first established with EBS. They also suggest a correlation between filament disorganization and either cytokinesis or nuclear shape, giving rise to the seemingly binucleate cells typical of EH.

Mutations in the rod domains of keratins 1 and 10 in epidermolytic hyperkeratosis

Epidermolytic hyperkeratosis is a hereditary skin disorder characterized by blistering and a marked thickening of the stratum corneum. In one family, affected individuals exhibited a mutation in the highly conserved carboxyl terminal of the rod domain of keratin 1. In two other families, affected individuals had mutations in the highly conserved amino terminal of the rod domain of keratin 10. Structural analysis of these mutations predicts that heterodimer formation would be unaffected, although filament assembly and elongation would be severely compromised. These data imply that an intact keratin intermediate filament network is required for the maintenance of both cellular and tissue integrity.

Transgenic mice expressing a mutant keratin 10 gene reveal the likely genetic basis for epidermolytic hyperkeratosis

Epidermolytic hyperkeratosis (EH; previously called bullous congenital ichthyosiform erythroderma) is an autosomal dominant skin disease of unknown etiology, affecting approximately 1 out of 300,000 people. It is typified by hyperkeratotic scaliness, blistering due to cytolysis within suprabasal epidermal cells, and hyperproliferation in basal cells. Histologically, EH epidermis exhibits a thickened stratum corneum and granular layer, with enlarged and irregular-shaped cells. Ultrastructurally, only suprabasal layers are affected, with three major aberrancies: (i) tonofilament clumping, (ii) nuclei and keratohyalin granules of irregular shape and size, and (iii) cell degeneration. We have discovered that transgenic mice expressing a mutant keratin 10 gene have the EH phenotype, thereby suggesting that a genetic basis for human EH residues in mutations in genes encoding suprabasal keratins K1 and K10. In addition, we show that (i) stimulation of basal cell proliferation can arise from a defect in suprabasal cells, and (ii) distortion of nuclear shape or aberrations in cytokinesis can occur when an intermediate filament network is perturbed.