Intermediate filaments in non-neuronal cells of invertebrates: isolation and biochemical characterization of intermediate filaments from the esophageal epithelium of the mollusc Helix pomatia

Intermediate filament genes as differentiation markers in the leech Helobdella

The intermediate filament (IF) cytoskeleton is a general feature of differentiated cells. Its molecular components, IF proteins, constitute a large family including the evolutionarily conserved nuclear lamins and the more diverse collection of cytoplasmic intermediate filament (CIF) proteins. In vertebrates, genes encoding CIFs exhibit cell/tissue type-specific expression profiles and are thus useful as differentiation markers. The expression of invertebrate CIFs, however, is not well documented. Here, we report a whole-genome survey of IF genes and their developmental expression patterns in the leech Helobdella, a lophotrochozoan model for developmental biology research. We found that, as in vertebrates, each of the leech CIF genes is expressed in a specific set of cell/tissue types. This allows us to detect earliest points of differentiation for multiple cell types in leech development and to use CIFs as molecular markers for studying cell fate specification in leech embryos. In addition, to determine the feasibility of using CIFs as universal metazoan differentiation markers, we examined phylogenetic relationships of IF genes from various species. Our results suggest that CIFs, and thus their cell/tissue-specific expression patterns, have expanded several times independently during metazoan evolution. Moreover, comparing the expression patterns of CIF orthologs between two leech species suggests that rapid evolutionary changes in the cell or tissue specificity of CIFs have occurred among leeches. Hence, CIFs are not suitable for identifying cell or tissue homology except among very closely related species, but they are nevertheless useful species-specific differentiation markers.

Yolk nucleus--the complex assemblage of cytoskeleton and ER is a site of lipid droplet formation in spider oocytes

Oocytes (future egg cells) of various animal groups often contain complex organelle assemblages (Balbiani bodies, yolk nuclei). The molecular composition and function of Balbiani bodies, such as those found in the oocytes of Xenopus laevis, have been recently recognized. In contrast, the functional significance of more complex and highly ordered yolk nuclei has not been elucidated to date. In this report we describe the structure, cytochemical content and evolution of the yolk nucleus in the oocytes of a common spider, Clubiona sp. We show that the yolk nucleus is a spherical, rather compact and persistent cytoplasmic accumulation of several different organelles. It consists predominantly of a highly elaborate cytoskeletal scaffold of condensed filamentous actin and a dense meshwork of intermediate-sized filaments. The yolk nucleus also comprises cisterns of endoplasmic reticulum, mitochondria, lipid droplets and other organelles. Nascent lipid droplets are regularly found in the cortical regions of the yolk nucleus in association with the endoplasmic reticulum. Single lipid droplets become surrounded by filamentous cages formed by intermediate filaments. Coexistence of the forming lipid droplets with the endoplasmic reticulum in the cortical zone of the yolk nucleus and their later investment by intermediate-sized filamentous cages suggest that the yolk nucleus is the birthplace of lipid droplets.

The desmosomal plaque and the cytoskeleton

Two major plasma membrane domains are involved in the architectural organization of the cytoskeleton. Both are junctions of the adherens category characterized by the presence of dense plaques associated with the cytoplasmic surface of their membranes. The plaques serve as specific anchorage structures for two different types of cytoplasmic filaments. Intermediate-sized filaments (IF) of several types, i.e. cytokeratin IF in epithelial cells, desmin IF in cardiac myocytes and vimentin IF in arachnoidal cells of meninges, meningiomas and several other cells, attach to the desmosomal plaques, whereas actin-containing microfilaments associate with non-desmosomal adhering junctions such as the zonula adherens, fascia adherens and punctum adherens. The plaques of both kinds of adhering junctions contain a common acidic polypeptide of Mr 83,000 identical to 'band 5 protein' of bovine snout epidermal desmosomes. However, other plaque components are mutually exclusive to one of the two subclasses of adhering junctions. The desmosomal plaque structure, which does not contain vinculin and alpha-actinin, comprises representatives of cytoplasmic, non-membrane-integrated proteins such as desmoplakin(s) and the cytoplasmic portions of transmembrane glycoproteins such as 'band 3 glycoprotein'. The analysis of both categories of junction-associated plaques should provide a basis for understanding the establishment and the dynamics of junction-cytoskeleton interaction.

Identification of a neurofilament-like protein in the protocerebral tract of the crab Ucides cordatus

Neurofilaments (NFs) have not been observed in crustaceans using conventional electron microscopy, and intermediate filaments have never been described in crustaceans and other arthropods by immunocytochemistry. Since polypeptides, labeled by the NN18-clone antibody, were revealed on microtubule side-arms of crayfish, we have tested, in this study, whether proteins similar to mammalian NFs are present in the protocerebral tract (PCT) of the crab Ucides cordatus. We used immunohistochemistry for light microscopy with monoclonal antibodies against three different NF subunits, high (NF-H), medium (NF-M), and light (NF-L). Labeling was observed with the NN18-clone, which recognizes NF-M. In order to confirm the results obtained with the immunohistochemical reactions, Western blotting, using the three primary antibodies, was performed and the presence of NF-M was confirmed. The NN18-clone monoclonal antibody recognized a protein of approximately 160 kDa, similar to the mammalian NF-M protein, but NF-L and NF-H were not recognized. Conventional transmission electron microscopy was used to observe the ultrastructural components of the axons and immunoelectron microscopy was used to show the distribution of the NF-M-like polypeptides along cytoskeletal elements of the PCT. Our results agree with previous studies on crustacean NF proteins that have reported negative immunoreactions against NF-H and NF-L subunits and positive immunoreactions against the mammalian NF-M subunit. However, the protein previously referred to as P600 and recognized by the NN18-clone, has a very high molecular weight, thus, being different from mammalian NF-M subunit and from the protein revealed now in our study.

