Phosphorylation of subunit proteins of intermediate filaments from chicken muscle and nonmuscle cells

Desmin and its molecular chaperone, the αB-crystallin: How post-translational modifications modulate their functions in heart and skeletal muscles?

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network through protein-protein interactions providing an effective mechanochemical integrator of morphology and function. Through a continuous and complex trans-cytoplasmic network, desmin intermediate filaments ensure this essential role in heart and in skeletal muscle. Besides their role in the maintenance of cell shape and architecture (permitting contractile activity efficiency and conferring resistance towards mechanical stress), desmin intermediate filaments are also key actors of cell and tissue homeostasis. Desmin participates to several cellular processes such as differentiation, apoptosis, intracellular signalisation, mechanotransduction, vesicle trafficking, organelle biogenesis and/or positioning, calcium homeostasis, protein homeostasis, cell adhesion, metabolism and gene expression. Desmin intermediate filaments assembly requires αB-crystallin, a small heat shock protein. Over its chaperone activity, αB-crystallin is involved in several cellular functions such as cell integrity, cytoskeleton stabilization, apoptosis, autophagy, differentiation, mitochondria function or aggresome formation. Importantly, both proteins are known to be strongly associated to the aetiology of several cardiac and skeletal muscles pathologies related to desmin filaments disorganization and a strong disturbance of desmin interactome. Note that these key proteins of cytoskeleton architecture are extensively modified by post-translational modifications that could affect their functional properties. Therefore, we reviewed in the herein paper the impact of post-translational modifications on the modulation of cellular functions of desmin and its molecular chaperone, the αB-crystallin.

Tropomyosin isoforms and reagents

Tropomyosins are rod-like dimers which form head-to-tail polymers along the length of actin filaments and regulate the access of actin binding proteins to the filaments.1 The diversity of tropomyosin isoforms, over 40 in mammals, and their role in an increasing number of biological processes presents a challenge both to experienced researchers and those new to this field. The increased appreciation that the role of these isoforms expands beyond that of simply stabilizing actin filaments has lead to a surge of reagents and techniques to study their function and mechanisms of action. This report is designed to provide a basic guide to the genes and proteins and the availability of reagents which allow effective study of this family of proteins. We highlight the value of combining multiple techniques to better evaluate the function of different tm isoforms and discuss the limitations of selected reagents. Brief background material is included to demystify some of the unfortunate complexity regarding this multi-gene family of proteins including the unconventional nomenclature of the isoforms and the evolutionary relationships of isoforms between species. Additionally, we present step-by-step detailed experimental protocols used in our laboratory to assist new comers to the field and experts alike.

Tropomyosin variants describe distinct functional subcellular domains in differentiated vascular smooth muscle cells

Tropomyosin (Tm) is known to be an important gatekeeper of actin function. Tm isoforms are encoded by four genes, and each gene produces several variants by alternative splicing, which have been proposed to play roles in motility, proliferation, and apoptosis. Smooth muscle studies have focused on gizzard smooth muscle, where a heterodimer of Tm from the α-gene (Tmsm-α) and from the β-gene (Tmsm-β) is associated with contractile filaments. In this study we examined Tm in differentiated mammalian vascular smooth muscle (dVSM). Liquid chromatography-tandem mass spectrometry (LC MS/MS) analysis and Western blot screening with variant-specific antibodies revealed that at least five different Tm proteins are expressed in this tissue: Tm6 (Tmsm-α) and Tm2 from the α-gene, Tm1 (Tmsm-β) from the β-gene, Tm5NM1 from the γ-gene, and Tm4 from the δ-gene. Tm6 is by far most abundant in dVSM followed by Tm1, Tm2, Tm5NM1, and Tm4. Coimmunoprecipitation and coimmunofluorescence studies demonstrate that Tm1 and Tm6 coassociate with different actin isoforms and display different intracellular localizations. Using an antibody specific for cytoplasmic γ-actin, we report here the presence of a γ-actin cortical cytoskeleton in dVSM cells. Tm1 colocalizes with cortical cytoplasmic γ-actin and coprecipitates with γ-actin. Tm6, on the other hand, is located on contractile bundles. These data indicate that Tm1 and Tm6 do not form a classical heterodimer in dVSM but rather describe different functional cellular compartments.

