Association of a Mr 50,000 cap-binding protein with the cytoskeleton in baby hamster kidney cells

Actin cytoskeleton in myofibroblast differentiation: ultrastructure defining form and driving function

Myofibroblasts are modified fibroblasts characterized by the presence of a well-developed contractile apparatus and the formation of robust actin stress fibers. These mechanically active cells are thought to orchestrate extracellular matrix remodeling during normal wound healing in response to tissue injury; these cells are found also in aberrant tissue remodeling in fibrosing disorders. This review surveys the understanding of the role of actin stress fibers in myofibroblast biology. Actin stress fibers are discussed as a defining ultrastructural and morphologic feature and well-accepted observations demonstrating its participation in contraction, focal adhesion maturation, and extracellular matrix reorganization are presented. Finally, more recent observations are reviewed, demonstrating its role in transducing mechanical force into biochemical signals, transcriptional control of genes involved in locomotion, contraction, and matrix reorganization, as well as the localized regulation of messenger RNA (mRNA) translation. This breadth of functionality of the actin stress fiber serves to reinforce and amplify its mechanical function, via induced expression of proteins that themselves augment contraction, focal adhesion formation, and matrix remodeling. In composite, the functions of the actin cytoskeleton are most often aligned, allowing for the integration and amplification of signals promoting both myofibroblast differentiation and matrix remodeling during fibrogenesis.

Emerging role for the cytoskeleton as an organizer and regulator of translation

The cytoskeleton is an intricate and dynamic fibrous network that has an essential role in the generation and regulation of cell architecture and cellular mechanical properties. The cytoskeleton also evolved as a scaffold that supports diverse biochemical pathways. Recent evidence favours the hypothesis that the cytoskeleton participates in the spatial organization and regulation of translation, at both the global and local level, in a manner that is crucial for cellular growth, proliferation and function.

Ureteral motility

The pyeloureteral function is to transport urine from the kidneys into the ureter toward the urinary bladder for storage until micturition. A set of mechanisms collaborates to achieve this purpose: the basic process regulating ureteral peristalsis is myogenic, initiated by active pacemaker cells located in the renal pelvis. Great emphasis has been given to hydrodynamic factors, such as urine flow rate in determining the size and pattern of urine boluses which, in turn, affect the mechanical aspects of peristaltic rhythm, rate, amplitude, and baseline pressure. Neurogenic contribution is thought to be limited to play a modulatory role in ureteral peristalsis. The myogenic theory of ureteral peristalsis can be traced back to Engelmann (1) who was able to localize the peristaltic pressure wave's origin in the renal pelvis and suggested that the ureteral contraction impulse passes from one ureteral cell to another, the whole ureter working as a functional syncitium. Recent studies of ureteral biomechanics, smooth muscle cell electrophysiology, membrane ionic currents, cytoskeletal components and pharmacophysiology much improved our understanding of the mechanism of how the urine bolus is propelled, how this process is disturbed in pathological states, and what could be done to improve it.

Improper organization of the actin cytoskeleton affects protein synthesis at initiation

Although the actin cytoskeleton and the translation machinery are considered to be separate cellular complexes, growing evidence supports overlapping regulation of the two systems. Because of its interaction with actin, the eukaryotic translation elongation factor 1A (eEF1A) is proposed to be a regulator or link between these processes. Using a genetic approach with the yeast Saccharomyces cerevisiae, specific regions of eEF1A responsible for actin interactions and bundling were identified. Five new mutations were identified along one face of eEF1A. Dramatic changes in cell growth, cell morphology, and actin cable and patch formation as well as a unique effect on total translation in strains expressing the F308L or S405P eEF1A mutant form were observed. The translation effects do not correlate with reduced translation elongation but instead include an initiation defect. Biochemical analysis of the eEF1A mutant forms demonstrated reduced actin-bundling activity in vitro. Reduced total translation and/or the accumulation of 80S ribosomes in strains with either a mutation or a null allele of genes encoding actin itself or actin-regulating proteins Tpm1p, Mdm20p, and Bnirp/Bni1p was observed. Our data demonstrate that eEF1A, other actin binding proteins, and actin mutants affect translation initiation through the actin cytoskeleton.

Both microtubule-stabilizing and microtubule-destabilizing drugs inhibit hypoxia-inducible factor-1alpha accumulation and activity by disrupting microtubule function

We have recently identified a mechanistic link between disruption of the microtubule cytoskeleton and inhibition of tumor angiogenesis via the hypoxia-inducible factor-1 (HIF-1) pathway. Based on this model, we hypothesized that other microtubule-targeting drugs may have a similar effect on HIF-1alpha. To test that hypothesis, we studied the effects of different clinically relevant microtubule-disrupting agents, including taxotere, epothilone B, discodermolide, vincristine, 2-methoxyestradiol, and colchicine. In all cases, HIF-1alpha protein, but not mRNA, was down-regulated in a drug dose-dependent manner. In addition, HIF-1alpha transcriptional activity was also inhibited by all drugs tested. To further examine whether these effects were dependent on microtubule network disruption, we tested the ability of epothilone B to inhibit HIF-1alpha protein in the human ovarian cancer cell line 1A9 and its beta-tubulin mutant epothilone-resistant subclone 1A9/A8. Our data showed that epothilone B treatment down-regulated HIF-1alpha protein in the parental 1A9 cells but had no effect in the resistant 1A9/A8 cells. These observations were confirmed by confocal microscopy, which showed impaired nuclear accumulation of HIF-1alpha in parental 1A9 cells at epothilone B concentrations that induced extensive microtubule stabilization. In contrast, epothilone B treatment had no effect on either microtubules or HIF-1alpha nuclear accumulation in the resistant 1A9/A8 cells. Furthermore, epothilone B inhibited HIF-1 transcriptional activity in 1A9 cells, as evidenced by a hypoxia response element-luciferase reporter assay, but had no effect on HIF-1 activity in the resistant 1A9/A8 cells. These data directly link beta-tubulin drug binding with HIF-1alpha protein inhibition. Our results further provide a strong rationale for testing taxanes and epothilones in clinical trials targeting HIF-1 in cancer patients.