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

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

Functional analysis of keratin components in the mouse hair follicle inner root sheath

BACKGROUND

Recently, a family of novel type I keratins of the inner root sheath of the hair follicle were discovered, increasing the number of keratins known to be expressed in the hair follicle. The mouse database shows three keratins that are possible orthologues of these inner root sheath keratins. The sequences of these keratins include rather unusual changes to a highly conserved motif at the end of the alpha-helical rod domain of the proteins, thought to be important in filament assembly.

OBJECTIVES

To investigate whether these keratins are expressed in the inner root sheath and to determine whether they assemble normally.

METHODS

To investigate this, polyclonal antibodies were raised for immunolocalization of the keratins and their cDNAs were cloned for transfection into cultured cells.

RESULTS

At least two of these keratins were expressed in the inner root sheath but the timing of expression of the different keratins was variable. Transfection of the relevant cDNAs into cells in culture indicated that these keratins were capable of integrating into existing keratin networks without disruption, but that de novo filament assembly with the type II inner root sheath keratin, mK6irs, was poor.

CONCLUSIONS

These results provide further evidence of the complexity of keratin expression in the three concentric layers of the inner root sheath.

The intermediate filament protein consensus motif of helix 2B: its atomic structure and contribution to assembly

Nearly all intermediate filament proteins exhibit a highly conserved amino acid motif (YRKLLEGEE) at the C-terminal end of their central alpha-helical rod domain. We have analyzed its contribution to the various stages of assembly by using truncated forms of Xenopus vimentin and mouse desmin, VimIAT and DesIAT, which terminate exactly before this motif, by comparing them with the wild-type and tailless proteins. It is surprising that in buffers of low ionic strength and high pH where the full-length proteins form tetramers, both VimIAT and DesIAT associated into various high molecular weight complexes. After initiation of assembly, both VimIAT and DesIAT aggregated into unit-length-type filaments, which rapidly longitudinally annealed to yield filaments of around 20 nm in diameter. Mass measurements by scanning transmission electron microscopy revealed that both VimIAT and DesIAT filaments contained considerably more subunits per cross-section than standard intermediate filaments. This indicated that the YRKLLEGEE-motif is crucial for the formation of authentic tetrameric complexes and also for the control of filament width, rather than elongation, during assembly. To determine the structure of the YRKLLEGEE domain, we grew crystals of peptides containing the last 28 amino acid residues of coil 2B, chimerically fused at its amino-terminal end to the 31 amino acid-long leucine zipper domain of the yeast transcription factor GCN4 to facilitate appropriate coiled-coil formation. The atomic structure shows that starting from Tyr400 the two helices gradually separate and that the coiled coil terminates with residue Glu405 while the downstream residues fold away from the coiled-coil axis.

Assembly and architecture of invertebrate cytoplasmic intermediate filaments reconcile features of vertebrate cytoplasmic and nuclear lamin-type intermediate filaments

The two major intermediate filament (IF) proteins from the esophagus epithelium of the snail Helix pomatia and the two major IF proteins from muscle tissue of the nematode Ascaris suum were investigated under a variety of assembly conditions. The lowest-order complexes from each of the four protostomic invertebrate (p-INV) IF proteins are parallel, unstaggered dimers involving two-stranded alpha-helical coiled coil formation of their approximately 350 amino acid residue central rod domain (i.e. long-rod). In the electron microscope these are readily recognized by their distinct approximately 56 nm long rod with two globular domains (i.e. representing the non-helical carboxy-terminal tail domain of the p-INV IF proteins) attached at one end, closely resembling vertebrate lamin dimers. The next-higher-order oligomers are tetramers, which are easily recognized by their two pairs of globular tail domains attached at either end of a approximately 72 nm long central rod portion. According to their size and shape, these tetramers are built from two dimers associated laterally in an antiparallel, approximately half-staggered fashion via the amino-terminal halves of their rod domains. This is similar to the NN-type tetramers found as the most abundant oligomer species in all types of vertebrate cytoplasmic IF proteins, which contain a approximately 310 amino acid residue central rod domain (i.e. short-rod). As a first step toward filament formation, the p-INV IF tetramers anneal longitudinally into protofilaments by antiparallel CC-type association of the carboxy-terminal halves of their dimer rods. The next step involves radial growth, occurring initially through lateral association of two four-chain protofilaments into octameric subfibrils, which then further associate into mature, full-width filaments. Head-to-tail polymers of dimers and paracrystalline fibers commonly observed with vertebrate lamins were only rarely seen with p-INV IF proteins. The globular domains residing at the carboxy-terminal end of p-INV IF dimers were studding the surface of the filaments at regular, approximately 24.5 nm intervals, thereby giving them a "beaded" appearance with an axial periodicity of about 24.5 nm, which is approximately 3 nm longer than the corresponding approximately 21.5 nm repeat pattern exhibited by short-rod vertebrate IFs.

