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

The 47-kD lens-specific protein phakinin is a tailless intermediate filament protein and an assembly partner of filensin

In previous studies we have characterized a lens-specific intermediate filament (IF) protein, termed filensin. Filensin does not self-assemble into regular IFs but is known to associate with another 47-kD lens-specific protein which has been suggested to represent its assembly partner. To address this possibility, we cloned and sequenced the cDNA coding for the bovine 47-kD protein which we have termed phakinin (from the greek phi alpha kappa omicron sigma = phakos = lens). The predicted sequence comprises 406 amino acids and shows significant similarity (31.3% identity over 358 residues) to type I cytokeratins. Phakinin possesses a 95-residue, non-helical domain (head) and a 311 amino acid long alpha-helical domain punctuated with heptad repeats (rod). Similar to cytokeratin 19, phakinin lacks a COOH-terminal tail domain and it therefore represents the second known example of a naturally tailless IF protein. Confocal microscopy on frozen lens sections reveals that phakinin colocalizes with filensin and is distributed along the periphery of the lens fiber cells. Quantitative immunoblotting with whole lens fiber cell preparations and fractions of washed lens membranes suggest that the natural stoichiometry of phakinin to filensin is approximately 3:1. Under in vitro conditions, phakinin self-assembles into metastable filamentous structures which tend to aggregate into thick bundles. However, mixing of phakinin and filensin at an optimal ratio of 3:1 yields stable 10-nm filaments which have a smooth surface and are ultrastructurally indistinguishable from "mainstream" IFs. Immunolabeling with specific antibodies shows that these filaments represent phakinin/filensin heteropolymers. Despite its homology to the cytokeratins, phakinin does not coassemble with acidic (type I), or basic (type II) cytokeratins. From these data we conclude that filensin and phakinin are obligate heteropolymers which constitute a new membrane-associated, lens-specific filament system related to, but distinct from the known classes of IFs.

Molecular characterization of the body site-specific human epidermal cytokeratin 9: cDNA cloning, amino acid sequence, and tissue specificity of gene expression

Differentiation of human plantar and palmar epidermis is characterized by the suprabasal synthesis of a major special intermediate-sized filament (IF) protein, the type I (acidic) cytokeratin 9 (CK 9). Using partial amino acid (aa) sequence information obtained by direct Edman sequencing of peptides resulting from proteolytic digestion of purified CK 9, we synthesized several redundant primers by 'back-translation'. Amplification by polymerase chain reaction (PCR) of cDNAs obtained by reverse transcription of mRNAs from human foot sole epidermis, including 5'-primer extension, resulted in multiple overlapping cDNA clones, from which the complete cDNA (2353 bp) could be constructed. This cDNA encoded the CK 9 polypeptide with a calculated molecular weight of 61,987 and an isoelectric point at about pH 5.0. The aa sequence deduced from cDNA was verified in several parts by comparison with the peptide sequences and showed the typical structure of type I CKs, with a head (153 aa), and alpha-helical coiled-coil-forming rod (306 aa), and a tail (163 aa) domain. The protein displayed the highest homology to human CK 10, not only in the highly conserved rod domain but also in large parts of the head and the tail domains. On the other hand, the aa sequence revealed some remarkable differences from CK 10 and other CKs, even in the most conserved segments of the rod domain. The nuclease digestion pattern seen on Southern blot analysis of human genomic DNA indicated the existence of a unique CK 9 gene. Using CK 9-specific riboprobes for hybridization on Northern blots of RNAs from various epithelia, a mRNA of about 2.4 kb in length could be identified only in foot sole epidermis, and a weaker cross-hybridization signal was seen in RNA from bovine heel pad epidermis at about 2.0 kb. A large number of tissues and cell cultures were examined by PCR of mRNA-derived cDNAs, using CK 9-specific primers. But even with this very sensitive signal amplification, only palmar/plantar epidermis was found positive. By in situ hybridization and immunolocalization we further showed that CK 9 is only expressed in the suprabasal cell layers of this special epidermal tissue. We discuss the molecular properties of CK 9 and its cell type- and body site-specific expression in relation to the special differentiation of palmar/plantar epidermis and to diseases specific for this body site.

