Specific in situ phosphorylation of plectin in detergent-resistant cytoskeletons from cultured Chinese hamster ovary cells

Identification of differentially expressed long noncoding RNAs in the ovarian tissue of ewes Shal and Sangsari using RNA-seq

Background

The ovary has an important role in reproductive function. Animal reproduction is dominated by numerous coding genes and noncoding elements. Although long noncoding RNAs (LncRNAs) are important in biological activity, little is known about their role in the ovary and fertility.

Methods

Three adult Shal ewes and three adult Sangsari ewes were used in this investigation. LncRNAs in ovarian tissue from two breeds were identified using bioinformatics analyses, and then target genes of LncRNAs were discovered. Target genes were annotated using the DAVID database, and their interactions were examined using the STRING database and Cytoscape software. The expression levels of seven LncRNAs with their target genes were assessed by real‐time PCR to confirm the RNA‐seq.

Results

Among all the identified LncRNAs, 124 LncRNAs were detected with different expression levels between the two breeds (FDR < 0.05). According to the DAVID database, target genes were discovered to be engaged in one biological process, one cellular component, and 21 KEGG pathways (FDR < 0.05). The PES1, RPS9, EF‐1, Plectin, SURF6, CYC1, PRKACA MAPK1, ITGB2 and BRD2 genes were some of the most crucial target genes (hub genes) in the ovary.

Conclusion

These results could pave the way for future efforts to address sheep prolificacy barriers.

Revelation of candidate genes and molecular mechanism of reproductive seasonality in female rohu (Labeo rohita Ham.) by RNA sequencing

BACKGROUND

Carp fish, rohu (Labeo rohita Ham.) is important freshwater aquaculture species of South-East Asia having seasonal reproductive rhythm. There is no holistic study at transcriptome level revealing key candidate genes involved in such circannual rhythm regulated by biological clock genes (BCGs). Seasonality manifestation has two contrasting phases of reproduction, i.e., post-spawning resting and initiation of gonadal activity appropriate for revealing the associated candidate genes. It can be deciphered by RNA sequencing of tissues involved in BPGL (Brain-Pituitary-Gonad-Liver) axis controlling seasonality. How far such BCGs of this fish are evolutionarily conserved across different phyla is unknown. Such study can be of further use to enhance fish productivity as seasonality restricts seed production beyond monsoon season.

RESULT

A total of ~ 150 Gb of transcriptomic data of four tissues viz., BPGL were generated using Illumina TruSeq. De-novo assembled BPGL tissues revealed 75,554 differentially expressed transcripts, 115,534 SSRs, 65,584 SNPs, 514 pathways, 5379 transcription factors, 187 mature miRNA which regulates candidate genes represented by 1576 differentially expressed transcripts are available in the form of web-genomic resources. Findings were validated by qPCR. This is the first report in carp fish having 32 BCGs, found widely conserved in fish, amphibian, reptile, birds, prototheria, marsupials and placental mammals. This is due to universal mechanism of rhythmicity in response to environment and earth rotation having adaptive and reproductive significance.

CONCLUSION

This study elucidates evolutionary conserved mechanism of photo-periodism sensing, neuroendocrine secretion, metabolism and yolk synthesis in liver, gonadal maturation, muscular growth with sensory and auditory perception in this fish. Study reveals fish as a good model for research on biological clock besides its relevance in reproductive efficiency enhancement.

Plectin in the Central Nervous System and a Putative Role in Brain Astrocytes

Plectin, a high-molecular-mass cytolinker, is abundantly expressed in the central nervous system (CNS). Currently, a limited amount of data about plectin in the CNS prevents us from seeing the complete picture of how plectin affects the functioning of the CNS as a whole. Yet, by analogy to its role in other tissues, it is anticipated that, in the CNS, plectin also functions as the key cytoskeleton interlinking molecule. Thus, it is likely involved in signalling processes, thereby affecting numerous fundamental functions in the brain and spinal cord. Versatile direct and indirect interactions of plectin with cytoskeletal filaments and enzymes in the cells of the CNS in normal physiological and in pathologic conditions remain to be fully addressed. Several pathologies of the CNS related to plectin have been discovered in patients with plectinopathies. However, in view of plectin as an integrator of a cohesive mesh of cellular proteins, it is important that the role of plectin is also considered in other CNS pathologies. This review summarizes the current knowledge of plectin in the CNS, focusing on plectin isoforms that have been detected in the CNS, along with its expression profile and distribution alongside diverse cytoskeleton filaments in CNS cell types. Considering that the bidirectional communication between neurons and glial cells, especially astrocytes, is crucial for proper functioning of the CNS, we place particular emphasis on the known roles of plectin in neurons, and we propose possible roles of plectin in astrocytes.