In vitro assembly of Drosophila lamin Dm0--lamin polymerization properties are conserved

Vertebrate nuclear lamins exhibit polymerization properties that are remarkably different from those of vertebrate cytoplasmic intermediate filament (IF) proteins. Notably, under conditions where vertebrate cytoplasmic IF proteins form tetramers consisting of laterally associated dimers, nuclear lamin dimers associate longitudinally into head-to-tail polymers. Also, in vitro, nuclear lamins readily form paracrystalline fibers, rather than stable 10-nm filaments. To investigate whether these properties are also shared with invertebrate nuclear lamins, we analyzed in considerable detail the polymerization behavior of recombinant full-length lamin Dm0 from the invertebrate Drosophila melanogaster. This lamin differs substantially from vertebrate lamins in its primary structure. We also analyzed lamin Dm0-derived fragments lacking either the head domain (headless), the tail domain (tailless), or both (rod). Like vertebrate lamins, full-length Drosophila lamin Dm0 assembled into head-to-tail polymers, with little or no formation of tetramers by lateral association of dimers. This longitudinal assembly was severely inhibited by deletion of the head domain. Removal of the tail domain led to increased formation of filamentous polymers. Under appropriate conditions, full-length Drosophila lamin Dm0 as well as the three lamin Dm0-derived fragments assembled into paracrystalline fibers. No steady-state condition tested yielded assembly of 10-nm filaments resembling those formed by vertebrate cytoplasmic IF proteins. These findings indicate that the in vitro assembly behavior of nuclear lamins is highly conserved but distinct from that of cytoplasmic IF proteins, thus evidencing its functional importance.

A cDNA from Drosophila melanogaster encodes a lamin C-like intermediate filament protein

A novel intermediate filament cDNA, pG-IF, has been isolated from a Drosophila melanogaster embryonic expression library screened with a polyclonal antiserum produced against a 46 kDa cytoskeletal protein isolated from Kc cells. This 46 kDa protein is known to be immunologically related to vertebrate intermediate filament proteins. The screen resulted in the isolation of four different cDNA groups. Of these, one has been identified as the previously characterized Drosophila nuclear lamin cDNA, Dm0, and a second, pG-IF, demonstrates homology to Dm0 by cross hybridization on Southern blots. DNA sequence analysis reveals that pG-IF encodes a newly identified intermediate filament protein in Drosophila. Its nucleotide sequence is highly homologous to nuclear lamins with lower homology to cytoplasmic intermediate filament proteins. pG-IF predicts a protein of 621 amino acids with a predicted molecular mass of 69,855 daltons. In vitro transcription and translation of pG-IF yielded a protein with a SDS-PAGE estimated molecular weight of approximately 70 kDa. It contains sequence principles characteristic of class V intermediate filament proteins. Its near neutral pI (6.83) and the lack of a terminal CaaX motif suggests that it may represent a lamin C subtype in Drosophila. In situ hybridization to polytene chromosomes detects one band of hybridization on the right arm of chromosome 2 at or near 51A. This in conjunction with Southern blot analysis of various genomic digests suggests one or more closely placed genes while Northern blot analysis detects two messages in Kc cells.

Immunological detection of cytoskeletal proteins in the exoerythrocytic stages of malaria by fluorescence and confocal laser scanning microscopy

Using monospecific antibodies, the presence and distribution of tubulin, actin, myosin, intermediate filaments, and lamins were examined in the exoerythrocytic liver schizont of Plasmodium berghei by conventional indirect fluorescent antibody methods and confocal laser scanning microscopy. The binding reactivity of the antibodies to parasite proteins was determined by Western blot analysis. The localisation of all antibodies in control host hepatocytes followed expected distributions in both uninfected and infected hepatocytes; by contrast, reactivity to the exoerythrocytic schizont was variable. The parasite reacted positively with selected anti-tubulin, -actin, and -myosin antibodies in both fluorescence and Western blot analysis. Anti-lamin antibodies were positive by confocal indirect fluorescent antibody labelling, but no labelling was detected with anti-intermediate filament antibody. Within the technical limits of resolution of the methods as applied to asynchronous parasite infections, not one of the antibodies reacting positively with the parasite by the indirect fluorescent antibody technique could be shown to identify unequivocally the classic architectural features associated with their respective target organelles, i.e. microtubules, stress-fibres or the nuclear envelope.

Reorganization of the cytoskeleton during Drosophila oogenesis: implications for axis specification and intercellular transport

Inhibitor studies have implicated microtubules in at least three important developmental processes during Drosophila oogenesis: oocyte determination and growth during stages 1 through 6, positioning of the anterior determinant bicoid mRNA during stages 9 through 12, and ooplasmic streaming during stages 10b through 12. We have used fluorescence cytochemistry together with laser scanning confocal microscopy to identify distinct microtubule structures at each of the above three periods that are likely to be involved in these processes. During stages 1 through 7, maternal components synthesized in nurse cells are transported through cytoplasmic bridges to the oocyte. At this time, microtubules that appear to originate in the oocyte pass through these cytoplasmic bridges into the adjacent nurse cells; these microtubules are likely to serve as a polarized scaffold on which maternal RNAs and proteins are transported. During stages 7 and 8, microtubules in the oocyte cortex reorganize to form an anterior-to-posterior gradient, suggesting a role for microtubules in the localization of morphogenetic determinants. Finally, when ooplasmic streaming begins during stage 10 b, it is accompanied by the assembly of subsurface microtubule arrays that spiral around the oocyte; these arrays disassemble as the oocyte matures and streaming stops. During ooplasmic streaming, many vesicles are closely associated with the subsurface microtubules, suggesting that streaming is driven by vesicle translocation along microtubules. We believe that actin plays a secondary role in each of these morphogenetic events, based on our parallel studies of actin organization during each of the above stages of oogenesis.