Molecular interaction between human tumor marker protein p150, the largest subunit of eIF3, and intermediate filament protein K7

The human tumor marker protein p150 was identified as the largest subunit of eukaryotic translation initiation factor 3 (eIF3) (also known as p170/p180). Its expression level is not only upregulated in many transformed cell lines, but also in several human cancers including breast, cervical, esophageal, and stomach carcinomas. The function of p150 in cancer and initiation of translation are not well understood. Using the yeast two-hybrid genetic screen, we found that a portion of p150 interacts with hPrt1, another subunit of eIF3, and cytokeratin 7, an intermediate filament protein. The interactions between p150 and hPrt1, and between p150 and cytokeratin 7 were verified both in vivo and in vitro. The interaction site for hPrt1 was mapped to the carboxyl half of the coiled-coil region of the p150 protein between amino acids 664-835. The expression of hPrt1 was clearly upregulated in cancer tissue, similarly to that of p150. By contrast, no substantial difference in the expression level of cytokeratin 7 was observed between cancer and normal breast tissue, suggesting that cytokeratin 7 expression is not co-regulated with p150. Taken together, our studies suggest a new role for p150 in translation initiation, possibly by acting as an adapter molecule between the translation initiation apparatus and the cytoskeleton structure in the cell.

A general RNA-binding protein complex that includes the cytoskeleton-associated protein MAP 1A

Association of mRNA with the cytoskeleton represents a fundamental aspect of RNA physiology likely involved in mRNA transport, anchoring, translation, and turnover. We report the initial characterization of a protein complex that binds RNA in a sequence-independent but size-dependent manner in vitro. The complex includes a approximately 160-kDa protein that is bound directly to mRNA and that appears to be either identical or highly related to a approximately 1600-kDa protein that binds directly to mRNA in vivo. In addition, the microtubule-associated protein, MAP 1A, a cytoskeletal associated protein is a component of this complex. We suggest that the general attachment of mRNA to the cytoskeleton may be mediated, in part, through the formation of this ribonucleoprotein complex.

Translation and the cytoskeleton: a mechanism for targeted protein synthesis

This review describes the critical evidence that in eukaryotic cells polyribosomes, mRNAs and components of the protein synthetic machinery are associated with the cytoskeleton. The role of microtubules, intermediate filaments and microfilaments are discussed; at present most evidence suggests that polyribosomes interact with the actin filaments. The use of non-ionic detergent/deoxycholate treatment in the isolation of cytoskeletal-bound polysomes is described and the conclusion reached that at low salt concentrations this leads to mixed preparations of polysomes derived from both the cytoskeleton and the endoplasmic reticulum. At present the best approach for isolation of cytoskeletal-bound polysomes appears to involve extraction with salt concentrations greater than 130 mM after an initial non-ionic detergent treatment. Such polysomes appear to be enriched in certain mRNAs and thus it is suggested that they are involved in translation of a unique set of proteins. The evidence for mRNA localisation is presented and the role of the cytoskeleton in transport and localisation of RNA discussed. Recent data on the role of the 3' untranslated region in the targeting of mRNAs both to particular regions of the cell and for translation on cytoskeletal-bound polysomes is described. The hypothesis is developed that the association of polysomes with the cytoskeleton is the basis of a mechanism for the targeting of mRNAs and the compartmentalization of protein synthesis.

Beta-actin mRNA-binding proteins associated with the cytoskeletal framework

Association of mRNA with the cytoskeletal framework (CSK) is thought to play a strategic role in the placement of mRNA in the cytoplasm. However, the molecular determinants underlying mRNA/CSK association are completely unknown. To begin addressing this issue, we have employed a binding assay to identify proteins of the CSK compartment of NIH 3T3 cells that bind in-vitro-transcribed 32P-labelled beta-actin mRNA with high affinity. Three proteins, of approximate molecular masses 27, 50 and 97 kDa, were observed to exhibit strong binding. Binding to these proteins took place at physiological salt concentration and withstood washing in 0.5 M salt. Furthermore, binding was unaffected by heparin but was inhibited by unlabelled beta-actin mRNA. Treatment of isolated CSKs with the microfilament-severing agent DNase I abolished all beta-actin mRNA-binding activities, thus suggesting a possible association of beta-actin mRNA with the microfilament network in situ. Removal of the 3' untranslated region (UTR) significantly reduced beta-actin mRNA binding to all three CSK proteins but removal of the 5' UTR mainly affected binding to the 97-kDa species and that to a lesser extent. beta-Tubulin mRNA bound to the same three CSK proteins as did beta-actin mRNA, but with considerably less avidity. In contrast, vimentin mRNA strongly recognized these CSK proteins, and further bound to a group of smaller proteins (< 29 kDa). As beta-actin mRNA, beta-tubulin mRNA and vimentin mRNA have been observed to occupy separate cytoplasmic locales, the proteins detected here may be operative both in binding mRNAs to the CSK in situ, as well as in localizing mRNA in the cytoplasm.