The Diversity of Intermediate Filaments in Astrocytes

Despite the remarkable complexity of the individual neuron and of neuronal circuits, it has been clear for quite a while that, in order to understand the functioning of the brain, the contribution of other cell types in the brain have to be accounted for. Among glial cells, astrocytes have multiple roles in orchestrating neuronal functions. Their communication with neurons by exchanging signaling molecules and removing molecules from extracellular space takes place at several levels and is governed by different cellular processes, supported by multiple cellular structures, including the cytoskeleton. Intermediate filaments in astrocytes are emerging as important integrators of cellular processes. Astrocytes express five types of intermediate filaments: glial fibrillary acidic protein (GFAP); vimentin; nestin; synemin; lamins. Variability, interactions with different cellular structures and the particular roles of individual intermediate filaments in astrocytes have been studied extensively in the case of GFAP and vimentin, but far less attention has been given to nestin, synemin and lamins. Similarly, the interplay between different types of cytoskeleton and the interaction between the cytoskeleton and membranous structures, which is mediated by cytolinker proteins, are understudied in astrocytes. The present review summarizes the basic properties of astrocytic intermediate filaments and of other cytoskeletal macromolecules, such as cytolinker proteins, and describes the current knowledge of their roles in normal physiological and pathological conditions.

On the role of tubulin, plectin, desmin, and vimentin in the regulation of mitochondrial energy fluxes in muscle cells

Mitochondria perform a central role in life and death of the eukaryotic cell. They are major players in the generation of macroergic compounds and function as integrated signaling pathways, including the regulation of Ca²⁺ signals and apoptosis. A growing amount of evidence is demonstrating that mitochondria of muscle cells use cytoskeletal proteins (both microtubules and intermediate filaments) not only for their movement and proper cellular positioning, but also to maintain their biogenesis, morphology, function, and regulation of energy fluxes through the outer mitochondrial membrane (MOM). Here we consider the known literature data concerning the role of tubulin, plectin, desmin and vimentin in bioenergetic function of mitochondria in striated muscle cells, as well as in controlling the permeability of MOM for adenine nucleotides (ADNs). This is of great interest since dysfunctionality of these cytoskeletal proteins has been shown to result in severe myopathy associated with pronounced mitochondrial dysfunction. Further efforts are needed to uncover the pathways by which the cytoskeleton supports the functional capacity of mitochondria and transport of ADN(s) across the MOM (through voltage-dependent anion channel).

Plectin-intermediate filament partnership in skin, skeletal muscle, and peripheral nerve

Plectin is a large, 500-kDa, intermediate filament (IF)-associated protein. It acts as a cytoskeletal crosslinker and signaling scaffold, affecting mechanical as well as dynamic properties of the cytoskeleton. As a member of the plakin family of cytolinker proteins, plectin has a multidomain structure that is responsible for its vast binding portfolio. It not only binds to all types of IFs, actin filaments and microtubules, but also to transmembrane receptors, proteins of the subplasma membrane protein skeleton, components of the nuclear envelope, and several kinases with known roles in migration, proliferation, and energy metabolism of cells. Due to alternative splicing, plectin is expressed as various isoforms with differing N-terminal heads that dictate their differential subcellular targeting. Through specific interactions with other proteins at their target sites and their ability to bind to all types of IFs, plectin molecules provide strategically located IF anchorage sites within the cytoplasm of cells. In this review, we will present an overview of the structural features and functional properties of plectin and discuss recent progress in defining the role of its isoforms in stress-prone tissues and the implicated diseases, with focus on skin, skeletal muscle, and Schwann cells of peripheral nerve.

Mycoplasma pneumoniae cytoskeletal protein HMW2 and the architecture of the terminal organelle

The terminal organelle of Mycoplasma pneumoniae mediates cytadherence and gliding motility and functions in cell division. The defining feature of this complex membrane-bound cell extension is an electron-dense core of two segmented rods oriented longitudinally and enlarging to form a bulb at the distal end. While the components of the core have not been comprehensively identified, previous evidence suggested that the cytoskeletal protein HMW2 forms parallel bundles oriented lengthwise to yield the major rod of the core. In the present study, we tested predictions emerging from that model by ultrastructural and immunoelectron microscopy analyses of cores from wild-type M. pneumoniae and mutants producing HMW2 derivatives. Antibodies specific for the N or C terminus of HMW2 labeled primarily peripheral to the core along its entire length. Furthermore, truncation of HMW2 did not correlate specifically with core length. However, mutant analysis correlated specific HMW2 domains with core assembly, and examination of core-enriched preparations confirmed that HMW2 was a major component of these fractions. Taken together, these findings yielded a revised model for HMW2 in terminal organelle architecture.

Phosphoproteome analysis of the human Chang liver cells using SCX and a complementary mass spectrometric strategy

The liver is the largest organ in the body, with many complex, essential functions, such as metabolism, deintoxication, and secretion, often regulated via post-translational modifications, especially phosphorylation. Thus, the detection of phosphoproteins and phosphorylation sites is important to comprehensively explore human liver biological function. The human Chang liver cell line is among the first derived from non-malignant tissue, and its phosphoproteome profile has never been globally analyzed. To develop the complete phosphoproteome and probe the roles of protein phosphorylation in normal human liver, we adopted a shotgun strategy based on strong cation exchange chromatograph, titanium dioxide and LC-MS/MS to isolate and identify phosphorylated proteins. Two types of MS approach, Q-TOF and IT, were used and compared to identify phosphosites from complex protein mixtures of these cells. A total of 1035 phosphorylation sites and 686 phosphorylated peptides were identified from 607 phosphoproteins. A search using the public database of PhosphoSite showed that approximately 344 phosphoproteins and 760 phosphorylation sites appeared to be novel. In addition, N-terminal phosphorylated peptides were a greater fraction of all identified phosphopeptides. With GOfact analysis, we found that most of the identified phosphoproteins are involved in regulating metabolism, consistent with the liver's role as a key metabolic organ.