Intermediate filaments in Sertoli cells

Using immunohistochemical techniques both at light and electron microscopic levels, the arrangement and distribution of intermediate filaments in Sertoli cells of normal testis (in rat and human), during pre- and postnatal development (in rabbit, rat, and mouse) and under experimental and pathological conditions (human, rat), have been studied and related to the pertinent literature. Intermediate filaments are centered around the nucleus, where they apparently terminate in the nuclear envelope providing a perinuclear stable core area. From this area they radiate to the plasma membranes; apically often a close association with microtubules is seen. Basally, direct contacts of the filaments with focal adhesions occur, while the relationship to the different junctions of Sertoli cells is only incompletely elucidated. In the rat (not in human) a group of filaments is closely associated with the ectoplasmic specializations surrounding the head of elongating spermatids. Both in rat and human, changes in cell shape during the spermatogenic cycle are associated with a redistribution of intermediate filaments. As inferred from in vitro studies reported in the literature, these changes are at least partly hormone-dependent (vimentin phosphorylation subsequent to FSH stimulation) and influenced by local factors (basal lamina, germ cells). Intermediate filaments, therefore, are suggested to be involved in the hormone-dependent mechanical integration of exogenous and endogenous cell shaping forces. They permit a cycle-dependent compartmentation of the Sertoli cell into a perinuclear stable zone and a peripheral trafficking zone with fluctuating shape. The latter is important with respect to the germ cell-supporting surface of the cell which seems to limit the spermatogenetic potential of the male gonad.

The role of the head and tail domain in lamin structure and assembly: analysis of bacterially expressed chicken lamin A and truncated B2 lamins

Nuclear lamins like cytoplasmic intermediate filament proteins exhibit a characteristic tripartite domain structure with a segmented alpha-helical rod domain flanked by an N-terminal head and a C-terminal tail domain. To examine the influence of the head and tail domains on the structure and assembly properties of nuclear lamins, we have engineered "headless," "tailless," and "rod" chicken lamin B2 cDNAs and expressed them in Escherichia coli. A full-length chicken lamin A cDNA was also expressed in E. coli, and the recombinant protein compared with the structure and assembly properties of full-length chicken lamin B2 (E. Heitlinger et al. (1991) J. Cell Biol. 113, 485-495). As with lamin B2, at their first level of structural organization, lamin A and the headless lamin B2 formed myosin-like dimers consisting of a 51- to 52-nm-long tail flanked by two globular heads at one end. Similarly, the tailless and rod lamin B2 fragments formed tropomyosin-like dimers consisting of a 51 to 52-nm-long rod. In contrast to the lateral mode of association of cytoplasmic IF dimers into four-chain tetramers, at their second level of structural organization, lamin A dimers, just as lamin B2 dimers (E. Heitlinger et al. (1991) J. Cell Biol. 113, 485-495), associated longitudinally to form polar head-to-tail polymers. Whereas dimers made of the truncated B2 headless and rod lamins had lost their propensity to associate head-to-tail, tailless lamin B2 dimers revealed an enhanced head-to-tail association. Finally, at their third level of structural organization, rather than assembling into stable 10-nm filaments, both lamin A and the three truncated B2 lamins formed paracrystalline arrays exhibiting distinct transverse banding patterns with axial repeats of either 24 or 48-49 nm depending on the species.

The crayfish neuronal cytoskeleton: an investigation of proteins having neurofilament-like immunoreactivity

We have evaluated the possibility that proteins similar to mammalian neurofilament proteins (NFPs) are present in crustacean neurons. A panel of monoclonal antibodies (mAbs), raised against mammalian NFP, was used to identify candidate proteins. The degree to which these proteins are similar to mammalian NFPs was further evaluated using the following criteria: tissue specificity, recognition by the neurofilament-specific Bodian silver strain, recognition by the intermediate filament-specific Pruss mAb, and insolubility following detergent-extraction. Three candidate polypeptides were identified by mAb screening: a very high molecular weight polypeptide (Mr greater than 300 kDa), a 40 kDa polypeptide, and a group of 4 bands at Mr = 66-84 kDa. Although all of these polypeptides were recognized by one or more anti-NFP mAb, not one of them was found exclusively in neuronal tissue, not one was stained by the NFP-specific Bodian method, and all were soluble under conditions in which mammalian NFPs are insoluble. As a result of this thorough evaluation, we conclude that crayfish neurons do not contain neurofilament-like proteins. Although not closely related to mammalian neurofilaments, the very high molecular weight crayfish polypeptide which was strongly labeled by a commercially available anti-NF-M mAb (clone NN18) during the mAb screening, may be a novel cytoskeletal protein. The evidence for this conclusion comes from immunocytochemical labeling experiments. Indirect immunofluorescence labeling of this protein differentially labeled axons, such that labeling intensity of specific axons was proportional to the relative concentration of cytoskeletal organelles in those axons. Labeling of neuronal cell bodies delineated a fibrous network throughout the cytoplasm, and intensely labeled microtubule-rich regions of cytoplasm which are characteristic of larger neuronal somata. Immunogold labeling and electron microscopic analysis of the distribution of this protein revealed that the NN18-clone antibody bound to an antigen located on microtubule side-arms.