Tektin B1 from ciliary microtubules: primary structure as deduced from the cDNA sequence and comparison with tektin A1

Tektins are a class of proteins that form filamentous polymers in the walls of ciliary and flagellar microtubules, and they may also be present in centrioles, centrosomes and mitotic spindles. We report here the cloning and sequencing of a cDNA for ciliary tektin B1. Comparison of the predicted amino acid sequence of tektin B1 with the previously published sequence for tektin A1 reveals several features that better define this class of proteins. Like tektin A1, the central region of the tektin B1 polypeptide chain is predicted to form a coiled-coil rod, consisting of four major alpha-helical regions that are separated by non-helical linkers. Between the central rod domains of tektins A and B there is a 34%/20% amino acid sequence identity/similarity, including equivalent 50-residue segments containing 36 identities, and a high probability of long-range structural homology. The tektin polypeptide chains are divided into two major segments that have significant sequence homology to each other, both within a given tektin chain and between tektins A and B, indicative of gene duplication events. The tektins have a secondary structure and molecular design similar to, but a low primary sequence homology with, intermediate filament proteins. Unlike tektin A1, tektin B1 lacks any part of the C-terminal IFP consensus sequence.

Conservation of the structure of keratin intermediate filaments: molecular mechanism by which different keratin molecules integrate into preexisting keratin intermediate filaments during differentiation

During development and differentiation, the intermediate filament component of the cytoskeleton of many cells and tissues is rebuilt by a dynamic exchange process in which one set of protein chains is replaced by another, without recourse to creation of a new network. One major example is the replacement of keratin 5/keratin 14 (K5/K14) keratin intermediate filaments (KIFs) by K1/K10 KIFs during terminal differentiation in the epidermis. The present work was undertaken to explore how this may occur. We have induced lysine-lysine cross-links with disulfosuccinimidyl tartrate in K5/K14 KIFs in order to determine the axial dimensions and relative axial alignments of the K5/K14 molecules. Many of the cross-links induced in subfilamentous oligomers containing one, two, or three molecules were also found in the intact KIF, indicating that the body of data thus generated provides physiologically relevant information on the structural organization in the KIF. A least-squares analysis using as data the positions of lysine residues involved in 23 induced cross-links has allowed the axial alignments of the various coiled-coil segments in the rod domain to be determined. Three modes of antiparallel alignment of two neighboring molecules were found: A11 (staggered by -16.7 nm), A22 (staggered by 28.8 nm), and A12 (almost in register; staggered by only 0.3 nm). Since the axial repeat length is about 1 nm less than the molecular length, the data require a fourth mode of molecule alignment, termed ACN, in which similarly directed molecules are overlapped by the equivalent of about 5-10 residues.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Human hair keratins

Human hair keratins were among the first to be studied but it is only recently that sufficient information has been obtained to gain a basic biologic perspective of these proteins. Hair keratins are members of the intermediate filament family of proteins, yet are sufficiently divergent from epidermal keratins to warrant separate classification: type Ia and IIa ("hard"/hair keratins) and type Ib and IIb (epidermal and other "soft" keratins). As with hair keratins from other species, the human proteins may be distinguished from their epidermal counterparts by a relatively higher cysteine content, 7.6% versus 2.9%, respectively. This feature reflects utilization of disulfide bonding in producing a tougher, more durable structure in the tissues in which the hair keratins are distributed. Although prominent in hair, their distribution is not strictly limited to this tissue. A number of molecular characteristics have been elucidated from human hair keratin gene studies including amino acid sequence data for a type Ia hair keratin. Studies of various pedigrees has revealed a fairly wide latitude of variation in human hair keratin expression that is tolerated without associated obvious hair phenotypic change. Thus, a foundation of knowledge regarding these proteins has emerged and continues to evolve.

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.