Poly(A) RNA codistribution with microfilaments: evaluation by in situ hybridization and quantitative digital imaging microscopy

The distribution of poly(A) RNA has been visualized in single cells using high-resolution fluorescent in situ hybridization. Digital imaging microscopy was used to quantitate the signal in various cellular compartments. Most of the poly(A) signal remained associated with the cellular filament systems after solubilization of membranes with Triton, dissociation of ribosomes with puromycin, and digestion of non-poly(A) RNA with ribonuclease A and T1. The actin filaments were shown to be the predominant cellular structural elements associating with the poly(A) because low doses of cytochalasin released about two- thirds of the poly(A). An approach to assess the extent of colocalization of two images was devised using in situ hybridization to poly(A) in combination with probes for ribosomes, membranes, or F- actin. Digital imaging microscopy showed that most poly(A) spatially distributes most significantly with ribosomes, slightly less with F- actin, and least of all with membranes. The results suggest a mechanism for anchoring (and perhaps moving) much of the cellular mRNA utilizing the interaction between actin filaments and poly(A).

Regulation of vimentin expression in cultured epithelial cells

Most cell types start expressing vimentin when brought into tissue culture. Using both vimentin-expressing (HeLa) and vimentin-negative (MCF-7) epithelial cell lines, we have identified the cis-regulatory DNA elements involved in this process. Sequences located 1.1-0.6 kb upstream of the vimentin transcription-initiation site strongly enhance expression in HeLa cells, but are silenced in MCF-7 cells. Other regulatory elements in the vimentin promoter (an enhancer 3.2-2.6 kb upstream and a minimal promoter region including the CAAT-box) are potentially active in both cell types, but are silenced by the 0.5-kb fragment in MCF-7 cells. Deletion of this fragment restores transcriptional activity of a transfected vimentin promoter. Our data indicate that a double AP 1/jun-binding site present in the 0.5-kb fragment mediates the induction of vimentin expression in cultured epithelial cells, while silencing sequences located within the same fragment are responsible for the absence of vimentin expression in MCF-7 cells. In contrast to MCF-7 cells, a transfected vimentin promoter and gene are transcriptionally active in the vimentin-negative epithelial cell line T24. Transfection studies show that type-III-intermediate-filament expression is not impaired at any level in these cells. Upon transfection and expression of a desmin construct in T24 cells not only desmin, but also vimentin was detected. Both proteins assembled into intermediate filaments. This induction of vimentin expression appeared to be regulated at the post-transcriptional level.

Hepatic morphologic and biochemical changes induced by subacute cocaine administration in mice

The initial event and site of cocaine-induced hepatic injury have not been elucidated. In an attempt to identify the minimal effective dose and the site of injury, we have examined the livers of mice exposed to small daily doses of cocaine, using morphological and biochemical methods. All doses of cocaine greater than 5 mg/kg were able to cause significant elevation of serum glutamic pyruvic transaminase. Light microscopy revealed a progression of centrilobular necrosis as the dose increased from 10-30 mg/kg. The initial morphologic changes observed prior to necrosis included aggregation of intermediate filaments and dilation of rough endoplasmic reticulum with loss of ribosomes. Immunohistochemistry, using antibodies to cytokeratins, showed staining of individual hepatocytes in livers from cocaine-treated animals but not in controls. In contrast to earlier reports, we found little, if any, disruption of mitochondria. In vitro, the direct application of cocaine, norcocaine, and N-hydroxynorcocaine on isolated mitochondria had no effect on the ADP:O or respiratory control ratios, at concentrations up to 2.0 mM. Our studies demonstrate that any early cellular alterations in cocaine-induced hepatic injury are manifested in intermediate filaments and endoplasmic reticulum with no evidence of mitochondrial involvement.

mRNA association with the cytoskeletal framework likely represents a physiological binding event

A multitude of studies has indicated that the vast majority of mRNA and polyribosomes is associated with the detergent-resistant cytoskeletal framework (CSK). However, the nature and purpose of this association remain unclear. To begin unraveling the factors which may mediate this phenomenon, we examined the extent of association of four mRNAs (tubulin, vimentin, actin, and histone mRNA) with the CSKs of NIH 3T3 cells over a wide range of salt concentrations. Results indicate that the vast majority (greater than 90%) of each of these mRNAs remains associated with the CSK after detergent extraction of cells in low ionic strength buffer (25 mM NaCl). This association is manifest under conditions that cause the complete depolymerization of microtubules but that leave microfilaments and intermediate filaments intact. Even after extensive washing in buffer of approximately physiological ionic strength (150 mM NaCl), 75-85% of these mRNAs still remain associated with the CSK. However, at least 50% of each of these mRNAs can be eluted from the CSK by washing with buffer containing 250 mM NaCl. Not all the mRNAs, though, display the same elution profile. This suggests that different binding sites and/or different binding affinities may exist for different mRNAs. Surprisingly, close to 50% of the polyribosome population remains bound to the CSK despite washing in as much as 1.0 M NaCl. These adherent polyribosomes appear to be of the same size as those that are eluted, allaying the possibility that they are retained by the CSK simply due to size exclusion. Collectively, these data strongly imply that mRNAs are neither weakly adsorbed to the CSK nor physically trapped within the meshwork of cytoskeletal filaments.(ABSTRACT TRUNCATED AT 250 WORDS)