The nuclear lamina

The inner face of the nuclear envelope of metazoan cells is covered by a thin lamina consisting of a one-layered network of intermediate filaments interconnecting with a complex set of transmembrane proteins and chromatin associating factors. The constituent proteins, the lamins, have recently gained tremendous recognition, because mutations in the lamin A gene, LMNA, are the cause of a complex group of at least 10 different diseases in human, including the Hutchinson-Gilford progeria syndrome. The analysis of these disease entities has made it clear that besides cytoskeletal functions, the lamina has an important role in the "behaviour" of the genome and is, probably as a consequence of this function, intimately involved in cell fate decisions. Furthermore, these functions are related to the involvement of lamins in organizing the position and functional state of interphase chromosomes as well as to the occurrence of lamins and lamina-associated proteins within the nucleoplasm. However, the structural features of these lamins and the nature of the factors that assist them in genome organization present an exciting challenge to modern biochemistry and cell biology.

Identification of a novel, highly variable amino-terminal amino acid sequence element in the nuclear intermediate filament protein lamin B(2) from higher vertebrates

By comparing newly available cDNA sequences of the human intermediate filament protein lamin B(2) with published sequences, we have identified an additional translation initiation codon 60 nucleotides upstream of the previously assumed translation start. In addition, corresponding sequences were identified in the chimpanzee, mouse, rat and bovine genes and cDNAs, respectively. Therefore, we generated antibodies against these potential 20 new amino acids of the human sequence. By immunoblot analysis and immunofluorescence microscopy we show that human lamin B(2) is indeed synthesized as a longer version than previously reported, because it contains these additional 20 amino acids. Notably, the sequence homology to mouse, rat and bovine lamin B(2) is significantly lower in this segment than in that between the second methionine codon and the start of the alpha-helical rod indicating that the tip of the "head" is engaged in more species-specific functions. Forced expression of the GFP-tagged authentic "long" and the 20 amino acid shorter version of lamin B(2) in human cultured SW-13 cells demonstrated that both the longer and the shorter version are properly integrated into the nuclear lamina, although the shorter version exhibited a tendency to disturb envelope architecture at higher expression levels.

Intermediate filaments mediate cytoskeletal crosstalk

Intermediate filaments, actin-containing microfilaments and microtubules are the three main cytoskeletal systems of vertebrate and many invertebrate cells. Although these systems are composed of distinctly different proteins, they are in constant and intimate communication with one another. Understanding the molecular basis of this cytoskeletal crosstalk is essential for determining the mechanisms that underlie many cell-biological phenomena. Recent studies have revealed that intermediate filaments and their associated proteins are important components in mediating this crosstalk.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

Interaction of periplakin and envoplakin with intermediate filaments

Periplakin is a component of desmosomes and the epidermal cornified envelope. Its N-terminal domain interacts with the plasma membrane; it heterodimerises with envoplakin via its rod domain; and its C-terminus interacts with intermediate filaments. Periplakin has the shortest C-terminus of the plakin family, comprising only the linker domain found in all conventional plakins. By transient transfection of COS7 cells and primary human epidermal keratinocytes with deletion mutants of the periplakin C-terminus we mapped sequences required for intermediate filament interaction to two regions of the linker motif that are most highly conserved amongst the plakins. The results were confirmed by overlay assays of the binding of in vitro translated periplakin constructs to keratins and vimentin. We found that envoplakin and periplakin could still associate with each other when parts of their rod domains were deleted and, surprisingly, that removal of the entire rod domain did not completely inhibit their interaction. Co-transfection of constructs containing the C-termini of envoplakin and periplakin suggested that the periplakin C-terminus may stabilise the interaction of the envoplakin C-terminus with intermediate filaments. We conclude that the periplakin C-terminus plays an important role in linking periplakin and envoplakin to intermediate filaments.

Investigation of nuclear architecture with a domain-presenting expression system

We have investigated the topogenic properties of the nucleus by ectopic expression of chimeric proteins consisting of a NLS-modified cytoplasmic filament-forming protein, Xenopus laevis vimentin, and domains of inner nuclear membrane proteins. Whereas the "carrier" without cargo, the NLS-vimentin alone, is deposited in a few nuclear body-type structures (J.M. Bridger, H. Herrmann, C. Münkel, P. Lichter, J. Cell Sci., 111, 1241-1253), the distribution is entirely changed upon coupling with the evolutionarily conserved domain of the lamin B tail, the entire lamin B tail, the amino-terminal nucleoplasmic segment of the lamin B receptor (LBR), and the LEM domain of emerin, respectively. Remarkably, every individual chimeric protein exhibits a completely different distribution. Therefore, we assume that the chimeric parts are specifically recognized by factors engaged in nucleus-specific topogenesis. Thus, the conserved domain of the lamin B tail results in the formation of many small accumulations spread all over the nucleus. The chimera with the complete lamin B tail is deposited in short fibrillar aggregates within the nucleus. It does not mediate the integration of the chimeric protein into the nuclear membrane in cultured cells, indicating that the lamin tail alone is not sufficient to direct the integration of a protein into the lamina in vivo. In contrast, in the nuclear assembly system of Xenopus laevis the recombinant NLS-vimentin-lamin tail protein is concentrated at the nuclear membrane. The LBR chimera is arranged in a "beaded-chain"-type fashion, quite different from the more random deposition of NLS-vimentin alone. To our surprise, the LEM domain of emerin induces the retention of most of the chimeric proteins within the cytoplasm. Hence, it appears to be engaged in a strong cytoplasmic interaction that overrides the nuclear localization signal. Finally, the lamin chimera with the conserved part of the lamin B tail is shown to recruit LBR to the nuclear vimentin bodies and, vice versa, the LBR chimera attracts lamin B in transfected cells, thereby demonstrating their bona fide interaction in vivo.