Allelic variations of human keratins K4 and K5 provide polymorphic markers within the type II keratin gene cluster on chromosome 12

To appreciate point mutations in keratin genes as causes for hereditary epithelial diseases, the normal variation of these gene sequences in the population must be known. Because genetic polymorphism of keratins at the protein level due to allelic variation has been described for the type II keratins 4 and 5, we have analyzed their corresponding genes using single-strand conformation polymorphism gel electrophoresis and sequence analysis of polymerase chain reaction amplified genomic DNA. Although no sequence variations were found in the carboxyl-terminal and rod domains we were able to map the molecular differences among the alleles to their amino-terminal domains. In particular, we have identified three alleles of keratin 4. Two alleles differed by a nucleotide transition causing a neutral amino acid substitution (alanine to valine) and one allele had a 42-bp in-frame deletion corresponding to 14 amino acids within the V1 subdomain. Three alleles were also recognized for the keratin 5 locus, all being elicited by single nucleotide substitutions. Of these, only one altered the amino acid sequence, replacing an uncharged (glycine) with a charged (glutamic acid) amino acid in the H1 subdomain. Pedigree analyses in three families showed the alleles to be inherited as autosomal Mendelian traits. Thus, these normal alleles of keratins 4 and 5 will provide favorable polymorphic markers for linkage analysis directly within the cluster of type II keratin genes located on chromosome 12q to elucidate the potential involvement of these and other keratin genes in disorders of squamous cell differentiation.

Structure, function, and dynamics of keratin intermediate filaments

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

SF-assemblin, the structural protein of the 2-nm filaments from striated microtubule associated fibers of algal flagellar roots, forms a segmented coiled coil

The microtubule associated system I fibers of the basal apparatus of the flagellate green alga Spermatozopsis similis are noncontractile and display a 28-nm periodicity. Paracrystals with similar periodicities are formed in vitro by SF-assemblin, which is the major protein component of system I fibers. We have determined the amino acid sequence of SF-assemblin and show that it contains two structural domains. The NH2-terminal 31 residues form a nonhelical domain rich in proline. The rod domain of 253 residues is alpha-helical and seems to form a segmented coiled coil with a 29-residue repeat pattern based on four heptads followed by a skip residue. The distinct cluster of acidic residues at the COOH-terminal end of the motifs (periodicity about 4 nm) may be related to tubulin binding of SF-assemblin and/or its self assembly. A similar structure has been predicted from cDNA cloning of beta-giardin, a protein of the complex microtubular apparatus of the sucking disc in the protozoan flagellate Giardia lamblia. Although the rod domains of SF-assemblin and beta-giardin share only 20% sequence identity, they have exactly the same length and display 42% sequence similarity. These results predict that system I fibers and related microtubule associated structures arise from molecules able to form a special segmented coiled coil which can pack into 2-nm filaments. Such molecules seem subject to a strong evolutionary drift in sequence but not in sequence principles and length. This conservation of molecular architecture may have important implications for microtubule binding.

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.

Suprabasal change and subsequent formation of disulfide-stabilized homo- and hetero-dimers of keratins during esophageal epithelial differentiation

Rabbit esophageal epithelium, a parakeratinized stratified epithelium, synthesizes as one of its major differentiation products a keratin pair consisting of a basic K4 (59 kDa) and an acidic K13 (41 kDa) keratin. Although immunohistochemical staining data suggest that in esophageal epithelia of some other species these two keratins are suprabasally located, antigenic masking of the epitopes in the basal cells has not been ruled out. Using several well-characterized monoclonal antibodies including AE8, which specifically recognizes K13, coupled with biochemical analysis of keratins of basal and suprabasal cells isolated from confluent rabbit esophageal epithelial culture, we have obtained direct evidence that K4 and K13 keratins are largely absent in the undifferentiated basal cells, but are present in large amounts in suprabasal cells. We also show that in the cornified cell layers that are formed during the terminal stage of esophageal epithelial differentiation, K4 and K13 keratins become disulfide-crosslinked to form three different dimers. Two of them (110 kDa and 100 kDa) are heterodimers and consist of equimolar amounts of K4 and K13; they presumably represent isomers crosslinked via different cysteine residues. The third dimer (90 kDa) was found to be a homodimer of the acidic K13 keratin. Trypsinization experiment established that at least some of the disulfide crosslinks in the K4/K13 heterodimer must involve cysteine residues residing in the trypsin-resistant rod domains of keratins. Air-oxidation of in vitro reconstituted filaments reproduced the two heterodimers, which most likely involve the crosslinking between type I and type II keratins of different coiled coils. The formation of these disulfide-crosslinked keratin dimers, instead of higher molecular mass oligomers or polymers as occurring in the epidermis and hair, may contribute to the formation of cornified cells with a physical stability and rigidity that are optimal for esophageal function. Our data also suggest that interactions involved in the formation of homodimers, thought to be metastable and unimportant during the initial step of filament assembly (i.e. tetramer formation), may actually play an important role in stabilizing a higher order structure in mature keratin filaments.