The cap and poly(A) tail function synergistically to regulate mRNA translational efficiency

The cap structure and the poly(A) tail are important regulatory determinants in establishing the translational efficiency of a messenger RNA. Although the mechanism by which either determinant functions remains poorly characterized, the interaction between the poly(A) tail-poly(A)-binding protein complex and events occurring at the 5' terminus during translation initiation has been an intriguing possibility. In this report, the mutual dependence of the cap and the poly(A) tail was studied. Poly(A)+ and poly(A)- luciferase (Luc) mRNAs generated in vitro containing or lacking a cap were translated in vivo in tobacco protoplasts, Chinese hamster ovary cells, and yeast following delivery by electroporation. The poly(A) tail-mediated regulation of translational efficiency was wholly dependent on the cap for function. Moreover, cap function was enhanced over an order of magnitude by the presence of a poly(A) tail. The relative differences in stability between the mRNAs could not account for the synergism. The synergism between the cap and poly(A) tail was not observed in yeast cells in which active translation had been disrupted. In addition, the synergism was not observed in in vitro translation lysates. These data demonstrate that the cap and the poly(A) tail are interdependent for optimal function in vivo and suggest that communication between the two regulatory determinants may be important in establishing efficient translation.

Interaction between mRNA, ribosomes and the cytoskeleton

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"In situ" translation: use of the cytoskeletal framework to direct cell-free protein synthesis

We have developed a novel, "in situ" translation system derived from cultured cells that are subject to mild detergent extraction. By using a low concentration of nonionic detergent to gently permeabilize cells while they remain adherent to a substrate, cytoskeletal frameworks are obtained that are devoid of membraneous barriers yet retain much the same topological arrangement of mRNA, ribosomes and cytostructure that exists "in vivo". Data indicate that when these cytoskeletal frameworks are supported by a ribosome-depleted, nuclease-treated, reticulocyte lysate supernatant, they are capable of resuming translation of their attached polysomes for at least 40 minutes. Emulsion autoradiography of ongoing protein synthesis demonstrates that protein synthetic activity is ubiquitous throughout the population of extracted cells, and not confined to a less well-extracted subset. Computer-assisted, two-dimensional gel analysis reveals that the pattern of proteins produced by such extracted cells is approximately 70% coincident with that produced by unextracted cells, including proteins of molecular weight as great as 200 kilodaltons. Furthermore, a continued increase in intensity of almost all proteins during the first 40 minutes of translation suggests that translational re-initiation, in addition to polysome run-off, is also taking place. Collectively, these findings indicate that much of the translational machinery remains both intact and competent in this cytoskeletal-based translation system. As such, this system should prove extremely useful in identifying molecular factors operant during certain types of translation control and in further examining the role played by the cytoskeleton in regulating gene expression.

Multiple types of mRNA-cytoskeleton interactions

Nearly all actively translated mRNAs are associated with the cytoskeleton in HeLa cells and the nature of this association is poorly understood. To gain insight into this association, we have examined and compared the cytoskeleton-mRNA interactions of a signal peptide-histone fusion mRNA (membrane-bound polysomal mRNA) to those of endogenous histone mRNA (nonmembrane-bound polysomal mRNA). We report here the detection of a cytoskeleton attachment site within the signal peptide-histone fusion mRNP/mRNA nucleotide sequence that is not present in wild-type histone mRNA or in HLA-B7 and chorionic gonadotropin-alpha membrane-bound polysomal mRNAs. These results support the possibility that there are multiple mechanisms for the attachment of specific classes of mRNAs to the cytoskeleton.

Cytochalasin D-mediated hyperinduction of the substrate-associated 52-kilodalton protein p52 in rat kidney fibroblasts

Regulation of certain differentiated and housekeeping functions in cultured mammalian cells is significantly influenced by cell shape. The shape-modulating agent cytochalasin D (CD) was used, therefore, to elucidate potential cytoarchitectural influences affecting synthesis of a major 52 kDa secreted/substrate-associated protein (p52) of normal rat kidney (NRK) fibroblasts. Biosynthetic labeling experiments indicated that treatment of NRK cells with CD increased, by 10-18-fold, the medium content of an Mr 52,000 protein. Two-dimensional gel electrophoresis and peptide fragment mapping confirmed that the 52 kDa protein produced in abundance as a consequence of CD treatment was identical to p52 constitutively expressed by NRK cells. A lower mw protein (p50; Mr 50,000) was also resolved which, based on pl microheterogeneity, protease fragmentation profile, and sensitivity to tunicamycin, could be identified as a less-glycosylated form of p52. p50 and p52 were both detected in the matrix and medium compartments of NRK and NRK/CD cells. The matrix p52 content of CD-induced and uninduced cells, however, was significantly greater (by 200-500-fold) than the corresponding medium levels. This differential compartmentalization, the time course of p52 accumulation in the matrix of NRK/CD cells compared to its appearance in the medium, and the kinetics of p52 pulse-chase from the matrix collectively indicated that the matrix is the initial site of p52 deposition. Low levels of CD (1 microM) produced extensive disruptions of cellular microfilaments but did not result in an overall cell shape change nor a hyperinduction of p52. Morphologic rounding (seen in 10-100 microM CD) coincided with augmented p52 production. Transition from a flat to a round phenotype in NRK cells, or at least the generation of sufficient microfilament fragmentation to compromise cell-substrate adhesivity, appears to be an essential aspect of CD-mediated p52 hyperinduction.