Evolutionary trends of neurofilament proteins in fish

1. Neurofilament complement was studied in an early chordate (Ciona intestinalis) and six fish species by immunoblot with antisera specific for each of the three mammalian NF subunits. 2. The anti-NF-H and anti-NF-M antisera were characterized as strictly specific for phosphorylated epitopes located in the carboxyterminal domain. 3. The NF-L subunit is absent in primitive chordates and appears first in fish; it can be identified on the basis of its apparent mol. wt, its reactivity with the anti-IFA antibody and with polyclonal antibodies raised to the NF-L subunit of mammals. 4. Primitive chordate neurofilaments are constituted by a single polypeptide of ca 160,000 mol. wt exhibiting only M-type phosphorylation-dependent epitopes. 5. Primitive fish (Acipenser transmontanus, Salmo gairdneri, Scorpaena porcus, Serranus scriba) possess only a single high mol. wt NF subunit reacting with both anti-NF-H and anti-NF-M antiserum while more recent species (Mugil saliens, Perca fluviatilis) possess two high mol. wt NF subunits which are immunologically distinct as to their phosphorylation structures. 6. The existence in some fish species of two high mol. wt NF polypeptides suggests that the process of gene duplication and diversification supposed to have given rise to the two high mol. wt NF subunits of mammals and birds has occurred repeatedly in vertebrate evolution, and may be regarded as a case of convergent evolution.

Cellular intermediate filament networks and their derangement in alcoholic hepatitis

Intermediate filaments are major components of most eukaryotic cells that form from the polymerization of protein subunits that are expressed in tissue and development specific fashions. The interactions of intermediate filaments with a myriad of other cellular proteins and structures give rise to a complex overall cellular architecture that is likely responsible for cellular well-being. The mature 10-nm filaments are relatively stable cellular structures, but the intermediate filaments undergo major morphological and biochemical changes, especially during mitosis, differentiation, and in response to certain drugs. Evidence exists that hepatocyte intermediate filaments (keratin filaments) are deranged in alcoholic hepatitis, an inflammatory liver disease of alcoholics and heavy spree drinkers. The classical and characteristic pathological hepatocyte inclusion bodies of alcoholic hepatitis, Mallory bodies, are composed in part of normal keratins that likely derive from the pre-existing hepatocyte intermediate filament network. It is unclear if intermediate filament network derangement in alcoholic hepatitis is directly caused by the actions of ethanol or its metabolites on intermediate filaments or their associated structures, or whether alcohol causes a cellular insult or injury elsewhere and a subsequent response (e.g., immune) causes intermediate filament network derangement. The precise mechanisms responsible for intermediate filament derangement remain to be elucidated; however, experimental data exist that support and refute several hypotheses. Hopefully, further studies will help determine a better overall understanding of the abnormalities of intermediate filaments and their relationship to the pathophysiology of alcoholic hepatitis and other diseases.

Identification of protein IT of the intestinal cytoskeleton as a novel type I cytokeratin with unusual properties and expression patterns

A major cytoskeletal polypeptide (Mr approximately 46,000; protein IT) of human intestinal epithelium was characterized by biochemical and immunological methods. The polypeptide, which was identified as a specific and genuine mRNA product by translation in vitro, reacted, in immunoblotting after SDS-PAGE, only with one of numerous cytokeratin (CK) antisera tested but with none of many monoclonal CK antibodies. In vitro, it formed heterotypic complexes with the type II CK 8, as shown by blot binding assays and gel electrophoresis in 4 M urea, and these complexes assembled into intermediate filaments (IFs) under appropriate conditions. A chymotrypsin-resistant Mr approximately 38,000 core fragment of protein IT could be obtained from cytoskeletal IFs, indicating its inclusion in a coiled coil. Antibodies raised against protein IT decorated typical CK fibril arrays in normal and transformed intestinal cells. Four proteolytic peptide fragments obtained from purified polypeptide IT exhibited significant amino acid sequence homology with corresponding regions of coils I and II of the rod domain of several other type I CKs. Immunocytochemically, the protein was specifically detected as a prominent component of intestinal and gastric foveolar epithelium, urothelial umbrella cells, and Merkel cells of epidermis. Sparse positive epithelial cells were noted in the thymus, bronchus, gall bladder, and prostate gland. The expression of protein IT was generally maintained in primary and metastatic colorectal carcinomas as well as in cell cultures derived therefrom. A corresponding protein was also found in several other mammalian species. We conclude that polypeptide IT is an integral IF component which is related, though somewhat distantly, to type I CKs, and, therefore, we propose to add it to the human CK catalogue as CK 20.