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.

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.

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

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

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.

Changes in keratin expression during fetal and postnatal development of intestinal epithelial cells

We have investigated keratin expression in fetal, newborn and adult rat intestines by immunofluorescence staining, immunoblotting of two-dimensional gels and Northern blot analysis of total cellular RNAs. Keratin-type intermediate filaments, composed predominantly of keratin no. 19, were observed already in the undifferentiated stratified epithelium present at 15-16 days of gestation. The marked maturation and differentiation of the epithelium taking place at 18-19 days of gestation was characterized by the appearance of the differentiation-specific keratin no. 21 and by a significant increase in the relative amount of keratin no. 8. The keratin pattern typical of adult villus cells became established at the time of birth, and was marked by a considerable increase in the complexity of the keratin-related polypeptides detected on two-dimensional gels, indicative of extensive post-translational modification of all keratins. Starting at 20 days of gestation there was a major increase in the relative abundance of mRNAs coding for keratin nos. 8, 19 and 21; in contrast, the relative amount of keratin no. 18 mRNA reached a peak shortly after birth and declined to very low levels in adult intestine. These results demonstrated marked changes in keratin expression and post-translational processing taking place at key stages of intestinal development. The appearance of keratin no. 21 in coincidence with the formation of an adult-type brush border and terminal web would be consistent with it having an important role in the organization of the intermediate filament network in the apical cytoplasm of the differentiated intestinal cells.

Chemical cross-linking indicates a staggered and antiparallel protofilament of desmin intermediate filaments and characterizes one higher-level complex between protofilaments

Tetrameric rods, protofilaments and assembled filaments of desmin, the intermediate filament protein of muscle, have been chemically cross-linked with the lysine specific cross-linkers EGS [ethylene glycol bis(succinimidylsuccinate), 1.61 nm span] and bis(sulfosuccinimidyl) suberate (1.14 nm span). One bis(sulfosuccinimidyl)suberate and two EGS cross-links were isolated from the rod and characterized. They show that the two coiled coils in the rod tetramer are staggered by approximately 15-20 nm and strongly indicate an antiparallel arrangement in which the inner overlapping part of the rod is formed by the amino-terminal helices 1A, 1B and 2A. Both EGS cross-links identified in the rod were also isolated from cross-linked filaments. The isolated rod, therefore, represents a complex also present in identical, or very similar form in protofilaments and in assembled filaments. Cross-linked filaments yielded a third EGS cross-link that must have been formed between neighboring protofilaments. It connects the highly conserved carboxy-terminus of helix 2B of the first protofilament to the overlap region formed by helices 1A and 2A of the second protofilament. The restrictions posed by these cross-links on current filament models are discussed.

Truncation of recombinant vimentin by ompT. Identification of a short motif in the head domain necessary for assembly of type III intermediate filament proteins

Recombinant vimentin expressed in E. coli JM 101 cells is cleaved after cell lysis between arginines 11 and 12. The truncated vimentin is assembly incompetent. Expression of the same cDNA construct in BL21 cells, which lack the protease ompT, provides intact and polymerization-competent vimentin. The ompT cleavage site is contained in a short sequence motif (YRRMF) shared by the head domains of type III and IV intermediate filament (IF) proteins. We propose that a related motif present in the N-terminal 32 residues of lambda CII accounts for the known IF formation of a fusion protein formed with a truncated GFAP.