The association of prosomes with some of the intermediate filament networks of the animal cell

The small RNP complexes of defined morphology and biochemical composition termed prosomes, first isolated from the cytoplasm associated with repressed mRNA (Martins de Sa, C., M.-F. Grossi de Sa, O. Akhayat, F. Broders, and K. Scherrer. J. Mol. Biol. 1986. 187:47-493), were found also in the nucleus (Grossi de Sa, M.-F., C. Martins de Sa, F. Harper, O. Coux, O. Akhayat, P. Gounon, J. K. Pal, Y. Florentin, and K. Scherrer. 1988. J. Cell Sci. 89:151-165). Immunofluorescence, immunoelectron microscopy, and immunochemical studies using mAbs directed against some of the prosomal proteins of duck erythroblasts indicate that in the cytoplasm of HeLa and PtK cells, prosome antigens are associated with the intermediate filament network of the cytokeratin type.

Cortical cytoskeleton of giant moth eggs

Unfertilized eggs of several species of giant moths contain a substantial cortical cytoskeleton. This structure is assembled during oogenesis, and contains actin as a major fibrillar component. The presence of actin was confirmed by gel electrophoresis and binding to phalloidin, DNase I, and a monoclonal antibody against cytoskeletal actin. Several lines of evidence suggest that the fat body is a source of the actin in the oocyte and that the transport and acquisition of actin by the ovary are similar to the mechanism of vitellogenin acquisition. A possible role for the cortical cytoskeleton in directing early embryogenesis is discussed.

Monoclonal antibody-aided characterization of cellular p220 in uninfected and poliovirus-infected HeLa cells: subcellular distribution and identification of conformers

A monoclonal antibody directed against the Mr-220,000 subunit (p220) of the mRNA cap-binding complex has been prepared and used to analyze the sucrose gradient sedimentation and subcellular location of p220 and its poliovirus-induced cleavage products. The antibody reacted with p220 on immunoblots of cell lysates from uninfected cells, but only with several smaller polypeptides, the p220 cleavage products, in cell lysates from poliovirus-infected cells. The sedimentation of p220 antigens from uninfected or infected cells was analyzed by immunoblot and by enzyme-linked immunosorbent assay (ELISA) of sucrose gradient fractions. The results indicate that antibody reactivity was partially influenced by antigen conformation. Major forms of intact p220 and cleaved p220 were identified by immunoblot, and these had similar sedimentation properties. ELISA analysis of the same gradient fractions detected only uncleaved p220; p220 cleavage products were not recognized. Furthermore, the antibody recognized two forms of native uncleaved p220, one of which appeared to bind antibody with greater affinity. This result suggested the existence of conformational variants of p220. The differential reactivity of the antibody for cleaved versus uncleaved p220 served as a useful control during indirect immunofluorescence analysis to determine the subcellular distribution of p220 antigens. The distribution of p220 in uninfected cells was mainly cytoplasmic, but some nuclear antigens were also apparent. After poliovirus infection only the nuclear pattern remained. Disappearance of the cytoplasmic pattern confirmed the inability of the antibody to react with native p220 cleavage products. The cytoplasmic pattern also disappeared after human rhinovirus 14 infection, but not after mengovirus infection, results which correlated with the ability of human rhinovirus 14 and the inability of mengovirus to induce the cleavage of p220. The results demonstrate that p220 is not likely to be associated with the cytoskeleton and hint at the possibility of a partially nuclear location.

Amino acid sequence of the mRNA cap-binding protein from human tissues

The 25-kDa mRNA cap-binding protein (CBP) involved in translation was purified by affinity chromatography from human erythrocytes and rabbit reticulocytes. The sequences of eight human and seven rabbit tryptic and V8 proteolytic peptides were determined. Based on the peptide sequence data, oligodeoxynucleotide probes were synthesized and used to screen human fibroblast and lymphocyte lambda cDNA libraries. The DNA sequence obtained from recombinant lambda phage inserts was found to code for all but one peptide. A 23-base oligonucleotide was synthesized based on the DNA sequence and used to prime synthesis of cDNA from human placental mRNA to construct a third library in lambda gt10. Screening with a 22-base oligonucleotide, whose sequence was upstream from the 23-base primer, yielded numerous recombinant phages with approximately equal to 250-base inserts. The 1900-base-pair cDNA sequence compiled from all phage inserts appeared to represent the entire primary sequence of CBP (Mr 25,117). Blot analysis of human placental and HeLa mRNA revealed multiple CBP mRNA species ranging from 1925 to 2250 bases. The amino acid sequence of CBP showed homology to the cap-binding PB2 protein of influenza virus.

Cytochalasin releases mRNA from the cytoskeletal framework and inhibits protein synthesis

Cytochalasin D was shown to be a reversible inhibitor of protein synthesis in HeLa cells. The inhibition was detectable at drug levels typically used to perturb cell structure and increased in a dose-dependent manner. The drug also released mRNA from the cytoskeletal framework in direct proportion to the inhibition of protein synthesis. The released mRNA was unaltered in its translatability as measured in vitro but was no longer translated in the cytochalasin-treated HeLa cells. The residual protein synthesis occurred on polyribosomes that were reduced in amount but displayed a normal sedimentation distribution. The results support the hypothesis that mRNA binding to the cytoskeletal framework is necessary although not sufficient for translation. Analysis of the cytoskeletal framework, which binds the polyribosomes, revealed no alterations in composition or amount of protein as a result of treatment with cytochalasin D. Electron microscopy with embedment-free sections shows the framework in great detail. The micrographs revealed the profound reorganization effected by the drug but did not indicate substantial disaggregation of the cytoskeletal elements.