Molecular characterization of Calymmin, a novel notochord sheath-associated extracellular matrix protein in the zebrafish embryo

During the screening of a zebrafish postsomitogenesis embryo cDNA library, we have identified a cDNA corresponding to a novel type of protein localized to the notochordal sheath-associated extracellular matrix (ECM) of the embryo. The 4.049-kb mRNA encodes a predicted polypeptide of 1,207 amino acids (122 kDa, pI 10.50) with a potential signal peptide of 20 amino acids. After the signal peptide, the mature protein consists of 1,187 amino acids (119 kDa, pI 10.46), for which the name "Calymmin" (from Greek chialphalambdanumumualpha, to envelop, to cover) is proposed. The Calymmin mRNA is highly and transiently expressed by the notochord cells of the embryo from the 10- to 12-somite stage to the pharyngula period (13 and 24 hours postfertilization, respectively), and light and electron microscopical immunolocalization analysis revealed that the protein was specifically localized within a granular and filamentous layer of the ECM compartment surrounding the notochord. In zebrafish no tail mutants (ntl(tc41)), in which the notochord precursor cells are present but fail to differentiate, the Calymmin protein was not detected, confirming the notochord origin of Calymmin. These results indicate that Calymmin is a novel constitutive protein of the ECM compartment associated to the perinotochordal sheath in the zebrafish embryo, which is specifically expressed by the differentiating notochord cells.

Defining the mycoplasma 'cytoskeleton': the protein composition of the Triton X-100 insoluble fraction of the bacterium Mycoplasma pneumoniae determined by 2-D gel electrophoresis and mass spectrometry

After treating Mycoplasma pneumoniae cells with the nonionic detergent Triton X-100, an undefined, structured protein complex remains that is called the 'Triton X-100 insoluble fraction' or 'Triton shell'. By analogy with eukaryotic cells and supported by ultrastructural analyses it is supposed that this fraction contains the components of a bacterial cytoskeleton-like structure. In this study, the composition of the Triton X-100 insoluble fraction was defined by electron microscopic screening for possible structural elements, and by two-dimensional (2-D) gel electrophoresis and MS to identify the proteins present. Silver staining of 2-D gels revealed about 100 protein spots. By staining with colloidal Coomassie blue, about 50 protein spots were visualized, of which 41 were identified by determining the mass and partial sequence of tryptic peptides of individual proteins. The identified proteins belonged to several functional categories, mainly energy metabolism, translation and heat-shock response. In addition, lipoproteins were found and most of the proteins involved in cytadherence that were previously shown to be components of the Triton X-100 insoluble fraction. There were also 11 functionally unassigned proteins. Based on sequence-derived predictions, some of these might be potential candidates for structural components. Quantitatively, the most prevalent proteins were the heat-shock protein DnaK, elongation factor Tu and subunits alpha and beta of the pyruvate dehydrogenase complex (PdhA, PdhB), but definite conclusions regarding the composition of the observed structures can only be drawn after specific proteins are assigned to them, for example by immunocytochemistry.

Identification of the cytolinker plectin as a major early in vivo substrate for caspase 8 during CD95- and tumor necrosis factor receptor-mediated apoptosis

Caspase 8 plays an essential role in the execution of death receptor-mediated apoptosis. To determine the localization of endogenous caspase 8, we used a panel of subunit-specific anti-caspase 8 monoclonal antibodies in confocal immunofluorescence microscopy. In the human breast carcinoma cell line MCF7, caspase 8 predominantly colocalized with and bound to mitochondria. After induction of apoptosis through CD95 or tumor necrosis factor receptor I, active caspase 8 translocated to plectin, a major cross-linking protein of the three main cytoplasmic filament systems, whereas the caspase 8 prodomain remained bound to mitochondria. Plectin was quantitatively cleaved by caspase 8 at Asp 2395 in the center of the molecule in all cells tested. Cleavage of plectin clearly preceded that of other caspase substrates such as poly(ADP-ribose) polymerase, gelsolin, cytokeratins, or lamin B. In primary fibroblasts from plectin-deficient mice, apoptosis-induced reorganization of the actin cytoskeleton, as seen in wild-type cells, was severely impaired, suggesting that during apoptosis, plectin is required for the reorganization of the microfilament system.