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

Intermediate filament (IF) assembly is remarkable, in that it appears to be self-driven by the primary sequence of IF proteins, a family (40- 220 kd) with diverse sequences, but similar secondary structures. Each IF polypeptide has a central 310 amino acid residue alpha-helical rod domain, involved in coiled-coil dinner formation. Two short (approximately 10 amino acid residue) stretches at the ends of this rod are more highly conserved than the rest, although the molecular basis for this is unknown. In addition, the rod is segmented by three short nonhelical linkers of conserved location, but not sequence. To examine the degree to which different conserved helical and nonhelical rod sequences contribute to dimer, tetramer, and higher ordered interactions, we introduced proline mutations in residues throughout the rod of a type I keratin, and we removed existing proline residues from the linker regions. To further probe the role of the rod ends, we introduced more subtle mutations near the COOH-terminus. We examined the consequences of these mutations on (a) IF network formation in vivo, and (b) 10-nm filament assembly in vitro. Surprisingly, all proline mutations located deep in the coiled-coil rod segment showed rather modest effects on filament network formation and 10-nm filament assembly. In addition, removing the existing proline residues was without apparent effect in vivo, and in vitro, these mutants assembled into 10-nm filaments with a tendency to aggregate, but with otherwise normal appearance. The most striking effects on filament network formation and IF assembly were observed with mutations at the very ends of the rod. These data indicate that sequences throughout the rod are not equal with respect to their role in filament network formation and in 10-nm filament assembly. Specifically, while the internal rod segments seem able to tolerate considerable changes in alpha-helical conformation, the conserved ends seem to be essential for creating a very specific structure, in which even small perturbations can lead to loss of IF stability and disruption of normal cellular interactions. These findings have important implications for the disease Epidermolysis Bullosa Simplex, arising from point mutations in keratins K5 or K14.

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

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

Intermediate filament structure

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

Central Rod Domain Insertion and Carboxy-Terminal Fusion Mutants of Human Cytokeratin K19 Are Incorporated into Endogenous Keratin Filaments

Keratins comprise a multigene family of structural proteins that form the 10-nm filaments present in epithelial cells. Keratin filament formation requires the presence of stoichiometric quantities of type I and type II keratin peptides. Each keratin peptide contains an N-terminal "head" segment, a C-terminal "tail" segment, and a highly conserved, alpha-helical central rod domain. To investigate the importance of these domains in situ, we have altered the DNA coding sequence of human cytokeratin K19 and transiently expressed the mutants in PtK2 cells that contain an endogenous keratin filament system. Interestingly, K19 mutants containing 4, 8, 12, and 24 amino acid insertions in the non-alpha-helical L1 region of the central rod domain successfully integrate into the endogenous PtK2 keratin filaments. Another K19 mutant, K19-bGAL, that encodes bacterial beta-galactosidase (bGAL) fused in phase to the 3' end of the K19 central rod domain, also integrates into the endogenous PtK2 keratin filaments. Our results demonstrate 1) that the spacing between the highly conserved amino and carboxy terminal ends of the K19 central rod domain can be increased without significantly effecting K19's ability to interact with keratin filaments and 2) that addition of a highly soluble 66-kDa tail to K19 does not impede its interaction with the filament system.

Polarized and functional epithelia can form after the targeted inactivation of both mouse keratin 8 alleles

We have tested the requirement of keratin intermediate filaments for the formation and function of a simple epithelium. We disrupted both alleles of the mouse keratin 8 (mK8) gene in embryonic stem cells, and subsequently analyzed the phenotype in developing embryoid bodies in suspension culture. After the inactivation of the mouse keratin 8 (mK8) gene by a targeted insertion, mK8 protein synthesis was undetectable. In the absence of mK8 its complementary partners mK18 and mK19 were unable to form filaments within differentiated cells. Surprisingly, these ES cells differentiate to both simple and cystic embryoid bodies with apparently normal epithelia. Ultrastructural analysis shows an apparently normal epithelium with microvilli on the apical membrane, tight junctions and desmosomes on the lateral membrane, and an underlying basal membrane. No significant differences in the synthesis or secretion of alpha 1-fetoprotein and laminin were observed between the mK8- or wild-type embryoid bodies. Our data show that mK8 is not required for simple epithelium formation of extraembryonic endoderm.