Comparative immunohistochemical study of glial filament proteins (glial fibrillary acidic protein and vimentin) in goldfish, octopus, and snail

Glial fibrillary acidic protein (GFAP) and vimentin proteins are known to be component proteins of glial filaments in the CNS of many vertebrates. The nature of the filaments present in the glial cells of the goldfish optic tectum and in the CNS of two members of the Mollusca (Helix pomatia and Octopus vulgaris) were investigated using immunocytochemical localization of monoclonal antibodies to GFAP and vimentin. Immunoblots visualized by the alkaline phosphatase method showed cross-reactive protein bands to GFAP and vimentin antibodies in total brain homogenates of the goldfish, octopus, and snail. Immunofluorescence staining of the goldfish optic tectum showed GFAP immunoreactivity, primarily in the ependymal cell processes. Immunogold labelling at the ultrastructural level verified that GFAP antibodies were bound to glial filaments. Immunolabelling of the optic lobe of Octopus vulgaris and the cerebral ganglia of Helix pomatia suggests that a protein exhibiting antigenic properties similar to GFAP is a component protein in the filaments of the protoplasmic and filamentous glia randomly distributed throughout the CNS. Unlike anti-GFAP antibodies, which stained relatively specific to filaments, vimentin staining in the CNS tissues of the three organisms studied did not appear to be exclusive to filamentous structures. As vimentin protein has been shown, in previous studies as well as our own, to exist in many tissue types, this suggests that it does not appear to be confined to glial filaments but is shared with other subcellular components. The proteins GFAP and vimentin which are thought to be well conserved in vertebrate evolution also appear to be expressed in the nervous system of some lower organisms.

The major component of the paraflagellar rod of Trypanosoma brucei is a helical protein that is encoded by two identical, tandemly linked genes

The flagellum of the parasitic hemoflagellate Trypanosoma brucei contains two major structures: (a) the microtubule axoneme, and (b) a highly ordered, filamentous array, the paraflagellar rod (PFR). This is a complex, three-dimensional structure, of yet unknown function, that extends along most of the axoneme and is closely linked to it. Its major structural component is a single protein of 600 amino acids. This PFR protein can assume two different conformations, resulting in two distinct bands of apparent molecular masses of 73 and 69 kD in SDS-gel electrophoresis. Secondary structure predictions indicate a very high helix content. Despite its biochemical similarity to the intermediate filament proteins (solubility properties, amino acid composition, and high degree of helicity), the PFR protein does not belong in this class of cytoskeletal proteins. The PFR protein is coded for by two tandemly linked genes of identical nucleotide sequence. Both genes are transcribed into stable mRNAs of very similar length that carry the mini-exon sequence at their 5' termini.

Localization of cytokeratins in tissues of the rainbow trout: fundamental differences in expression pattern between fish and higher vertebrates

Using a panel of antibodies against different cytokeratins in immunofluorescence microscopy on frozen tissue sections and two-dimensional gel electrophoresis of cytoskeletal proteins from these tissues, we have studied the tissue distribution of cytokeratins in a fish, the rainbow trout Salmo gairdneri. We have distinguished at least 14 different cytokeratin polypeptides in only a limited number of tissues, thus demonstrating the great complexity of the cytokeratin pattern in a fish species. The simplest cytokeratin pattern was that present in hepatocytes, comprising one type-II (L1) and two type-I (L2, L3) polypeptides that appear to be related to mammalian cytokeratins 8 and 18, respectively. Two or all three cytokeratins of this group were also identified in several other epithelial tissues, such as kidney. Epithelia associated with the digestive tract contained, in addition, other major tissue-specific cytokeratins, such as components D1-D3 (stomach, intestine and swim bladder) and B1 and B2 (biliary tract). With the exception of D1, all these polypeptides were also found in a cultured cell line (RTG-2). Epidermal keratinocytes contained D1 and six other major cytokeratins, termed E1-E6. The most complex cytokeratin pattern was that found in the gill epithelium. Surprisingly, antibodies specific for cytokeratins of the L1-L3 group also reacted with certain cell-sheet-forming tissues that are not considered typical epithelia and in higher vertebrates express primarily, if not exclusively, vimentin. Such tissues were (a) endothelia, including the pillar cells of the "gill filaments", (b) scale-associated cells, and (c) the ocular lens epithelium, and also several nonepithelial cell types, such as (d) fibroblasts and other mesenchymal cells, (e) chondrocytes, (f) certain vascular smooth muscle cells, and (g) astroglial cells of the optic nerve. The differences between the patterns of cytokeratin expression in this fish species and those of higher vertebrates are discussed. It is concluded that the diversity of cytokeratins has already been established in lower vertebrates such as fish, but that the tissue-expression pattern of certain cytokeratins has been restricted during vertebrate evolution. We discuss the value of antibodies specific for individual cytokeratin polypeptides as marker molecules indicating cell and tissue differentiation in fish histology, embryology, and pathology.

Sequence and structure of the mouse gene coding for the largest neurofilament subunit

We have determined the complete nucleotide sequence of the mouse gene encoding the neurofilament NF-H protein. The C-terminal domain of NF-H is very rich in charged amino acids (aa) and contains a 3-aa sequence, Lys-Ser-Pro, that is repeated 51 times within a stretch of 368 aa. The location of this serine-rich repeat in the phosphorylated domain of NF-H indicates that it represents the major protein kinase recognition site. The nfh gene shares two common intron positions with the nfl and nfm genes, but has an additional intron that occurs at a location equivalent to one of the introns in non-neuronal intermediate filament-coding genes. This additional nfh intron may have been acquired via duplication of a primordial intermediate filament gene.