The role of tropomyosin isoforms and phosphorylation in force generation in thin-filament reconstituted bovine cardiac muscle fibres

The thin filament extraction and reconstitution protocol was used to investigate the functional roles of tropomyosin (Tm) isoforms and phosphorylation in bovine myocardium. The thin filament was extracted by gelsolin, reconstituted with G-actin, and further reconstituted with cardiac troponin together with one of three Tm varieties: phosphorylated alphaTm (alphaTm.P), dephosphorylated alphaTm (alphaTm.deP), and dephosphorylated betaTm (betaTm.deP). The effects of Ca, phosphate, MgATP and MgADP concentrations were examined in the reconstituted fibres at pH 7.0 and 25 degrees C. Our data show that Ca(2+) sensitivity (pCa(50): half saturation point) was increased by 0.19 +/- 0.07 units when betaTm.deP was used instead of alphaTm.deP (P < 0.05), and by 0.27 +/- 0.06 units when phosphorylated alphaTm was used (P < 0.005). The cooperativity (Hill factor) decreased (but insignificantly) from 3.2 +/- 0.3 (5) to 2.8 +/- 0.2 (7) with phosphorylation. The cooperativity decreased significantly from 3.2 +/- 0.3 (5) to 2.1 +/- 0.2 (9) with isoform change from alphaTm.deP to betaTm.deP. There was no significant difference in isometric tension or stiffness between alphaTm.P, alphaTm.deP, and betaTm.deP muscle fibres at saturating [Ca(2+)] or after rigor induction. Based on the six-state cross-bridge model, sinusoidal analysis indicated that the equilibrium constants of elementary steps differed up to 1.7x between alphaTm.deP and betaTm.deP, and up to 2.0x between alphaTm.deP and alphaTm.P. The rate constants differed up to 1.5x between alphaTm.deP and betaTm.deP, and up to 2.4x between alphaTm.deP and alphaTm.P. We conclude that tension and stiffness per cross-bridge are not significantly different among the three muscle models.

Desmin mediates TNF-alpha-induced aggregate formation and intercalated disk reorganization in heart failure

We explored the involvement of the muscle-specific intermediate filament protein desmin in the model of tumor necrosis factor alpha (TNF-alpha)-induced cardiomyopathy. We demonstrate that in mice overexpressing TNF-alpha in the heart (alpha-myosin heavy chain promoter-driven secretable TNF-alpha [MHCsTNF]), desmin is modified, loses its intercalated disk (ID) localization, and forms aggregates that colocalize with heat shock protein 25 and ubiquitin. Additionally, other ID proteins such as desmoplakin and beta-catenin show similar localization changes in a desmin-dependent fashion. To address underlying mechanisms, we examined whether desmin is a substrate for caspase-6 in vivo as well as the implications of desmin cleavage in MHCsTNF mice. We generated transgenic mice with cardiac-restricted expression of a desmin mutant (D263E) and proved that it is resistant to caspase cleavage in the MHCsTNF myocardium. The aggregates are diminished in these mice, and D263E desmin, desmoplakin, and beta-catenin largely retain their proper ID localization. Importantly, D263E desmin expression attenuated cardiomyocyte apoptosis, prevented left ventricular wall thinning, and improved the function of MHCsTNF hearts.

Desmin filaments studied by quasi-elastic light scattering

We studied polymers of desmin, a muscle-specific type III intermediate filament protein, using quasi-elastic light scattering. Desmin was purified from chicken gizzard. Polymerization was induced either by 2 mM MgCl(2) or 150 mM NaCl. The polymer solutions were in the semidilute regime. We concluded that the persistence length of the filaments is between 0.1 and 1 microm. In all cases, we found a hydrodynamic diameter of desmin filaments of 16-18 nm. The filament dynamics exhibits a characteristic frequency in the sense that correlation functions measured on one sample but at different scattering vectors collapse onto a single master curve when time is normalized by the experimentally determined initial decay rate.

Implications of intermediate filament protein phosphorylation

Intermediate filament (IF) proteins, a large family of tissue specific proteins, undergo several posttranslational modifications, with phosphorylation being the most studied modification. IF protein phosphorylation is highly dynamic and involves the head and/or tail domains of these proteins, which are the domains that impart most of the structural heterogeneity and hence presumed tissue specific functions. Although the function of IF proteins remains poorly understood, several regulatory roles for IF protein phosphorylation have been identified or are emerging. Those roles include filament disassembly and reorganization, solubility, localization within specific cellular domains, association with other cytoplasmic or membrane associated proteins, protection against physiologic stress and mediation of tissue-specific functions. Understanding the mechanistic and functional aspects of IF protein phosphorylation is providing insights not only regarding the function of this modification, but also regarding the function of IF proteins.

