The Cytoskeleton in Normal and in Trans
Transcrição
The Cytoskeleton in Normal and in Trans
Research Program Cell Differentiation and Carcinogenesis Division Cell Biology Division Cell Biology (A0100) Head: Prof. Dr. rer. nat. Werner W. Franke 16 The Cytoskeleton in Normal and in Transformed Cells: Molecular Characterization of the Main Components, Functional Domain Analysis and Tumor Diagnosis Scientists: Dr. Leonid Eshkind (- 3/00) Dr. Hans Heid PD Dr. Harald Herrmann-Lerdon Dr. Ilse Hofmann Prof. Dr. Jürgen Kartenbeck Dr. Lutz Langbein Dr. Hans-Richard Rackwitz (- 6/00) Dr. Thorsten Ralle (5/99 - 4/00) Dr. Ansgar Schmidt (- 9/00) PD Dr. Marion Schmidt-Zachmann PD Dr. Reimer Stick (10/98 - 8/00) Graduate students: Kemal Akat (2/2000 -) Christine Dreger (5/2000 -) Sandra Kneissel Jochen Köser (- 11/99) Michaela Reichenzeller Wiebke Peitsch (-8/2000) Qi Tian W .W. Franke, H. Herrmann-Lerdon, L. Langbein Carola Borrmann Jens Eilbracht Alexandra König (5/00 -) Claudia Mertens (- 8/00) Rainer Paffenholz (-6/99) Beate Straub Visiting scientists / guests: Dr. Barbara Deumling (Germany; - 8/00) Dr. Natalia Kazakova (Russia; - 12/99) Dr. Birgit Kräling (Germany/USA; - 4/00) Technical assistants: Jutta Arlt (85%) Peter Eichhorn Katrin Götzke (65%; - 9/00) Christine Grund (80%) Michaela Hergt Astrid Hofmann Andreas Hunziker Cäcilia Kuhn Monika Mauermann (- 2/00) Edeltraut Noffz (50%) Jutta Osterholt Silke Prätzel Michaela Rode ( - 9/99) Heiderose Schumacher (50%) Tatjana Wedig (1/00 -) Stefanie Winter-Simanowski (50%) Ralf Zimbelmann The Division of Cell Biology investigates intracellular architectonic elements, both in the nucleus (karyoskeleton) and the cytoplasm (cytoskeleton), and their interactions with the nuclear envelope and cell junctions, respectively. The constituting proteins, their interactions and functions are characterized by use of biochemical methods, recombinant DNA technology and morphological analyses, including electron microscopy and immunolocalization techniques, and their patterns of synthesis in normal and malignant cells and tissues are determined. To elucidate possible functions of these proteins and structures gene transfer and expression techniques are applied, including stably transfected cell lines as well as transgenic and gene-targeted mice, to solve principal problems of cell biology that are central to the understanding of cell and tissue differentiation, carcinogenesis and metastasis. The reagents generated, in particular monoclonal antibodies developed in the course of these studies, are also examined for their diagnostic value in the detection and classification of tumors. In cooperation with: U. Aebi, A. Engel, A. Lustig, Biozentrum, Basel, Switzerland; M. Amagai, T. Hashimoto, Keio University School of Medicine, Tokyo, Japan; H. Baribault, La Jolla Cancer Research Foundation, USA; A. Ben Ze’ev, B. Geiger, Weizmann Institute of Science, Rehovot, Israel; W. Birchmeier, P. Ruiz, MDC für Molekulare Medizin, Berlin; B. Cribier, Dept. Dermatology, Université de Strasbourg, France; H. Denk, K. Zatloukal, Dept. Pathology, University of Graz, Austria; V.E. Gould, Department of Pathology, Rush-Presbyterian-St. Luke’s Medical Center, Chicago, USA; L. Jahn, W. Kübler, Medizinische Universitätsklinik Heidelberg; P. Krammer, M.E. Peter, DKFZ; M. Krawczak, Inst. Medical Genetics, University of Wales, Cardiff, U.K.; H. Kurzen; Dept. Dermatology, Heidelberg University Medical School; I.M. Leigh, Skin Tumor Lab., Royal School of Medicine, London, U.K.; P. Lichter, D. & A. Olins, DKFZ; T. Magin, Institut für Genetik, Universität Bonn; J. Markl, J.R. Harris, Zoologisches Institut, Universität Mainz; I. Moll, W. Peitsch, Hautklinik und Poliklinik, UKE Hamburg; R. Moll, A. Schmidt, Medizinisches Zentrum für Pathologie, Universität Marburg; V. Romano, Laboratorio di Genetica Moleculare, Troina, Italy; A. Starzinski-Powitz, Humangenetik, Universität Frankfurt; J. Schweizer, M.A. Rogers, H. Winter, DKFZ; D. Roop, Dept. Molecular and Cellular Biology and Dermatology, Baylor College, Houston, USA; H. Spring, DKFZ; O. Swensson, Hautklinik, Universität Kiel; J.