Intermediate filaments and their associates: multi-talented structural elements specifying cytoarchitecture and cytodynamics

The assembly of intermediate filament (IF) arrays involves the recruitment of a complex set of cell-type-specific IF-associated proteins. Some of them are integral membrane proteins, others act as crosslinking proteins with vectorial binding activities, and yet others comprise motor proteins. In vivo IFs appear to be predominantly heteropolymers, although in vitro several IF proteins (e.g. vimentin, desmin, neurofilament (NF)-L and the nuclear lamins) do self-assemble into IF-like polymers. In contrast, NF-M, NF-H, nestin, synemin and paranemin, all bona fide IF proteins, are unable to self-assemble into IFs either in vitro or in vivo. The individual IF proteins of this large multigene family are chemically heterogeneous, exhibiting different assembly kinetics and yielding discrete types of filaments. The unique physical properties and interaction capabilities of these distinct IF molecular building blocks, in combination with accessory proteins, mediate the generation of a highly dynamic and interconnected, cell-type-specific cytoarchitecture.

Retinoic acid differentiation of HL-60 cells promotes cytoskeletal polarization

Retinoic acid (RA) treatment of HL-60 cells in vitro induces granulocytic differentiation, involving reorganization of the nucleus and cytoplasm, development of chemoattractant-directed migration, and eventual apoptosis. The present studies with HL-60/S4 cells document that major elements of the cytoskeleton are changed: actin increases by 50%; vimentin decreases by more than 95%. The cellular content of alpha-tubulin does not significantly change; but the centrosomal-microtubule (MT) array moves away from the lobulating nucleus. Cytoskeletal-modifying chemicals modulate this polarized reorganization: Taxol and cytochalasin D enhance centrosome movement; nocodazole reverses it. Cytoskeletal-modifying chemicals do not appear to affect nuclear lobulation or the integrity of envelope-limited chromatin sheets (ELCS). Employing bcl-2-overexpressing HL-60 cells permitted demonstration of nuclear lobulation, ELCS formation, and centrosome-MT movement concomitantly during RA-induced differentiation, implying independence between the cellular reorganization and apoptotic programs. RA appears to promote an inherent potential in HL-60 cells for cytoskeletal polarization, likely to be important for chemoattractant-directed cell migration, an established characteristic of mature granulocytes.

Plectin: a cytolinker by design

Plectin is a cytoskeletal protein of >500 kDa that forms dumbbell-shaped homodimers comprising a central parallel alpha-helical coiled coil rod domain flanked by globular domains, thus providing a molecular backbone ideally suited to mediate the protein's interactions with an array of other cytoskeletal elements. Plectin self-associates and interacts with actin and intermediate filament cytoskeleton networks at opposite ends, and it binds at both ends to the hemidesmosomal transmembrane protein integrin beta-4, and likely to other junctional proteins. The central coiled coil rod domain can form bridges over long stretches and serves as a flexible linker between the structurally diverse N-terminal domain and the highly conserved C-terminal domain. Plectin is also a target of p34cdc2 kinase that regulates its dissociation from intermediate filaments during mitosis.

Role of plectin in cytoskeleton organization and dynamics

Plectin and its isoforms are versatile cytoskeletal linker proteins of very large size (&gt;500 kDa) that are abundantly expressed in a wide variety of mammalian tissues and cell types. Earlier studies indicated that plectin molecules were associated with and/or directly bound to subcomponents of all three major cytoskeletal filament networks, the subplasma membrane protein skeleton, and a variety of plasma membrane-cytoskeleton junctional complexes, including those found in epithelia, various types of muscle, and fibroblasts. In conjunction with biochemical data, this led to the concept that plectin plays an important role in cytoskeleton network organization, with consequences for viscoelastic properties of the cytoplasm and the mechanical integrity and resistance of cells and tissues. Several recent findings lent strong support to this concept. One was that a hereditary disease, epidermolysis bullosa simplex (EBS)-MD, characterized by severe skin blistering combined with muscular dystrophy, is caused by defects in the plectin gene. Another was the generation of plectin-deficient mice by targeted inactivation of the gene. Dying shortly after birth, these animals exhibited severe defects in skin, skeletal muscle and heart. Moreover, in vitro studies with cells derived from such animals unmasked an essential new role of plectin as regulator of cellular processes involving actin stress fibers dynamics. Comprehensive analyses of the gene locus in man, mouse, and rat point towards a complex gene expression machinery, comprising an unprecedented diversity of differentially spliced transcripts with distinct 5′ starting exons, probably regulated by different promoters. This could provide a basis for cell type-dependent and/or developmentally-controlled expression of plectin isoforms, exerting different functions through binding to distinct partners. Based on its versatile functions and structural diversification plectin emerges as a prototype cytolinker protein among a family of proteins sharing partial structural homology and functions.