Remodeling of the cytoskeletal lattice in denervated skeletal muscle

The effect of denervation-induced atrophy on the cytoskeletal lattice in rat fast- and slow-twitch skeletal muscle has been investigated. Immunochemical analyses and immunofluorescence microscopy experiments employing monospecific antibodies to dystrophin, desmin, and alpha-tubulin were carried out on intact and denervated muscles. The relative cellular content of dystrophin and desmin were reduced in the soleus muscle (slow-twitch), while significant increases were shown in the gastrocnemius muscle (fast-twitch). In both muscles, alpha-tubulin levels increased up to 12-fold as a function of time compared to control values. Immunofluorescence microscopy revealed a distinct rearrangement of the microtubule network toward a predominantly longitudinal alignment, which was accompanied by an increase in the density of the fluorescence. It is concluded that the relative increase of the three structural proteins in the fast-twitch gastrocnemius muscle may be related to the apparent resistance of this muscle type to denervation-induced atrophy. The increased alpha-tubulin content in denervated slow- and fast-twitch muscles could be indicative of an adaptive mechanism designed to maintain the integrity of the muscle fiber in view of eventual regenerative activities.

Desmin degradation and Ca(2+)-dependent proteolysis during myoblast fusion

It has already been reported that, in vitro, intermediate filaments such as desmin and vimentin are very susceptible to proteolysis by calpains (Ca(2+)-activated cysteine proteinases). On the other hand, desmin and m-calpain are both present at the onset of myoblast fusion and throughout this phenomenon. Based on these observations, the aim of this study was to demonstrate, with cultured rat myoblasts, that the amount of desmin decreased significantly as multinucleated myotubes were formed. Using immunoblot analysis, it has been shown that the desmin concentration decreased 41% as myoblasts fuse. Moreover, under conditions which stimulate myoblast fusion, desmin concentration was reduced by 21% compared to the control culture. Under our experimental conditions, which lead to a reduced desmin level, the amount of m-calpain was increased about three-fold. These results suggested that m-calpain could be involved in myoblast fusion via desmin cleavage. This hypothesis was confirmed by the results obtained after calpeptin treatment. In the presence of this cell-penetrating inhibitor of calpains, desmin seems not to be degraded. Taking into account the observations obtained after different hydrolysis assays and as compared to those observed on cultured cells, it seems conceivable that m-calpain would be able to initiate desmin cleavage leading to the formation of proteolytic fragments which should be immediately degraded.

Effect of the substrate on neurofilament phosphorylation in mixed cultures of rat embryo spinal cord and dorsal root ganglia

The effect of the substrate on neurofilament phosphorylation was studied in primary cultures of spinal cord and dorsal root ganglia dissociated from 15-day-old rat embryos. On polylysine and Primaria substrates, spinal cord neurons formed aggregates connected by bundles of neurites. (Primaria dishes have a modified plastic surface with a net positive charge). On both polylysine and Primaria substrates, spinal cord neurons were stained with neurofilament monoclonal antibodies reacting with phosphorylated epitopes appearing early in rat embryo development, i.e. soon after neurofilament expression. Conversely, immunoreactivity with antibodies recognizing late phosphorylation events was only observed on Primaria substrates. As reported by many investigators, fibronectin and laminin were excellent substrates for dorsal root ganglia neurons in culture. However, on both laminin and fibronectin substrates immunoreactivity with antibodies recognizing late phosphorylation events, was only observed on Primaria substrates. As reported by many investigators, fibronectin and laminin were excellent substrates for dorsal root ganglia neurons in culture. However, on both laminin and fibronectin substrates immunoreactivity with antibodies recognizing late phosphorylation events, only occurred after several days in culture, at a time when non-neuronal cells (mainly astrocytes) had formed a confluent monolayer.