-P. Thiery, CNRS, Laboratoire de Physiopathologie du Développement, Paris, France; E. Yaoita, Institute of Nephrology, Niigata University, Japan. Cellular architecture and the interaction of individual cells with neighbouring cells or extracellular matrix components is mediated by cell type-specific protein complexes termed the “cytoskeleton”. The mechanisms of the cell type-specific synthesis of sets of cytoskeletal proteins, their assembly and coordinated integration into various structures differ considerably in different kinds of cells and tissues, both in the normal and the transformed situation. In order to understand a specific developmental process and the maintenance of the differentiated state it is of fundamental importance to elucidate the contribution and localization of individual architectonic molecules, their modifications, and the nature of their functional domains that contribute to the specific cell structure and the functions. Moreover, we have recently identified several proteins, originally identified as constitutive elements of the cytoskeleton, as integral components of karyoplasmic complexes, indicating that they may be engaged in gene regulatory activities. I. Assembly, structure and function of intermediate filaments Intermediate filament (IF) proteins represent - like the other filament-forming proteins - a multigene family. Its more than 50 members, which are expressed in cell typespecific patterns, constitute scaffolds with essential structural functions both in the cytoplasm (e.g., cytokeratins, vimentin, desmin, glial and neurofilament proteins) and the karyoplasm (e.g., lamins) [24]. Specific functions of the DKFZ 2001: Research Report 1999/2000 Research Program Cell Differentiation and Carcinogenesis various cytokeratins have recently been identified using gene elimination and replacement strategies (e.g. [43, 78]). The (“soft”) cytokeratins (CK) and the (“hard”) hair keratins (HK) are subgroups of the keratin IF-protein multigene family. Extending previous work, a major aim was the completion of the human catalog of CKs and HKs. The cytokeratins include ~23 polypeptides expressed in epithelial cells in a cell type-specific and differentiation-dependent manner [15, 16]. All of the type II keratins, CKs and HKs, are located in a single gene cluster on chromosome 17 [58]. Recently we have found a new CK, CK6hf, that is specifically expressed in the companion layer (“innermost layer of outer root sheath”) of the hair. As the synthesis of CK6hf starts at the base of the hair follicle and continues in an apical direction, this has also allowed us to clearly demonstrate that the companion layer is a single-layered differentiation sui generis and not part of the outer root sheath, as formerly assumed. Interestingly, the newly detected CK6hf seems to be involved in a hair disease named “loose anagen” [6]. Using polypeptide analysis, previously only eight members of the human HK subfamily had been distinguished, four type I (hHa) and four type II (hHb) polypeptides. After systematic sequencing of the relevant genomic clusters and cloning of cDNAs the gene family now was shown to consist of 9 hHa (hHa1-8; including the two rather related proteins hHa3-I, hHa3-II) and 6 hHb (hHb1-6) polypeptides [58]. By examining the expression of these HK genes we have clearly shown that there are - comparable to the CKs - patterns of sequentially and differentiation-dependent expression within the hair forming compartment (hair follicle): The expression of some members (hHa5, hHb5) begins in the upper matrix cells, a region formerly assumed to be free of HKs. The synthesis of other HKs follows in the lower/mid cortex (hHa1, hHa3-I, hHa3-II; hHb1, hHb3, hHb6) and ends in the upper cortex hHa4. Moreover, two of the HKs, hHa2 and Hhb2, are exclusively synthesized in the hair cuticle. Another HK, hHa7, not detectable in the anagen hair follicle, is expressed in vellus hair only and hHa8 is detectable only in single cortex cells. In contrast, hHb4, although a true HK, is not expressed in the hair follicle but in the filiform papillae of tongue. Based on these results a catalog of human type-I HKs [40] and type-II HKs (manuscript submitted) has been established. One of these type I HKs, the pseudogene, ;hHaA, is transcribed in humans but not stable at the protein level because of a premature stop codon; it is, however, clearly detectable as a functional gene in chimpanzee hairs. This indicates that during evolution only about 240,000 years ago, this HK was homozygoteously mutated and lost in the hominide lineage. Moreover, our studies on chimpanzee hair follicles have revealed a much more complex pattern when compared to humans [76]. Based on our gene expression studies in humans, we have recently characterized the mutated hHb6 (and hHb1) as being causative for the heritable hair disease, monilethrix. The