Identification of a basic protein of Mr 75,000 as an accessory desmosomal plaque protein in stratified and complex epithelia

Desmosomes are intercellular adhering junctions characterized by a special structure and certain obligatory constituent proteins such as the cytoplasmic protein, desmoglein. Desmosomal fractions from bovine muzzle epidermis contain, in addition, a major polypeptide of Mr approximately 75,000 ("band 6 protein") which differs from all other desmosomal proteins so far identified by its positive charge (isoelectric at pH approximately 8.5 in the denatured state) and its avidity to bind certain type I cytokeratins under stringent conditions. We purified this protein from bovine muzzle epidermis and raised antibodies to it. Using affinity-purified antibodies, we identified a protein of identical SDS-PAGE mobility and isoelectric pH in all epithelia of higher complexity, including representatives of stratified, complex (pseudostratified) and transitional epithelia as well as benign and malignant human tumors derived from such epithelia. Immunolocalization studies revealed the location of this protein along cell boundaries in stratified and complex epithelia, often resolved into punctate arrays. In some epithelia it seemed to be restricted to certain cell types and layers; in rat cornea, for example, it was only detected in upper strata. Electron microscopic immunolocalization showed that this protein is a component of the desmosomal plaque. However, it was not found in the desmosomes of all simple epithelia examined, in the tumors and cultured cells derived thereof, in myocardiac and Purkinje fiber cells, in arachnoideal cells and meningiomas, and in dendritic reticulum cells of lymphoid tissue, i.e., all cells containing typical desmosomes. The protein was also absent in all nondesmosomal adhering junctions. From these results we conclude that this basic protein is not an obligatory desmosomal plaque constituent but an accessory component specific to the desmosomes of certain kinds of epithelial cells with stratified tissue architecture. This suggests that the Mr 75,000 basic protein does not serve general desmosomal functions but rather cell type-specific ones and that the composition of the desmosomal plaque can be different in different cell types. The possible diagnostic value of this protein as a marker in cell typing is discussed.

Phosphorylated epitopes of neurofilaments have been conserved during chordate evolution

We have characterized some rabbit polyclonal responses as strictly specific for phosphorylated epitopes located in the carboxyterminal (tail) domain of the H or the M subunits of mammalian neurofilaments. These antibodies have been used to confirm the occurrence in lizard neurofilaments of a single heavy subunit cross-reacting with both H and M from mammals. A heavy subunit with similar cross-reactivity has been detected in neurofilaments preparations from fishes, whereas more primitive Chordata possess a HMW polypeptide cross-reacting with only the M subunit. We could also demonstrate in frog spinal cord two distinct heavy subunits cross-reacting with either the M or the H subunit from mammals, a fact which suggests a convergent evolution for phosphorylated epitopes of neurofilaments.

Evidence for intermediate filaments in squirrelfish erythrophores of Holocentrus ascensionus (Rufus)

We have documented the presence of intermediate filaments (IF) in cultured erythrophores of the squirrelfish Holocentrus ascensionus (Rufus). SDS-PAGE and Western blots with monoclonal antibodies T11 and R12 demonstrated that isolated IF consisted of a pair of polypeptides of 54 and 52 kDa. Immunofluorescent studies revealed that the two proteins formed prominent radially oriented IF networks in erythrophores. Immunoelectron microscopic studies showed that the IF were distributed in a "spider-web"-like network of filaments which occasionally intersected with the microtubule surfaces. The IF proteins also were found in fish iridiphores but not in fish epithelial cells which cocultured with the chromatophores.

Basic proteins of the perinuclear theca of mammalian spermatozoa and spermatids: a novel class of cytoskeletal elements

The nuclei of bovine spermatids and spermatozoa are surrounded by dense cytoplasmic webs sandwiched between the nuclear envelope and the acrosome and plasma membrane, respectively, filling most of the cytoplasmic space of the sperm head. This web contains a complex structure, the perinuclear theca, which is characterized by resistance to extractions in nondenaturing detergents and high salt buffers, and can be divided into two major subcomponents, the subacrosomal layer and the postacrosomal calyx. Using calyces isolated from bull and rat spermatozoa we have identified two kinds of basic proteins as major constituents of the thecal structure and have localized them by specific antibodies at the light and electron microscopic level. These are an Mr 60,000 protein, termed calicin, localized almost exclusively to the calyx, and a group of multiple-band polypeptides (MBP; Mr 56,000-74,000), which occur in both the calyx and the subacrosomal layer. The polypeptides of the MBP group are immunologically related to each other, but unrelated, by antibody reactions and peptide maps, to calicin. We show that these basic cytoskeletal proteins are first detectable in the round spermatid stage. As we have not detected any intermediate filament proteins and proteins related to nuclear lamins of somatic cells in sperm heads, we conclude that the perinuclear theca and its constituents, calicin and MBP proteins, are the predominant cytoskeletal elements of the sperm head. Immunologically cross-reacting polypeptides with similar properties have been identified in the heads of rat and human spermatozoa. We speculate that these insoluble basic proteins contribute, during spermiogenesis, to the formation of the perinuclear theca as an architectural element involved in the shape changes and the intimate association of the nucleus with the acrosome and the plasma membrane.

The structure of a human neurofilament gene (NF-L): a unique exon-intron organization in the intermediate filament gene family

We have cloned and determined the nucleotide sequence of the human gene for the neurofilament subunit NF-L. The cloned DNA contains the entire transcriptional unit and generates two mRNAs of approx. 2.6 and 4.3 kb after transfection into mouse L-cells. The NF-L gene has an unexpected intron-exon organization in that it entirely lacks introns at positions found in other members of the intermediate filament gene family. It contains only three introns that do not define protein domains. We discuss possible evolutionary schemes that could explain these results.