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

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

Cytokeratin phosphorylation, cytokeratin filament severing and the solubilization of the maternal mRNA Vg1

During meiotic maturation, the cortical cytokeratin filament system of the Xenopus oocyte disappears (Klymkowsky, M. W., and L. A. Maynell. 1989. Dev. Biol. 134:479). Here we demonstrate that this disappearance results from the severing of cytokeratin filaments into a heterogenous population of oligomers, with S- values ranging from 12S and greater. Cytokeratin filament severing correlates with the hyperphosphorylation of the type II cytokeratin of the oocyte. Both the severing of cytokeratin filaments and cytokeratin hyperphosphorylation are reversed by treatment with cycloheximide. These data suggest that fragmentation of cytokeratin filaments is controlled, at least in part, by the phosphorylation of the type II cytokeratin, and that the cytokeratin kinase activity responsible is biosynthetically labile. Cytokeratin filaments have been suggested to anchor the maternal mRNA Vg1 to the vegetal cortex of the oocyte (Pondel, M., and M. L. King. 1988. Proc. Natl. Acad. Sci. USA. 85:7216). By injecting fractions containing active maturation promoting factor or a purified, mutant cyclin protein, we find that the bulk of the Vg1 mRNA in the oocyte can be solubilized under conditions that block the fragmentation of cytokeratin filaments, and that the fragmentation of cytokeratin filaments itself leads to the solubilization of only a minor fraction of the Vg1 mRNA. Thus, at best, cytokeratin filaments directly anchor only a minor fraction of the Vg1 mRNA in the oocyte. Moreover, factors distinct from maturation promoting factor appear to be required for the complete solubilization of Vg1 mRNA during oocyte maturation.

Transient expression of mouse hair keratins in transfected HeLa cells: interactions between "hard" and "soft" keratins

Although it has been shown previously that an acidic (type I) "soft" keratin can interact with many basic (type II) "soft" keratins to form 10-nm intermediate filaments, it has been unclear whether "soft" keratins are compatible with the "hard" keratins typically found in hair and nail. To address this issue and to generate more structural information about hard keratins, we have isolated and sequenced a cDNA clone that encodes a mouse hair basic keratin (b4). Our sequence data revealed new information regarding the structural conservation of hard keratins as a group, being significantly different from soft keratins. Using expression vectors containing appropriate cDNA inserts, we studied the expression of this basic (b4) as well as an acidic (a1) mouse hair keratin in HeLa cells. The expression of these alien hair keratins in the transfected cells was surveyed using a panel of monoclonal and polyclonal antibodies. Our results indicated that the basic and acidic hair keratin readily incorporated into the existing endogenous soft keratin network of HeLa cells. Overproduction of hair keratin, however, occasionally led to the formation of cytoplasmic aggregates containing both hard and soft keratins. These data suggest that although small amounts of newly synthesized hair keratins can incorporate into the "scaffolding" of the preformed soft keratin filament network, possibly through dynamic subunit exchange, overproduction of hard keratins can lead to the partial collapse of the soft keratin network. These observations, along with the deduced amino acid sequence data, support and extend the concept that hard and soft keratins, although closely related, are divergent enough to justify their being divided into two separate subgroups.

Sorting out IF networks: consequences of domain swapping on IF recognition and assembly

Vimentin and keratin are coexpressed in many cells, but they segregate into two distinct intermediate filament (IF) networks. To understand the molecular basis for the sorting out of these IF subunits, we genetically engineered cDNAs encoding hybrid IF proteins composed of part vimentin and part type I keratin. When these cDNAs were transiently expressed in cells containing vimentin, keratin, or both IFs, the hybrid IF proteins all recognized one or the other or both networks. The ability to distinguish networks was dependent upon which segments of IF proteins were present in each construct. Constructs containing sequences encoding either helix 1B or helix 2B seemed to be the most critical in conferring IF recognition. At least for type I keratins, recognition was exerted at the level of dimer formation with wild-type type II keratin, as demonstrated by anion exchange chromatography. Interestingly, despite the fact that swapping of helical domains was not as deleterious to IF structure/function as deletion of helical domains, keratin/vimentin hybrids still caused structural aberrations in one or more of the cytoplasmic IF network. Thus, sequence diversity among IF proteins seems to influence not only coiled-coil but also higher ordered associations leading to 10-nm filament formation and/or IF interactions with other cellular organelles/proteins.