Distribution of intermediate filament proteins in normal and diseased human glomeruli

The distribution of intermediate filament proteins (vimentin, desmin, and cytokeratin) was studied by means of immunofluorescence in the normal human and rat glomerulus and in pathologic human glomeruli. Antifibronectin antibodies were used as mesangial markers. In normal human glomeruli, vimentin antibodies stained endothelial cells, podocytes, and mesangial cells; desmin antibodies, surprisingly, stained podocytes. In normal rat glomeruli, the pattern of vimentin staining was the same as in humans, but desmin antibodies stained both mesangial cells and podocytes. In human and rat glomeruli cytokeratin staining was confined to segments of Bowman's capsule. In human pathologic glomeruli, vimentin and desmin antibodies stained the structures that were positive in normal glomeruli, giving a characteristic pattern for each pathologic condition examined. These results are compatible with the mesenchymal origin of podocytes and mesangial cells and suggest that both cells have smooth muscle-like phenotypic features. Mesangial cells may have slightly different differentiation paths in humans and rats, leading to a distinct expression of intermediate filament proteins.

Association of La and Ro antigens with intracellular structures in HEp-2 carcinoma cells

Monoclonal antibodies were raised against homogeneous Ro and La antigens, two proteins associated with Ro and La ribonucleoproteins (RNPs). The specificity of the monoclonal antibodies was proven by immunoblot analysis and by immunoprecipitation. The anti-Ro antibody reacted with a Mr 95,000 protein in a mouse lymphoma cell extract and with a Mr 60,000 polypeptide in extracts from human spleen. The anti-La antibody recognized a Mr 50,000 polypeptide in the mouse L5178y cell extract. The two monoclonal antibodies precipitated RNPs that contained the typical RNA species of Ro or La RNPs. The localization of Ro and La antigen was performed by direct immunofluorescence microscopy. It was found that the anti-Ro antibody reacted with a fibrous network that behaves like cytokeratin, one of the intermediate filament systems. The anti-La antibody reacted with nuclear structures that gave a speckled-type pattern.

Disorganisation of intermediate filament structure in alcoholic and other liver diseases

The distribution of Mallory body antigens JMB1 and 2 was examined in 82 human fresh diagnostic needle liver biopsies and 28 necropsies by the indirect immunoperoxidase technique using 2 monoclonal antibodies (anti-JMB1 and 2) against Mallory bodies. The JMB1 antigen was detectable in bile duct epithelium and in hepatocytes of histologically normal livers. It was also found in all Mallory bodies in various hepatic disorders. This antigen was markedly increased in the cytoplasm of all liver cells in acute alcoholic hepatitis superimposed on alcoholic cirrhosis, in most cases of acute alcoholic hepatitis, and in severe fatty infiltration of the liver with or without Mallory body formation. Mallory bodies contained this antigen but the cytoplasm of Mallory body containing cells lacked JMB1. In normal liver the JMB2 antigen was localised on the cytoplasmic intermediate filament network of hepatocytes and bile duct epithelium; and almost all Mallory bodies also contained this antigen but the adjacent cytoplasm of these cells lacked JMB2. In severe alcoholic liver disease these antigens could not be detected in large zones of hepatocytes even when these hepatocytes did not contain Mallory bodies. It is evident that there is disorganisation of intermediate filament constituents in severe alcoholic liver disease.

Cytochalasin releases mRNA from the cytoskeletal framework and inhibits protein synthesis

Cytochalasin D was shown to be a reversible inhibitor of protein synthesis in HeLa cells. The inhibition was detectable at drug levels typically used to perturb cell structure and increased in a dose-dependent manner. The drug also released mRNA from the cytoskeletal framework in direct proportion to the inhibition of protein synthesis. The released mRNA was unaltered in its translatability as measured in vitro but was no longer translated in the cytochalasin-treated HeLa cells. The residual protein synthesis occurred on polyribosomes that were reduced in amount but displayed a normal sedimentation distribution. The results support the hypothesis that mRNA binding to the cytoskeletal framework is necessary although not sufficient for translation. Analysis of the cytoskeletal framework, which binds the polyribosomes, revealed no alterations in composition or amount of protein as a result of treatment with cytochalasin D. Electron microscopy with embedment-free sections shows the framework in great detail. The micrographs revealed the profound reorganization effected by the drug but did not indicate substantial disaggregation of the cytoskeletal elements.

Disruption of the three cytoskeletal networks in mammalian cells does not affect transcription, translation, or protein translocation changes induced by heat shock

Mammalian cells show a complex series of transcriptional and translational switching events in response to heat shock treatment which ultimately lead to the production and accumulation of a small number of proteins, the so-called heat shock (or stress) proteins. We investigated the heat shock response in both qualitative and quantitative ways in cells that were pretreated with drugs that specifically disrupt one or more of the three major cytoskeletal networks. (These drugs alone, cytochalasin E and colcemid, do not result in induction of the heat shock response.) Our results indicated that disruption of the actin microfilaments, the vimentin-containing intermediate filaments, or the microtubules in living cells does not hinder the ability of the cell to undergo an apparently normal heat shock response. Even when all three networks were simultaneously disrupted (resulting in a loose, baglike appearance of the cells), the cells still underwent a complete heat shock response as assayed by the appearance of the heat shock proteins. In addition, the major induced 72-kilodalton heat shock protein was efficiently translocated from the cytoplasm into its proper location in the nucleus and nucleolus irrespective of the condition of the three cytoskeletal elements.