Ligand-independent role of the beta 4 integrin subunit in the formation of hemidesmosomes

Recently, we have shown that a region within the beta4 cytoplasmic domain, encompassing the second fibronectin type III (FNIII) repeat and the first 27 amino acids of the connecting segment, is critical for the localization of alpha6 beta4 in hemidesmosomes. In addition, this region was shown to regulate the distribution of HD1/plectin in transfected cells. In order to investigate the function of the beta4 extracellular and cytoplasmic domains in the assembly and integrity of hemidesmosomes, we have constructed chimeric receptors consisting of the extracellular and transmembrane domains of the interleukin 2 receptor (IL2R), fused to different parts of the beta4 cytoplasmic domain. These chimeras are expressed as single subunits at the plasma membrane. The results show that the first and the second FNIII repeat, together with the first part of the connecting segment (in total a stretch of 241 amino acids spanning amino acids 1,115 to 1,356) are both essential and sufficient for the localization of beta4 in pre-existing hemidesmosomes. Moreover, expression of the IL2R/beta4 chimeric constructs in COS-7 and CHO cells, which do not express alpha6 beta4 or the bullous pemphigoid (BP) antigens but do express HD1/plectin, revealed that the stretch of 241 amino acids is sufficient for inducing the formation of type II hemidesmosomes. Expression of the IL2R/beta4 chimeras in a keratinocyte cell line derived from a patient lacking beta4 expression, showed that amino acids 1,115 to 1,356 can also induce the formation of type I hemidesmosomes. We further demonstrate that type I and II hemidesmosomes can also be formed upon adhesion of alpha6 beta4-expressing cells to fibronectin. These findings establish that the beta4 extracellular domain is not essential for the induction of hemidesmosome assembly. Moreover, they demonstrate that binding of alpha6 beta4 to ligand, and heterodimerization of alpha6 with beta4, are not required for hemidesmosome formation. This indicates that the assembly of hemidesmosomes can be regulated from within the cell.

Molecular characterization of a novel, widespread nuclear protein that colocalizes with spliceosome components

We report the identification and molecular characterization of a novel type of constitutive nuclear protein that is present in diverse vertebrate species, from Xenopus laevis to human. The cDNA-deduced amino acid sequence of the Xenopus protein defines a polypeptide of a calculated mass of 146.2 kDa and a isoelectric point of 6.8, with a conspicuous domain enriched in the dipeptide TP (threonine-proline) near its amino terminus. Immunolocalization studies in cultured cells and tissues sections of different origin revealed an exclusive nuclear localization of the protein. The protein is diffusely distributed in the nucleoplasm but concentrated in nuclear speckles, which represent a subnuclear compartment enriched in small nuclear ribonucleoprotein particles and other splicing factors, as confirmed by colocalization with certain splicing factors and Sm proteins. During mitosis, when transcription and splicing are downregulated, the protein is released from the nuclear speckles and transiently dispersed throughout the cytoplasm. Biochemical experiments have shown that the protein is recovered in a approximately 12S complex, and gel filtration studies confirm that the protein is part of a large particle. Immunoprecipitation and Western blot analysis of chromatographic fractions enriched in human U2 small nuclear ribonucleoprotein particles of distinct sizes (12S, 15S, and 17S), reflecting their variable association with splicing factors SF3a and SF3b, strongly suggests that the 146-kDa protein reported here is a constituent of the SF3b complex.

Polarisation-dependent association of plectin with desmoplakin and the lateral submembrane skeleton in MDCK cells

The intermediate filament-binding protein plectin and cytokeratin were localised at the cellular periphery of fully polarised Madin-Darby canine kidney (MDCK) cells, whereas vimentin was primarily found in a perinuclear network. Confocal and immunoelectron microscopy revealed that plectin was restricted to areas underlying the lateral plasma membrane. It colocalised with fodrin, a component of the submembrane skeleton, and was closely associated with desmosomal plaque structures. Biochemically, plectin was shown to interact directly with immunoprecipitated desmoplakin in vitro. Upon loss of cell polarity in low calcium medium, plectin redistributed to a cytoplasmic vimentin- and cytokeratin-related network, clearly distinct from diffusely distributed fodrin and internalised desmoplakin structures. The structural reorganisation of plectin was also reflected by an increased solubility of the protein in Triton X-100/high salt, and a decrease in its half-life from approximately 20 to approximately 5 hours. Furthermore, unlike cytokeratins and vimentin, desmoplakin and fodrin did not associate with plectin attached to magnetic beads in cell lysates of unpolarised cells, while all proteins formed a stable complex in polarised cells. Altogether, these data indicate that plectin is involved in the anchorage of intermediate filaments to desmosomes and to the submembrane skeleton in polarised MDCK cells.

Identification of plectin as a substrate of p34cdc2 kinase and mapping of a single phosphorylation site

Plectin is an in vitro substrate for various kinases present in cell lysates from mitotic and interphase Chinese hamster ovary cells. Sensitivity of plectin kinase activity to the inhibitor olomoucine, and two-dimensional tryptic peptide mapping of plectin phosphorylated by various kinase preparations suggested that the major plectin kinase activity in mitotic extracts is related to the cell cycle regulator kinase p34cdc2. Bacterial expression of various truncated plectin mutant proteins comprising different domains of the molecule and their phosphorylation by purified p34cdc2kinase revealed that the target site of this kinase resided within plectin's C-terminal globular domain. Among the subdomains of the C-terminal region (six repeats and a short tail sequence), only repeat 6 and the tail were phosphorylated by p34cdc2 kinase. As shown by two-dimensional phosphopeptide mapping, repeat 6, but not the tail, contained a mitosis-specific phosphorylation site targeted by p34cdc2 kinase in intact plectin molecules. By performing site-directed mutagenesis of a potential p34cdc2 recognition sequence motif within the repeat 6 domain, threonine 4542 was identified as the major target for the kinase. Protein kinase A, phosphorylating plectin also within repeat 6, targeted sites that were clearly different from those of p34cdc2 kinase.