involvement of HKs in tumorgenesis of follicle based neoplasias or BCCs has been investigated (manuscripts submitted). Division Cell Biology As a model protein to study general principles of IF assembly we have chosen the type III IF protein vimentin and shown it to polymerize in three distinct phases [19]. However, also cytokeratins, desmin and neurofilament proteins were shown to follow the same assembly schedule starting from so-called “unit-length filaments” [20, 23]. Most notably, we obtained the first atomic structure information of IF proteins crystallizing the IF consensus sequence of vimentin [25]. The assembly of certain IF proteins is temperature-dependent as has been shown for vimentin from various species [21]. This extraordinary degree of temperature-sensitivity of vimentin has been utilized in a novel context. The existence of a hitherto more or less hypothetical space between the chromosomes of the interphase nucleus (the interchromosomal domain compartment) was visualized and explored by directly employing a “nuclear space shuttle”, namely Xenopus vimentin carrying a nuclear localization signal introduced by recombinant DNA techniques [22, 46, 57]. In stably transfected cultured human cells, the transgene product is deposited in nuclear aggregates at 37°C. However, upon lowering the growth temperature to 28°C, filaments start to grow out of these aggregates connecting them within a few hours. The “network” of filaments formed this way is found exclusively outside of chromosomal territories colocalizing with various nuclear entities such as coiled bodies, PML bodies or newly transcribed RNA. The cytoskeleton is also intimately engaged in drastic changes and transitions of nuclear architecture, as observed during the differentiation of certain blood stem cells. Thus, during the generation of granulocytic cells from undifferentiated precursors, in the course of which nuclear shape and probably function changes by complex lobulations and evaginations, the vimentin in the cytoplasm and the lamin system in the nucleus as well as the lamin receptors are fundamentally affected [49]. Similarly, also intranuclear lamin filaments are intimately engaged in the architectural organization of the nucleus being reflected by the evolutionarily conserved molecular structure of these IF proteins and their central importance during oogenesis and embryogenesis [13, 26, 46, 56]. The integration of IFs into the cellular structure, i.e. the cytoskeleton, involves a number of associated proteins one of which is plectin [7, 64, 65]. Its central role in cellular structure and physiology has been shown in the context of the apoptotic reorganization process. Here, plectin is one of the first substrates to be attacked, and cleaved once in the middle of the molecules, by the regulator caspase 8 [67]. II. Desmosomes - intercellular connecting structures and anchoring sites of the cytoskeleton Our studies on the molecular composition and functions of intercellular junctions, particularly those of the “adhaerens” group, have focussed on the clarification of the complexity, localization and functions of proteins of the so-called “armrepeat” family, known to be important cytoplasmic components of junctional plaques. Here we have discovered and DKFZ 2001: Research Report 1999/2000 17 Research Program Cell Differentiation and Carcinogenesis 18 investigated in considerable detail the subfamily of the plakophilins of which three human genes (PKP1-3) and a total of five isoformic splice variants have been identified, cDNA-cloned, sequenced and localized by cell fractionation and immunolocalization methods [44, 45, 61]. Most remarkably, plakophilins 1 and 2 have been shown to occur systemically in two places, i.e. in the nucleoplasm in form of distinct small particles readily extractable in physiological buffer solutions, and in a largely insoluble form in desmosomal plaques. The nucleoplasmic occurrence is also seen in diverse cell types devoid of desmosomes whereas the recruitment to the plaque is highly differentiation- and isoform-specific. A series of sequence-defined mono- and polyclonal antibodies have been prepared and specific immunocytochemical protocols have been developed that allow the specific localization of these proteins, even the splice variants, in both compartments, the nucleoplasm and the desmosomes or related junctions [28, 44, 45, 61]. These antibodies have also been successfully used for cell typing in histology and pathology, especially