Keratin incorporation into intermediate filament networks is a rapid process

The properties of keratin-containing intermediate filament (IF) networks in vivo were studied following the microinjection of biotinylated keratin. Keratin-IFs were biotinylated, disassembled, and separated into type I and type II proteins by ion exchange chromatography. Recombination of these derivatized type I and type II keratins resulted in the formation of 10-nm diameter IF. The type I keratins were microinjected into epithelial cells and observed by immunofluorescence microscopy. Biotin-rich spots were found throughout the cytoplasm at 15-20 min after injection. Short biotinylated fibrous structures were seen at 30-45 min after injection, most of which colocalized with the endogenous bundles of IF (tono-filaments). By 1 1/2 to 2 h after microinjection, extensive biotinylated keratin IF-like networks were evident. These were highly coincident with the endogenous tonofilaments throughout the cell, including those at desmosomal junctions. These results suggest the existence of a relatively rapid subunit incorporation mechanism using numerous sites along the length of the endogenous tonofilament bundles. These observations support the idea that keratin-IFs are dynamic cytoskeletal elements.

Expression of chicken lamin B2 in Escherichia coli: characterization of its structure, assembly, and molecular interactions

Chicken lamin B2, a nuclear member of the intermediate-type filament (IF) protein family, was expressed as a full-length protein in Escherichia coli. After purification, its structure and assembly properties were explored by EM, using both glycerol spraying/low-angle rotary metal shadowing and negative staining for preparation, as well as by analytical ultracentrifugation. At its first level of structural organization, lamin B2 formed "myosin-like" 3.1S dimers consisting of a 52-nm-long tail flanked at one end by two globular heads. These myosin-like molecules are interpreted to represent two lamin polypeptides interacting via their 45-kD central rod domains to form a segmented, parallel and unstaggered 52-nm-long two-stranded alpha-helical coiled-coil, and their COOH-terminal end domains folding into globular heads. At the second level of organization, lamin B2 dimers associated longitudinally to form polar head-to-tail polymers. This longitudinal mode of association of laminin dimers is in striking contrast to the lateral mode of association observed previously for cytoplasmic IF dimers. At the third level of organization, these polar head-to-tail polymers further associated laterally, in an approximately half-staggered fashion, to form filamentous and eventually paracrystal-like structures revealing a pronounced 24.5-nm axial repeat. Finally, following up on recent studies implicating the mitotic cdc2 kinase in the control of lamin polymerization (Peter, M., J. Nakagawa, M. Dorée, J. C. Labbé, and E. A. Nigg. 1990. Cell. 61:591-602), we have examined the effect of phosphorylation by purified cdc2 kinase on the assembly properties and molecular interactions of the bacterially expressed lamin B2. Phosphorylation of chicken lamin B2 by cdc2 kinase interferes with the head-to-tail polymerization of the lamin dimers. This finding supports the notion that cdc2 kinase plays a major, direct role in triggering mitotic disassembly of the nuclear lamina.

Interactions of intermediate filament proteins from wool

Filaments of wool are heteropolymers formed by interaction of type I and type II intermediate filament (IF) proteins. There are four proteins in each of these two classes. Interaction of the reduced wool IF proteins was studied by two-dimensional electrophoresis which showed that complexes between type I and type II proteins were formed in solution at urea concentrations below 6 M. Complex formation between the carboxymethyl derivatives of wool IF proteins was studied using a filter binding assay in which radio-labelled individual components were allowed to react under various conditions with SDS-PAGE separated components after transfer to nitrocellulose. The results suggested that (i) absolute type specificity of interaction was maintained, (ii) fine specificity, i.e. preferential reaction between specific components is observed, (iii) wool IF proteins (hard keratins) also react, with the same type specificity, with soft keratins isolated from cow snout, (iv) the initial step in the polymerization sequence that leads to filament formation yields heterodimers.

Deletions in epidermal keratins leading to alterations in filament organization in vivo and in intermediate filament assembly in vitro