Biochemical characterization of contractile proteins of rat cultured mesangial cells

We examined characteristics of contractile proteins of rat cultured mesangial cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that actomyosin was a major protein component in the extract of mesangial cells. By Western blot, tropomyosin, caldesmon, alpha-actinin, and vinculin were recognized in mesangial cells. The molecular masses of alpha-actinin and vinculin were the same as aortic smooth muscle. Mesangial cells contained five tropomyosin isoforms: TM-1, -2, -3, -4, and -5. TM-1 was present as a major isoform in mesangial cells, and it had an immunological cross reactivity with beta-tropomyosin of aortic smooth muscle. On the other hand, the molecular mass of caldesmon was similar to dermal fibroblasts, and differed from aortic smooth muscle. Immunofluorescent studies showed that the staining patterns of tropomyosin and caldesmon between cultured mesangial cells and cultured vascular smooth muscle cells were somewhat different. From these results, we conclude that mesangial cells abound in contractile proteins, and that the compositions of these proteins are similar to those of aortic smooth muscle with minor differences. Thus, this study appears to biochemically support a hypothesis that mesangial cells are derived from vascular smooth muscle cells, but with a minor modification in their evolution.

Absence of calponin phosphorylation in contracting or resting arterial smooth muscle

We have tested the hypothesis of Winder and Walsh [(1990) J. Biol. Chem. 265, 10148] that the contractile state of smooth muscle is regulated by calponin phosphorylation. Porcine carotid arterial muscles were highly labeled with 32P, then contracted with four different agents for various times. No radioactivity was detected in calponin isolated by 2D or 1D gel electrophoresis from the muscles. Similarly, resting muscles showed no [32P]phosphate in calponin. Apparently the sites of calponin available for phosphorylation in vitro are rendered unavailable in the intact muscle.

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.

Phosphorylation modulates keratin structure

Bovine hoof keratin was shown to be a substrate for cAMP-dependent protein kinase using [gamma-32P]ATP. Natural-abundance cross-polarization (CP) MAS 13C NMR was used to examine the effect of phosphorylation on keratin structure. When short contact times were used, phosphorylation was shown to increase the number of residues in the motionally restricted portions of the protein; i.e., a portion(s) of the protein became more rigid upon phosphorylation. Circular dichroism (CD) spectra showed a spectral shape characteristic of alpha helix for this keratin. Phosphorylation of the keratin by cAMP-dependent protein kinase resulted in a CD spectrum with the same shape but of greater apparent intensity. This may have been the result of an increase in the alpha-helical content of the protein. These data showed that the structure of keratin changed significantly upon phosphorylation by cAMP-dependent protein kinase. The region of the keratin molecule most likely to be altering its structure was the end of the molecule, which was involved in the formation of, and intracellular attachment of, intermediate filaments. Therefore, these data suggested that cAMP-dependent phosphorylation may produce significant changes in the intracellular organization of intermediate filaments. When the keratin was phosphorylated using cold ATP, magic-angle spinning (MAS) 31P nuclear magnetic resonance (NMR) revealed two resonances arising from the phosphorylation sites on the keratin. The more shielded resonance was shown to arise from cAMP-dependent protein kinase phosphorylation. Static 31P NMR measurements suggested that at least two classes of cAMP-dependent sites existed with the same isotropic 31P chemical shift; one was considerably motionally restricted with respect to the other.

Modulation of keratin intermediate filament distribution in vivo by induced changes in cyclic AMP-dependent phosphorylation

Treatment of PtK1 cells with 5 mM acrylamide for 4 hr induces reversible dephosphorylation of keratin in concert with reversible aggregation of intermediate filaments (Eckert and Yeagle, Cell Motil. Cytoskeleton 11:24-30, 1988). We have examined this phenomenon by 1) in vitro phosphorylation of isolated PtK1 keratin filaments and 2) combined treatments of PtK1 cells with both acrylamide and agents which elevate intracellular cAMP levels. PtK1 keratins were incubated in gamma-32P-ATP in the presence or absence of cAMP-dependent kinase (A-kinase) and cAMP. Levels of phosphorylation were analyzed by electrophoresis and autoradiography. Phosphorylation of keratin polypeptides (56 kD, 53 kD, 45 kD, 40 kD) occurred without added kinase, suggesting the presence of an endogenous kinase which remains with intermediate filaments in residues of Triton X-100 extracted cells. Phosphorylation levels were increased by A-kinase but not by cAMP alone, indicating the presence of cAMP-dependent phosphorylation sites in addition to sites phosphorylated by the endogenous kinase. To study the possible role of cAMP-dependent phosphorylation in acrylamide-induced aggregation of keratin filaments, we treated cells with acrylamide in the presence of 8-bromo-cAMP (brcAMP), pertussis toxin (PT), isobutylmethylxanthine (IBMX), or forskolin, which increase intracellular cAMP levels. The distribution and phosphorylation levels of keratin filaments, as well as intracellular cAMP levels, were determined for each of these treatments. In addition to aggregation and dephosphorylation of keratin filaments reported previously, treatment of cells with acrylamide alone also results in reduced levels of intracellular cAMP. 8-bromo-cAMP, IBMX, and forskolin prevent acrylamide-induced aggregation of keratin filaments and result in both normal levels of keratin phosphorylation and normal intracellular cAMP levels. PT was apparently ineffective. These observations suggest that 1) PtK1 keratins are phosphorylated by cAMP-dependent kinase and an endogenous, cAMP-independent kinase and 2) alteration of levels of cAMP-dependent phosphorylation may be involved in aggregation of keratin filaments in response to acrylamide.