in dermatological questions [45, 61]. Further, the formation process of desmosomes was investigated and the “minimal” components were estimated [37] and interactions of desmosomal proteins with other components of the cytoskeleton as intermediate filaments were investigated [28, 29]. In the course of our studies on “arm-repeat” proteins we have also discovered, cDNA-cloned and sequenced a novel member of this multi-gene family which, however, by amino acid sequence comparison is more closely related to protein p120 than to the plakophilins and has been identified only in neural cells [51, 52]. This protein with the acronym NPRAP, then termed neurojungin, has also been identified in specific junctional plaques, most prominently in those of the “zona limitans externa” of the neural retina, connecting photoreceptor cells with the cells of the socalled “Müller glia” [52]. We have developed several types of antibodies specific for individual desmosomal cadherins, i.e. desmogleins 1-3 and desmocollins 1-3, and other junctional proteins, which are valuable in the immunohistochemical differentiation and identification of various cell types in normal development and in tumor diagnoses (e.g. [34, 39, 45] and several ms. submitted). This panel of antibodies to junction typespecific components of both the plaque and the extracellular segments has also allowed to detect and define novel kinds of adhering junctions which cannot be subsumed under the hitherto existing categories of “adherens” and desmosomal junctions [3, 4]. Division Cell Biology Karyoskeletal Elements, Intranuclear Architecture and Topogenesis of Nuclear/ Nucleolar Proteins M.S. Schmidt-Zachmann In cooperation with Michael Stöhr, Dr. Martina Schnölzer, PD Dr. Hanswalter Zentgraf, DKFZ; Prof. Dr. Joseph Gall, Carnegie Institution, Baltimore, USA; Prof. Dr. Angela Krämer, University of Geneva, Switzerland; Prof. Dr. Thoru Pederson, University of Massuchusetts Medical School, USA; Dr. Volker Cordes, Karolinska Institute, Stockholm, Sweden. The compartmentalization of the eukaryotic cell and its functions are based on the specific topogenesis of cellular proteins. While in the past decade important molecular principles concerning the nucleocytoplasmic distribution of proteins have been elucidated, such as the presence of a short, basic signal sequence, termed nuclear localization signal (NLS) in many proteins, it remains to be determined to what extent and how distinct amino acid sequence motifs govern the targeting of proteins to precise subnuclear structures. The nucleolus, which represents an accumulation of rDNA and its transcription products together with a characteristic set of proteins, is a complex nuclear substructure in which key steps of ribosome biogenesis as well as assembly of other kind of ribonucleoprotein particles take place. To study the role of primary sequences in nucleolar targeting of a constitutive nucleolar protein, we have analyzed the major non-ribosomal nucleolar protein NO38. By detailed mutational analyses of cDNAs coding for Xenopus NO38, individual domains essential for nuclear/nucleolar localization have been identified [80]. Our studies lead us to conclude that nucleolar accumulation of proteins is a two step process: (i) active transport into the cell nucleus via a functional nuclear localization signal (NLS), and (ii) accumulation in the nucleolus due to specific binding interactions between these proteins and other nucleolar components, particularly rDNA, rRNA and possibly other protein constituents. One major aim of our studies is the identification of so far unknown components of the nucleolus. While we had described the sequence elements required for the specific nucleolar accumulation of protein NO38/B23, its direct and constitutive binding partners within the nucleolus remained to be identified. In immunoprecipitation experiments and affinity chromatography using cellular and nuclear extracts of Xenopus laevis cultured cells and antibodies to protein NO38, we were able to isolate and cDNA-clone a novel nucleolar, very acidic protein with a SDS-PAGE mobility corresponding to Mr 29,000. The protein, which was termed NO29, is sequence-related to NO38 and to the histone-binding protein nucleoplasmin, i.e. it represents a new member of the nucleoplasmin-family. Protein NO29 was immunolocalized to nucleoli in Xenopus oocytes and diverse somatic cells. Moreover, biochemical studies revealed that NO29, which is partly complexed with protein