Rapid effects of EGF on cytoskeletal structures and adhesive properties of highly metastatic rat mammary adenocarcinoma cells

In the highly metastatic rat mammary adenocarcinoma cell clone MTLn3, EGF induced increased adhesion to fibronectin while in the human epidermoid carcinoma cell line A431 EGF induced diminished adhesive properties. Flattening of cells with extensive formation of filopodia was observed in MTLn3 cells within 5 min of EGF addition, while in A431 cells EGF induced rounding up and only occasional formation of filopodia. Immunofluorescent analysis revealed extension of microtubules (MT) into the filopodia and Western blot analysis demonstrated an EGF-induced 2- to 3-fold increase in the amount of assembled tubulin in MTLn3 but not in A431 cells. In MTLn3, but only marginally in A431 cells, EGF treatment resulted in phosphorylation of a 280 kD cytoskeleton-associated protein, which was rapid and dose-dependent. These results suggest differential signal transduction pathway of cytoskeleton-associated EGFRs in highly metastatic MTLn3 as compared with A431 cells.

Calyculin-A increases the level of protein phosphorylation and changes the shape of 3T3 fibroblasts

Calyculin-A, an inhibitor of type 1 and 2A phosphatases, was applied extracellularly to 3T3 fibroblasts. At 0.1 microM, calyculin-A caused a marked increase in protein phosphorylation in both the cytosolic and insoluble cellular fractions. This effect was independent of external Ca2+. An immunoprecipitate, formed with an antibody to myosin, contained several cytoskeletal components. Increased phosphorylation following treatment with calyculin-A was observed in vimentin, the 20-kD myosin light chain, and an unidentified 440-kD component. An enhanced level of vimentin phosphorylation was found in intermediate filament preparations from treated cells. Calyculin-A also caused marked shape changes of 3T3 cells. Within minutes after addition of calyculin-A (0.1 microM) cells became rounded and lost attachment to the substratum. Stress fibers, intermediate filaments, and microtubules, prominent in the attached control cells, were not evident in the rounded cells. Shape changes were reversible and after removal of calyculin-A the rounded cells attached to the substratum, resumed a flattened shape, and were active mitotically. In the cells treated with calyculin-A an unusual "ball-like" structure was observed with transmission electron microscopy. This unique structure was 2-3 microM in diameter and was located close to the nucleus. The use of calyculin-A adds further support to the idea that cell shape is controlled, at least in part, by concerted actions of a kinase-phosphatase couple.

Identification and characterization of a novel mammalian intermediate filament-associated protein

A novel monoclonal antibody, designated M1.4, recognizes the high molecular weight microtubule-associated protein MAP1A (ca. Mr 380 kD) in both bovine and rat brain. In HeLa cells, however, M1.4 binds to a 240 kD polypeptide on immunoblots and co-localizes with both vimentin and cytokeratin filaments using double-label immunofluorescence microscopy. Immunoelectron microscopy indicates that the 240 kD polypeptide localizes along bundled intermediate filaments in a periodic manner. Two-dimensional electrophoretic analysis indicates that the 240 kD polypeptide has a basic pI of 7.7. When HeLa cell intermediate filaments are isolated using standard non-ionic detergent/high-salt conditions the 240 kD polypeptide does not sediment with the intermediate filaments, unlike the established intermediate filament-associated protein plectin. Immunoblot analysis with M1.4 shows the 240 kD polypeptide is expressed in a number of mammalian cell lines. Additionally, double-label immunofluorescence shows the 240 kD polypeptide to associate with vimentin filaments in African Green Monkey kidney (CV-1) and JC neuroblastoma cells. Due to its unique biochemical and biological characteristics, the 240 kD polypeptide is clearly a novel intermediate filament-associated protein for which we have proposed the designation gyronemin (Gr. gyros: around; nemin: filament).

Modulation of a 190-kD microtubule-associated protein in pigment epithelium by VIP

Vasoactive intestinal peptide stimulates phosphorylation of six high molecular weight cytosolic proteins in the cultured retinal pigment epithelium (RPE). Of these, the 190-kD phosphoprotein is associated with the microtubules assembled by taxol/GTP and is immunologically related to the brain microtubule-associated protein 2 (mol.wt. = 280 kD). VIP is also shown here to stimulate secretion in the cultured RPE. VIP-stimulated phosphorylation of a 190-kD microtubule-associated protein is also demonstrated here in the retinal glia.

Plectin: general overview and appraisal of its potential role as a subunit protein of the cytomatrix

Plectin has recently been identified as a widespread and abundant cytoplasmic protein of mammalian cells. In this article the available data on plectin are reviewed, focusing on plectin's occurrence and localization in various cell types and tissues, its biochemical characterization, and its molecular interaction partners. Furthermore, the putative role of this protein has a multifunctional connecting link of the cytomatrix and its structural as well as functional relationship to other cytoskeletal proteins is discussed. It is concluded that plectin is potentially the most versatile crosslinking element of the cytomatrix reported to date.