Disruption of the three cytoskeletal networks in mammalian cells does not affect transcription, translation, or protein translocation changes induced by heat shock

Mammalian cells show a complex series of transcriptional and translational switching events in response to heat shock treatment which ultimately lead to the production and accumulation of a small number of proteins, the so-called heat shock (or stress) proteins. We investigated the heat shock response in both qualitative and quantitative ways in cells that were pretreated with drugs that specifically disrupt one or more of the three major cytoskeletal networks. (These drugs alone, cytochalasin E and colcemid, do not result in induction of the heat shock response.) Our results indicated that disruption of the actin microfilaments, the vimentin-containing intermediate filaments, or the microtubules in living cells does not hinder the ability of the cell to undergo an apparently normal heat shock response. Even when all three networks were simultaneously disrupted (resulting in a loose, baglike appearance of the cells), the cells still underwent a complete heat shock response as assayed by the appearance of the heat shock proteins. In addition, the major induced 72-kilodalton heat shock protein was efficiently translocated from the cytoplasm into its proper location in the nucleus and nucleolus irrespective of the condition of the three cytoskeletal elements.

Stability and movement of mRNAs and their encoded proteins in Xenopus oocytes

The stability and movement of several polyadenylated (poly A+) and nonpolyadenylated (poly A-) mRNAs in Xenopus oocytes have been examined. At least 50% of the poly A+ mRNA molecules (9S rabbit globin mRNA, chicken ovalbumin, and lysozyme) were stable in oocytes over a 48-h period, irrespective of the amount injected. About 50% of injected poly A- reovirus mRNAs was degraded within the first 24 h of injection, irrespective of the amount injected, although no further degradation was observed over an additional 24 h. The movement of all poly A+ mRNAs injected at either the animal or vegetal pole of the oocyte was very slow. Little movement of RNA from the animal half to the vegetal half was observed even 48 h after injection. In contrast, similar amounts of mRNA were present in both halves 48 h after vegetal pole injection. Similar results were obtained after injection of poly A- reovirus mRNAs. The movement of the proteins encoded by the poly A+ mRNAs was studied in the 6-h period after injection when little mRNA movement had occurred. 85% of the globin synthesized accumulated in the animal half irrespective of injection site. The movement of the sequestered secretory proteins ovalbumin and lysozyme in the same oocytes as globin was much slower; very little lysozyme appeared in the half of the oocyte opposite the site of injection.

Effect of viral infection on host protein synthesis and mRNA association with the cytoplasmic cytoskeletal structure

We studied the association of several eucaryotic viral and cellular mRNAs with cytoskeletal fractions derived from normal and virus-infected cells. We found that all mRNAs appear to associate with the cytoskeletal structure during protein synthesis, irrespective of their 5' and 3' terminal structures: e.g., poliovirus that lacks a 5' cap structure or reovirus and histone mRNAs that lack a 3' poly A tail associated with the cytoskeletal framework to the same extent as capped, polyadenylated actin mRNA. Cellular (actin) and viral (vesicular stomatitis virus and reovirus) mRNAs were released from the cytoskeletal framework and their translation was inhibited when cells were infected with poliovirus. In contrast, actin mRNA was not released from the cytoskeleton during vesicular stomatitis virus infection although actin synthesis was inhibited. In addition, several other conditions under which protein synthesis is inhibited did not result in the release of mRNAs from the cytoskeletal framework. We conclude that the association of mRNA with the cytoskeletal framework is required but is not sufficient for protein synthesis in eucaryotes. Furthermore, the shut-off of host protein synthesis during poliovirus infection and not vesicular stomatitis virus infection occurs by a unique mechanism that leads to the release of host mRNAs from the cytoskeleton.

Alcohol induced liver disease

Alcohol induces a variety of changes in the liver: fatty change, hepatitis, fibrosis, and cirrhosis. The histopathological appearances of these conditions are discussed, with special attention to differential diagnosis. Many forms of alcoholic liver disease are associated with Mallory body formation and fibrosis. Mallory bodies are formed, at least in part, from intermediate filaments. Associated changes in intermediate filament organisation in alcoholic liver disease also occur. Their significance in the pathogenesis of hepatocyte death may be related to abnormalities in messenger RNA function. The mechanisms underlying hepatic fibrogenesis are also discussed.