NO38, occurs in preribosomes but not in mature ribosomes. The location and the enormously high content of negatively charged amino acids (pI of 3.75) lead to the hypothesis that NO29 might be involved in nuclear and DKFZ 2001: Research Report 1999/2000 Research Program Cell Differentiation and Carcinogenesis Division Cell Biology nucleolar accumulation of ribosomal proteins and the coordinated assembly of pre-ribosomal particles [81]. also essential for the association of the protein with the U2snRNP [12]. Recently, we reported the identification, cDNA cloning and molecular characterization of another novel, constitutive nucleolar protein. The cDNA-deduced amino acid sequence of the human protein defines a polypeptide of a calculated mass of 61.5 kDa. Inspection of the primary sequence disclosed that the protein is a member of the family of “DEAD-box” proteins, representing a subgroup of putative ATP-dependent RNA helicases. ATPase activity of the recombinant protein is evident and stimulated by a variety of polynucleotides tested. Immunolocalization studies revealed that protein NOH61 (“nucleolar helicase of 61 kDa”) is highly conserved during evolution and shows a strong accumulation in nucleoli. Interestingly, protein NOH61 has been identified as a specific constituent of free nucleoplasmic 65S pre-ribosomal particles, but is absent from cytoplasmic ribosomes. Treatment of cultured cells with (i) the transcription inhibitor actinomycin D and (ii) RNase A results in a complete dissociation of NOH61 from nucleolar structures. The specific intracellular localization and its striking sequence homology to other known RNA helicases lead to the hypothesis that protein NOH61 is involved in ribosome synthesis, most likely during the assembly process of the large (60S) ribosomal subunit [82]. Nuclear pore complexes (NPCs) are constitutive structures of the nuclear envelope in all eukaryotic cells and represent gateways between the cytoplasm and the nucleus through which the exchange and bidirectional transport of molecules and particles take place. Attached to the nucleoplasmic side of the NPC are long bundles of filaments of approximately 5 nm diameter that can project into the nuclear interior for more than 350 nm in some cells. We have identified a polypeptide of 270 kDa, termed Tpr, as a major component of these intranuclear filaments [8]. Structural properties of this protein, notably the molecular regions involved in nuclear targeting and NPC association, have been determined [9], and its possible function in nucleocytoplasmic transport processes is currently under investigation. Another central part of our ongoing studies is the characterization of the nucleolar skeleton of amplified nucleoli from Xenopus oocytes. Since it has been shown that the basic nuclear activities - replication and transcription - are connected with the nuclear skeleton (reviewed by Cook, Bioessays 16 (1994) 425-430), it is likely that the same holds true for the nucleolar activities. Notably, the only candidate known so far, which probably contributes to the nucleolar framework visualized in the electron microscope upon certain treatments of interphase cells, is a polypeptide of 145 kDa described almost 20 years ago (e.g. Franke et al., J. Cell Biol. 90 (1981) 289-299). We have successfully isolated the corresponding cDNA clone, determined the encoded polypeptide as a novel type and have immunolocalized the protein in the cortices of amplified nucleoli. We then started with a detailed molecular characterization of this protein and its function(s). Recently, we have described the identification, cDNA cloning and immunodetection of a novel type of constitutive nuclear protein which occurs in diverse vertebrate species, from Xenopus to man. This 146-kDa protein shows a specific intranuclear distribution, i.e. it accumulates in nuclear speckles. This latter subnuclear component is known to be enriched in splicing factors. The protein has been identified as a subunit of the U2snRNP-associated splicing complex SF3b and is therefore designated as SF3b155 [63]. Subsequently, we addressed the question of the topogenic properties of different protein domains of SF3b155 and have identified molecular segments of SF3b155 that mediate its nuclear import and govern its accumulation in nuclear speckles. Moreover, our data indicate that this novel “speckles-targeting sequence”, a protein domain highly enriched in the dipeptide threonine/proline (TP-domain) is Our studies on the