To investigate the sequences important for assembly of keratins into 10-nm filaments, we used a combined approach of (a) transfection of mutant keratin cDNAs into epithelial cells in vivo, and (b) in vitro assembly of mutant and wild-type keratins. Keratin K14 mutants missing the nonhelical carboxy- and amino-terminal domains not only integrated without perturbation into endogenous keratin filament networks in vivo, but they also formed 10-nm filaments with K5 in vitro. Surprisingly, keratin mutants missing the highly conserved L L E G E sequence, common to all intermediate filament proteins and found at the carboxy end of the alpha-helical rod domain, also assembled into filaments with only a somewhat reduced efficiency. Even a carboxy K14 mutant missing approximately 10% of the rod assembled into filaments, although in this case filaments aggregated significantly. Despite the ability of these mutants to form filaments in vitro, they often perturbed keratin filament organization in vivo. In contrast, small truncations in the amino-terminal end of the rod domain more severely disrupted the filament assembly process in vitro as well as in vivo, and in particular restricted elongation. For both carboxy and amino rod deletions, the more extensive the deletion, the more severe the phenotype. Surprisingly, while elongation could be almost quantitatively blocked with large mutations, tetramer formation and higher ordered lateral interactions still occurred. Collectively, our in vitro data (a) provide a molecular basis for the dominance of our mutants in vivo, (b) offer new insights as to why different mutants may generate different phenotypes in vivo, and (c) delineate the limit sequences necessary for K14 to both incorporate properly into a preexisting keratin filament network in vivo and assemble efficiently into 10-nm keratin filaments in vitro.

A single human keratin 18 gene is expressed in diverse epithelial cells of transgenic mice

The expression of keratin 18 (K18) is restricted in humans primarily to a variety of single layered or simple epithelia. However, direct introduction of a cloned K18 gene into cultured, somatic cells by DNA transfection has been shown to result in the promiscuous expression of K18 even while the endogenous mouse form of K18 (Endo B) remains silent. To determine if the cloned K18 genomic DNA fragment contains sufficient information to be regulated appropriately when subjected to a normal developmental environment, and to determine if the cloned gene is expressed in diverse epithelia, the K18 gene, including 2.5 kb of 5' flanking sequence and 3.5 kb of 3' flanking sequence, has been introduced into the germ line of mice. Mice from all three resulting K18 transgenic lines express the gene in an appropriate tissue-specific pattern that includes hepatocytes, simple epithelia of the intestinal tract, ductal cells of several glands and epithelial cells of the thymus. No expression of K18 was found in muscle, heart, or in most of the brain even in mice carrying 18 copies of the K18 gene. In most tissues, the level of K18 RNA was directly proportional to copy number and was as efficiently expressed as the endogenous Endo B gene. The K18 protein was identified by both protein blotting methods and indirect immunofluorescence staining. No pathological consequences of overexpression of the K18 gene were observed. The cloned K18 gene appears to contain all cis-acting DNA sequences necessary for appropriate expression. In addition, diverse epithelial cell types are able to express this single human gene.

Retrovirus-mediated transgenic keratin expression in cultured fibroblasts: specific domain functions in keratin stabilization and filament formation

With retrovirus-mediated gene transfer, we used intact and deleted keratin proteins to investigate the molecular basis of intermediate filament function. Three levels of assembly show a different stringency for the involvement of individual keratin domains: protein accumulation requires the alpha helix domains; stable filament formation additionally requires both N- and C-terminal domains of either one of the two interacting keratins, suggesting that head to tail homotypic interaction is important for effective elongation; and higher order organization of the cytoplasmic network depends on correct type I-type II pairing of keratins. The presence of two distinct interaction sites along potentially different axes may explain the characteristic morphology of keratin intermediate filament networks.

Elucidating the early stages of keratin filament assembly

Because of extraordinarily tight coiled-coil associations of type I and type II keratins, the composition and structure of keratin subunits has been difficult to determine. We report here the use of novel genetic and biochemical methods to explore the early stages of keratin filament assembly. Using bacterially expressed humans K5 and K14, we show that remarkably, these keratins behave as 1:1 complexes even in 9 M urea and in the presence of a reducing agent. Gel filtration chromatography and chemical cross-linking were used to identify heterodimers and heterotetramers as the most stable building blocks of keratin filament assembly. EM suggested that the dimer consists of a coiled-coil of K5 and K14 aligned in register and in parallel fashion, and the tetramer consists of two dimers in antiparallel fashion, without polarity. In 4 M urea, both end-to-end and lateral packing of tetramers occurred, leading to a variety of larger heteromeric complexes. The coexistence of multiple, higher-ordered associations under strongly denaturing conditions suggests that there may not be a serial sequence of events leading to the assembly of keratin intermediate filaments, but rather a number of associations may take place in parallel.