Epinemin: a new protein associated with vimentin filaments in non-neural cells

In this report I describe a new protein, defined by a monoclonal antibody, which is associated with vimentin filaments in a variety of cultured cells and in skeletal muscle. By immunofluorescence it is absent in smooth muscle, in cells without vimentin, and in neural vimentin containing cells. This protein has a molecular weight of 44,500, a pl of 5, a two-dimensional tryptic peptide fingerprint pattern different from vimentin, is unrelated to actin by Cleveland peptide analysis and by light and electron microscopy, and is not recognized by either a polyclonal antivimentin antibody (Frank, E.D., and L. Warren, 1981, Proc. Natl. Acad. Sci. USA, 78:3020-3024) or a monoclonal antibody against all classes of intermediate filaments (Pruss, R.M., R. Mirsky, M.C. Raff, R. Thorpe, A.J. Dowding, and B.H. Anderton, 1981, Cell, 27:419-428). The protein is resistant to nonionic detergent extraction, is soluble in high salt and can thus be removed from vimentin filaments, but fragments with vimentin in either low salt or anionic detergent and collapses with vimentin in colchicine-treated cells. By light microscopy, the distribution of the protein is indistinguishable from vimentin filaments and appears uniform along them. In contrast, immunoferritin electron microscopy reveals that the molecule is distributed in an intermittent pattern on vimentin filaments. Adopting the terminology of Granger and Lazarides (1980, Cell, 30:263-275), the molecule is called epinemin, meaning "upon filaments."

An intermediate filament-associated protein, p50, recognized by monoclonal antibodies

Monoclonal antibodies (mAB) were raised to be used as probes to identify cytoplasmic components associated with intermediate filaments (IF). Four hybridomas (B27, B76, B78, and B100) secreting mAB were generated by fusing mouse myeloma cells with the spleen cells of mice immunized intraperitoneally with Triton-high salt insoluble materials from BHK-21 cells. This insoluble material consists mostly of IF, a small number of microfilaments, and some polyribosomes. Biochemical studies show that the Triton-insoluble materials contain many proteins, including vimentin (decamin) and desmin. Immunofluorescence microscopy of BHK-21 cells stained with the four mAB showed that these mAB decorate the IF in a dotted pattern. Double staining with polyclonal antibody to vimentin confirmed the reactivity of the mAB with the IF. These mAB also stained the vimentin-containing filament system in a variety of other cells including epithelial cells (PTK1 and HeLa) and cells of astroglial origin. Histological studies showed that mAB-B100 stained many types of tissue including epidermis, smooth muscle, and subdermis pericytes, but not the white matter nor the gray matter of the cerebellum and spinal cord. Immunoelectron microscopy with colloidal gold has shown that the mAB-B100 decorated the IF in clusters or aggregates around proteinaceous materials associated with the filaments. Results of immunoprecipitation indicate that mAB-B100 reacted with a protein of 50,000 daltons. These findings suggest that the mAB-B100 we have developed recognizes one of the many components of what appears to be an integrated cytoskeletal structure connected with intermediate filaments.

CAP binding proteins associated with the nucleus

Cap binding proteins of HeLa cells were identified by photo-affinity labelling using the cap analogue gamma-[32P]-[4-(benzoyl-phenyl)methylamido]-7-methylguanosine-5'- triphosphate. Photoreaction with whole cell homogenates resulted in specific labelling of five major polypeptides. The small molecular weight polypeptide appeared to be identical to the 24 000 to 26 000 dalton cap binding protein previously identified in initiation factors. A cap binding protein of 37 000 dalton was found in initiation factors as well as in preparations of crude nuclei. It was released from nuclei by washing with buffer of moderate salt concentration. Three high molecular weight cap binding proteins (approximately 120 000, approximately 89 000, approximately 80 000 dalton) were found in the nuclear fraction and were only partly released upon nuclease digestion and high salt extraction.

The role of cytoskeletal and cytocontractile elements in pathologic processes

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Association of cap binding protein-related polypeptides with cytoplasmic RNP particles of chick embryonic muscle

Cap binding protein (CBP)-related polypeptides were identified in different cytoplasmic RNP particles of embryonic chick muscles using monoclonal antibody to purified CBP. A single immunoreactive peptide (Mr 78 000) was present in preparations of both free mRNP particles and a novel 10 S translation inhibitory RNP particle. In contrast, proteins isolated from these particles showed two new low-Mr immunoreactive peptides (Mr 43 000 and Mr 29 000). No CBP related protein could be detected in polysomal mRNP, although an immunoreactive Mr 43 000 CBP-related protein was present in polysomes. The relevance of the association of different CBP-related polypeptides with cytoplasmic RNP particles and polysomes are discussed.

Polyribosomes are not associated with vimentin-type intermediate filaments in Ehrlich ascites tumor cells

Polyribosomes were isolated from Mg2-stabilized, Triton X-100-resistant residual cell structures and from a postmitochondrial supernatant of hypotonically swollen and homogenized Ehrlich ascites tumor cells and analysed for their vimentin contents by polyacrylamide gradient slab gel electrophoresis. Whereas polyribosomes from detergent-resistant cell residues were accompanied by a small amount of vimentin on sucrose gradients, polyribosomes isolated from a hypotonic, postmitochondrial supernatant were free of vimentin. Since the presence of vimentin in cell residue-derived polyribosomes might be interpreted as the result of unspecific adsorption or cosedimentation of vimentin filament fragments, both due to the presence of high Mg2 concentration during cell extraction, the results suggest that there is no direct and stable interaction of polyribosomes with vimentin-type intermediate filaments in suspension-grown Ehrlich ascites tumor cells. This is in accordance with the electron microscopic observation that cytoplasmic regions rich in intermediate filaments are virtually devoid of polyribosomes. When the extraction of the same cells with Triton X-100 was carried out at low ionic strength and in the presence of small amounts of Ca2, residual cell structures were completely destabilized and clean nuclei without any attached cytoplasmic material were obtained. This is due to the degradation of the vimentin-type filaments by the Ca2-activated thiol proteinase highly specific for vimentin.