intranuclear topogenesis of proteins, i.e. the specific accumulation in the nucleolus, speckles and NPCs, respectively, allow us to conclude that proteinspecific sequence elements determine the specific intranuclear distribution of a given protein. Functions and Dynamics of Defined Membrane Domains J. Kartenbeck In cooperation with: Prof. Dr. Angel Alonso, PD Dr. Ursula BantelSchaal, DKFZ; Dr. Franz Bosch, Dept. Otolaryngology, University of Heidelberg Medical School; Dr. Nikolaus Gassler, Inst. Pathology, University of Heidelberg; PD Dr. Chris Haller, University of Heidelberg Medical School; Prof. Dr. Ari Helenius, ETH-Zürich, Switzerland; Prof. Dr. Dietrich Keppler, DKFZ; Prof. Dr. Rudolf Leube, Inst. Anatomy, University of Mainz. Hemidesmosomal structures at basal layers of keratinocytes in normal squamous epithelia can colocalize Bullous pemphigoid antigen 1 (BPA1) and =6>4-integrins. In 25 cases the expression of these genes was analyzed during the process of tumor cell invasion and metastasis. With the onset of invasive growth, upregulation of BPA1 and =4>6 integrins extended to the active proliferative zone, also in tumors which no longer displayed hemidesmosomes. The loss of polarized expression and the redirected expression to the entire surface of many tumor cell layers was found to be a property of the metastastic phenotype of squamous cell carcinomas. The evaluation of reduced expressions of proteins engaged in the formation of intercellular contacts, including desmosomes, is generally taken as an indicator for malignancy and as a prognostic factor. Therefore, in a long-term project we analyzed head and neck squamous cell carcinomas (HNSCC) from 190 patients for the expression of the desmosmal proteins, desmoplakin and desmoglein, and for the adhaerens junction protein, E-cadherin. The expression patterns of these proteins from primary tumors were compared with metastases therefrom. We saw a great spectrum of tumor differentiation and variability of expression patterns. In most cases metastases revealed expression patterns as their primary tumors, and changed DKFZ 2001: Research Report 1999/2000 19 Research Program Cell Differentiation and Carcinogenesis expression of the various proteins and structures behaved in a parallel manner. By statistical evaluation we found no correlation between expression pattern and metastatic tumor events. In the great majority of cases junctional proteins of both intercellular contact types (desmosome, adhaerens junctions) were still present, although often with a decreased expression pattern. 20 To elucidate whether rapid degradation and instability of the mutated canalicular isoform (MRP2) mRNA or a defective protein processing could cause this lack of apical MRP2 protein (Dubin-Johnson syndrome), the expression and localization of mutant MRP2 proteins was studied in transfected cell lines (in cooperation with Prof. Dr. D. Keppler). It was found that an apparently immature form of MRP2 protein accumulated in the cisternae of the rough endoplasmic reticulum. Degradation of the misfolded or incorrectly assembled proteins might take place in proteasomes, as inhibition of the proteolytic activities with MG132 resulted in paranuclear accumulation of the mutant protein in aggresome-like structures [31]. Bile flow and Mrp2 distribution was also analyzed after intravenous injection of phalloidin, which is preferentially transported into hepatocytes. The resulting cholastasis is in part due to a loss of ATP-dependent export pumps, including MRP2 from the canalicular membrane [59]. MRP2 was further identified in rat kidney brush-border membrane domains of proximal tubules. Clear-cell renal carcinoma, the most common malignancy of human adult kidney showed MRP-2 expression in more than 90% of the cases [60]. Detection and Characterization of Novel Proteins, Protein Modifications and Genes H. Heid, H.-R. Rackwitz, A. Hunziker, W.W. Franke In cooperation with: R. Benavente, Biozentrum, Würzburg; R.M. Flügel, Tumorvirologie, DKFZ; W.W. Just, Biochemie-Zentrum, Universität Heidelberg; T.W. Keenan, Virginia Polytechnic Institute, Blacksburg, USA; M. Schnölzer, Zentrale Proteinanalytik, DKFZ; M. Volkmann, Medizinische Klinik und Poliklinik, Universität Heidelberg The experts in protein chemistry and synthesis of the Division of Cell Biology have cooperated with several other groups, using mass spectrometric characterization, PCR techniques, DNA sequencing, protein and peptide microsequencing, and peptide synthesis combined with antibody production, in a series of studies that have led to discoveries of several novel proteins, novel loalizations, and novel specific modifications and functions of proteins (e.g. [1, 2, 14, 17, 30, 33, 41, 48, 50, 53, 55, 68-71, 73, 74, 79]). Publications (* = external co-author) [1] Aleem, E.A., Flohr, t., Hunziker, A., Mayer, D., Bannasch, P., Thielmann, H.W. 2001. Detection and quantification of protein phosphatase inhibitor-1 gene expression in total rat liver and isolated hepatocytes. Mol. Cell. Biochem. 