Cytoplasmic intermediate filament proteins and the nuclear lamins A, B and C share the IFA epitope

The murine monoclonal antibody IFA isolated by Pruss et al. (Cell 27 (1981) 419) reacts with all major proteins of the cytoplasmic intermediate filament family (IF) albeit with different affinities but leaves the nucleus undecorated in standard immunofluorescence microscopy. Here we show that IFA reacts with all three nuclear lamins from rat and man in immunoblotting. This is most easily demonstrated in a cell line in which most cells lack cytoplasmic IFs. Thus the rather minor but ubiquitous 66 kD polypeptides identified by Pruss et al. as IF-associated proteins reflect the lamin triplet. While surprising at first, these results are in agreement with the approximate location of the IFA epitope on IF molecules and the recently discovered sequence homology along the rod domain between lamins A and C and IF proteins. Our results extend this relation to lamin B in spite of its unique behaviour during mitosis.

Drosophila melanogaster contains a set of polypeptides capable of polymerizing into intermediate-like filaments

A partial purification scheme applied to the Triton X-100 insoluble pellet of adult flies homogenates yielded a fraction which upon polymerization reproducibly produces filaments which in the electron microscope have all the typical properties of intermediate filaments. This is the first report of the presence of protein components with such properties in Drosophila. Thus, it is highly possible that insects and arthropodes, like other lower invertebrates, may contain the third extremely insoluble element of the cellular cytoskeleton.

Monoclonal antibodies against plant proteins recognise animal intermediate filaments

Four monoclonal antibodies were raised against polypeptides present in a high-salt detergent-insoluble fraction from cells of Chlamydomonas reinhardtii. Indirect immunofluorescence microscopy of fibroblasts and epithelial cells grown in culture using these plant antibodies revealed staining arrays identical to those obtained with well characterised antibodies to animal intermediate filaments. Immunofluorescence microscopy of Chlamydomonas with these monoclonal antibodies and a monoclonal antibody that recognises all animal intermediate filaments (anti-IFA) gave a diffuse, patchy cytoplasmic staining pattern. Both the plant antibodies and anti-IFA stained interphase onion root tip cells in a diffuse perinuclear pattern. In metaphase through to telophase, the labelling patterns colocalised with those of microtubules. Labelling of the phragmoplast was also detected but not staining of the preprophase band. On Western blots of various animal cell lines and tissues, all the antibodies labelled known intermediate filament proteins. On Western blots of whole Chlamydomonas proteins, all the antibodies labelled a broad band in the 57,000 Mr range, and three antibodies labelled bands around 66,000 and 140,000 Mr but with variable intensities. On Western blots of whole onion root tip proteins, all the antibodies labelled 50,000 Mr (two to three bands) polypeptides and a diffuse band around 60,000 Mr and three of the antibodies also labelled several polypeptides in the 90,000-200,000 Mr range. The consistent labelling of these different bands by several different monoclonal antibodies recognising animal intermediate filaments makes these polypeptides putative plant intermediate filament proteins.

Identification of the conserved, conformation-dependent cytokeratin epitope recognized by monoclonal antibody (lu-5)

The epitope recognized by the murine monoclonal antibody (mAB lu-5) recently described as a formaldehyde-resistant, "pan-epithelial marker" of great value in tumour diagnosis is located on the surface of cytokeratin filaments. It has been preserved during vertebrate evolution from amphibia to man. As this epitope is not reactive after SDS-polyacrylamide gel electrophoresis (SDS-PAGE), the epitope-bearing protein has been identified by a dot-blot antibody binding assay, using purified proteins in which the epitope is reconstituted. We show that the epitope is present in most cytokeratin polypeptides of both the acidic (type I) and basic (type II) subfamily but does not occur in other cytoskeletal proteins. The location of this widespread epitope is discussed with respect to homologies of amino acid sequences of cytokeratins and their conformations.

Intermediate filaments in muscle and epithelial cells of nematodes

Current concepts of the developmentally controlled multigene family of intermediate filament (IF) proteins expect the origin of their complexity in evolutionary precursors preceding all vertebrate classes. Among invertebrates, however, firm ultrastructural as well as molecular documentation of IFs is restricted to some giant axons and to epithelia of a few molluscs and annelids. As Ascaris lumbricoides is easily dissected into clean tissues, IF expression in this large nematode was analyzed by electron microscopic and biochemical procedures and a monoclonal antibody reacting with all mammalian IF proteins. We document for the first time the presence of IFs in muscle cells of an invertebrate. They occur in three muscle types (irregular striated pharynx muscle, obliquely striated body muscle, uterus smooth muscle). IFs are also found in the epithelia studied (syncytial epidermis, intestine, ovary, testis). Immunoblots on muscles, pharynx, intestine, uterus, and epidermis identify a pair of polypeptides (with apparent molecular masses of 71 and 63 kD) as IF constituents. In vitro reconstitution of filaments was obtained with the proteins purified from body muscle. In the small nematode Caenorhabditis elegans IF proteins are so far found only in the massive desmosome-anchored tonofilament bundles which traverse a special epithelial cell type, the marginal cells of the pharynx. We speculate that IFs may occur in most but perhaps not all invertebrates and that they may not occur in all cells in large amounts. As electron micrographs of the epidermis of a planarian--a member of the Platyhelminthes--reveal IFs, the evolutionary origin of this cytoplasmic structure can be expected either among the lowest metazoa or already in some unicellular eukaryotes.