Structure and hydrodynamic properties of plectin molecules

Plectin is a cytoskeletal, high molecular weight protein of widespread and abundant occurrence in cultured cells and tissues. To study its molecular structure, the protein was purified from rat glioma C6 cells and subjected to chemical and biophysical analyses. Plectin's polypeptide chains have an apparent molecular weight of 300,000, as shown by one-dimensional sodium dodecyl sulfate/polyacrylamide electrophoresis. Cross-linking of non-denatured plectin in solution with dimethyl suberimidate and electrophoretic analyses on sodium dodecyl sulfate/agarose gels revealed that the predominant soluble plectin species was a molecule of 1200 X 10(3) Mr consisting of four 300 X 10(3) Mr polypeptide chains. Hydrodynamic properties of plectin in solution were obtained by sedimentation velocity centrifugation and high-pressure liquid chromatography analysis yielding a sedimentation coefficient of 10 S and a Stokes radius of 27 nm. The high f/fmin ratio of 4.0 indicated a very elongated shape of plectin molecules and an axial ratio of about 50. Shadowing and negative staining electron microscopy of plectin molecules revealed multiple domains: a rigid rod of 184 nm in length and 2 nm in diameter, and two globular heads of 9 nm diameter at each end of the rod. Circular dichroism spectra suggested a composition of 30% alpha-helix, 9% beta-structure and 61% random coil or aperiodic structure. The rod-like shape, the alpha-helix content as well as the thermal transition within a midpoint of 45 degrees C and the transition enthalpy (168 kJ/mol) of secondary structure suggested a double-stranded, alpha-helical coiled coil rod domain. Based on the available data, we favor a model of native plectin as a dumb-bell-like association of four 300 X 10(3) Mr polypeptide chains. Electron microscopy and turbidity measurements showed that plectin molecules self-associate into various oligomeric states in solutions of nearly physiological ionic strength. These interactions apparently involved the globular end domains of the molecule. Given its rigidity and elongated shape, and its tendency towards self-association, plectin may well be an interlinking element of the cytoskeleton that may also form a network of its own.

Plectin from bovine lenses. Chemical properties, structural analysis and initial identification of interaction partners

Plectin was purified to near homogeneity from epithelial and cortical cell layers of bovine lenses using a simple and fast purification scheme that included as last step, gel permeation chromatography in the presence of 0.25% sodium N-lauroyl sarcosinate. Lens cell plectin showed extensive structural homology to plectin from cultured cells as revealed by immunoblotting experiments and amino acid analysis. Further characterization included solubility in various buffer solutions, codistribution with vimentin in repeated rounds of intermediate filament disassembly and assembly, and hydrodynamic behaviour in high-performance gel permeation chromatography. Electron microscopy of negatively stained and rotary shadowed plectin molecules revealed a dumb-bell-like structure with an estimated relative molecular mass of 1.16 X 10(6). Specific head-to-head self-interaction of plectin molecules at low salt concentrations and formation of large aggregates under high-salt and physiological conditions was also demonstrated. Isolation, as well as reconstitution of soluble protein complexes containing plectin, vimentin and other cytoskeletal and membrane skeleton proteins, provided first hints to plectin's role as an interlinking component of the cytoskeleton and the membrane skeleton of lens tissue.

In vivo phosphorylation of myelin basic proteins: single and double isotope incorporation in developmentally related myelin fractions

The phosphorylation of myelin basic proteins (MBPs) was studied in developing mouse brain. Based on our previous work we postulated that phosphorylation of MBPs takes place prior to their appearance in the myelin compartment as well as within the myelin sheath. To further test this hypothesis we utilized a subfractionation protocol that yields brain fractions enriched in myelin membranes of differing developmental stages. Incorporation of radioactive phosphate into MBPs was studied in each of the subcellular fractions. After 5- and 15-min incubations of isotope in vivo the highest specific radioactivities (SAs) of MBPs were found in the least mature myelin fractions. Incorporation of 32P in MBPs was greater into serine residues than threonine residues in all of the subcellular fractions studied. The relative turnover of MBP phosphates was studied in each of the subcellular myelin fractions using a time-staggered, double isotope methodology. The most rapid equilibration of MBP phosphates with the trichloroacetic acid (TCA)-soluble phosphate pool occurred in the most mature myelin fractions indicating that the highest turnover of MBP phosphates occurs in the most mature myelin fractions. The SAs and turnover rates of each of the four commonly observed mouse MBPs (14, 17, 18.5, and 21.5 kDa) were similar in any particular subfraction demonstrating that the MBP phosphotransferase system(s) acts on each of the MBPs in a similar manner.

Identification of plectin in different human cell types and immunolocalization at epithelial basal cell surface membranes

The occurrence of plectin in various human tissues and cell lines was investigated using immunofluorescence microscopy and antibody gel overlay/immunoblotting techniques. Plectin was identified in all tissues and cell lines tested, namely placenta, kidney, cornea, foreskin and eyelid skin, skin fibroblasts, monocytes, keratinocytes and HeLa cells. In frozen sections of cornea and skin, plectin was found to be enriched at epithelial basal cell surface membranes. Consequently, antibodies to plectin could serve as a tool in the classification of mechanobullous diseases.