217, 1-12. [2] *Alsheimer, M., *von Glasenapp, E., Schnölzer, M., Heid, H., *Benavente, R. 2000. Meiotic lamin C2: the unique amino-terminal hexapeptide GNAEGR is essential for nuclear envelope association. Proc. Natl. Acad. Sci. USA 97, 13120-13125. Division Cell Biology [3] Borrmann, C. 2000. Molekulare Charakterisierung der Adhärens-Zellverbindungen des Herzens: Identifizierung einer neuen Art, der Area composita. Ph.D. Thesis. University of Heidelberg. [4] Borrmann, C., Mertens, C., Schmidt, A., Langbein, L., Kuhn, C., Franke, W.W. 2000. Molecular diversity of plaques of epithelial adhering junctions. Ann. NY Acad. Sci. 915, 144-150. [5] Cerdà, J., Reidenbach, S., Prätzel, S., Franke, W.W. 1999. Cadherin-catenin complexes during zebrafish oogenesis: heterotypic junctions between oocytes and follicle cells. Biol. Reprod. 61, 692-704. [6] *Chapalain, V., Winter, H., Langbein, L., *LeRoy, J.M., *Labreze, C., *Nikolic, M., Schweizer, J., *Taieb, A. 2001. Is the loose anagen hair syndrome a keratin disorder? A clinical and molcular study. Arch. Dermatol., in press. [7] *Clubb, B.H., *Chou, Y.-H., Herrmann, H., *Svitkina, T.M., *Borisy, G.G., *Goldman, R.D. 2000. The 300-kDa intermediate filament-associated protein (IFAP300) is a hamster plectin ortholog. Biochem. Biophys. Res. Commun. 273, 183-187. [8] Cordes, V.C., Reidenbach, S., Rackwitz, H.-R., Franke, W.W. 1997. Identification of protein p270/Tpr as a constitutive component of the nuclear pore complex-attached intranuclear filaments. J. Cell Biol. 136, 515-529. [9] Cordes, V.C., *Hase, M.E., *Müller, L. 1998. Molecular segments of protein Tpr that confer nuclear targeting and association with the nuclear pore complex. Exp. Cell Res. 245, 43-56. [10] Dandekar, G. 2000. Charakterisierung endothelialer Zell-ZellVerbindungskomplexe und Einfluß einer Co-Kultur mit fibroblastoiden Zellen auf die endotheliale Differenzierung. Diploma Thesis. University of Heidelberg [11] Dreger. C. 2000. Ektopische Expression von Chimären aus Vimentin und Lamin B1, Emerin sowie Lamin B-Rezeptor im Zellkern von humanen Gewebekulturzellen. Diploma Thesis. University of Heidelberg. [12] Eilbracht, J., Schmidt-Zachmann, M. 2001. Identification of a novel sequence element directing a protein to nuclear speckles. Proc. Natl. Acad. Sci. USA98, 3849-3854. [13] *Erber, A., *Riemer, D., *Hofemeister, H., *Bovenschulte, M., Stick, R., *Panopoulou, G., *Lehrach, H., *Weber, K. 1999. Characterization of the Hydra lamin and its gene: a molecular phylogeny of metazoan lamins. J. Mol. Evol. 49, 260-271. [14] *Eschelbach, A., Hunziker, A., *Klimaschewski, L. 1998. Differential display PCR reveals induction of immediate early genes by vasoactive intestinal peptide in PC12 cells. Ann. N.Y. Acad. Sci. 865, 181-188. [15] Franke, W.W. 1999. Einheit des Lebens - Bau und Bild der Zelle. In: Jahrhundertwissenschaft Biologie: die großen Themen (P. Sitte, ed.), Beck Verlag, München; pp. 43-64. [16] Franke, W.W., Kartenbeck, J. 1999. Cytokeratins. In: Guidebook to the Cytoskeletal and Motor Proteins, 2. ed. (T. Kreis, R. Vale, eds.) Oxford University Press, Oxford; pp. 297-300. [17] *Geier, G., *Banaj H.-J., Heid, H., *Bini, L., *Pallini, V., *Zwilling, R. 1999. Aspartyl proteases in Caenorhabditis elegans. Isolation, identification and characterization by a combined use of affinity chromatography, two-dimensional gel electrophoresis, microsequencing and databank analysis. Eur. J. Biochem. 264, 872-879. [18] Haass, N. 1999. Genstruktur, Expression und intrazelluläre Lokalisierung von Pantophysin. M.D. Thesis. University of Heidelberg. [19] Herrmann, H., *Aebi, U. 1998. Intermediate filament assembly: fibrillogenesis is driven by decisive dimer-dimer interactions. Curr. Opin. Struct. Biol. 8, 177-185. [20] Herrmann, H., *Aebi, U. 1999. Neurofilament triplet proteins. In: Guidebook to the Cytoskeletal and Motor Proteins, 2. ed. (T. Kreis, R. Vale, eds.) 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