Expression of the chicken vimentin gene in transgenic mice: efficient assembly of the avian protein into the cytoskeleton

To study expression and function of the vimentin gene, transgenic mice were generated by microinjecting the entire chicken gene plus 2.4 kilobases of 5' and 2.6 kilobases of 3' flanking sequences. All the transgenic mice obtained had incorporated multiple copies of the gene. RNA analyses demonstrated that the chicken vimentin gene was efficiently expressed in an appropriate tissue-specific pattern and that the transcripts were properly processed, as in chicken, giving rise to two RNAs. The vimentin transgene was predominantly expressed in lens at levels of up to 10-fold the endogenous level in every transgenic line studied. The chicken vimentin transcripts were efficiently translated into polypeptides that were modified posttranslationally and could assemble into the mouse cytoskeleton. Overexpression of the chicken vimentin gene did not obviously affect the expression of the endogenous gene at the RNA or the protein level. Immunofluorescence microscopy further demonstrated that the chicken protein was properly expressed spatially in lens. However, the levels were much higher in the transgenic animals.

Phosphorylation of vimentin in mitotically selected cells. In vitro cyclic AMP-independent kinase and calcium-stimulated phosphatase activities

The phosphorylation of the intermediate filament protein vimentin was examined under in vitro conditions. Cell cytosol and Triton-insoluble cytoskeleton preparations from nonmitotic and mitotically selected mouse L-929 cells exhibited vimentin kinase activity that is apparently cAMP and Ca2+ independent. The level of vimentin kinase activity was greater in preparations from mitotically selected cells than nonmitotic cells. Addition of Ca2+ to mitotic cytosol decreased net vimentin phosphorylation. Dephosphorylation experiments indicated that there is phosphatase activity in these preparations which is stimulated by addition of Ca2+. Fractionation of cytosol from nonmitotic cells on DEAE-Sephacel and phosphocellulose revealed a single major vimentin kinase activity (peak I). Fractionation of cytosol from mitotically selected cells yielded a similar activity (peak I) and an additional vimentin kinase activity (peak II) that was not found in nonmitotic preparations. Based on substrate specificity and lack of inhibition to characteristic inhibitors, the semipurified peak I and II vimentin kinase activities appear to be cAMP-independent enzymes that are distinct from casein kinases I and II. Phosphopeptide mapping studies indicated that both peak I and peak II vimentin kinases phosphorylate tryptic peptides in the NH2-terminal region of vimentin that are phosphorylated in intact cells. Electron microscopic examination of reconstituted vimentin filaments phosphorylated with both semipurified kinases indicated that phosphorylation induced filament disassembly. These experiments indicate that the increased phosphorylation of vimentin during mitosis may be catalyzed by a discrete cAMP-independent protein kinase. In addition, preparations from mitotic cells exhibited a Ca2+-stimulated phosphatase activity, suggesting that Ca2+ may play a regulatory role in vimentin dephosphorylation during mitosis.

Cyclic AMP-dependent protein kinase-induced vimentin filament disassembly involves modification of the N-terminal domain of intermediate filament subunits

The intermediate filament protein vimentin was phosphorylated with cAMP-dependent protein kinase under conditions that induce filament disassembly. Digestion of phosphorylated vimentin with lysine-specific endoprotease and subsequent tryptic peptide mapping indicated that a 12 kDa N-terminal fragment contained all the phosphorylation sites found in the intact molecule. Analysis of cyanogen bromide digests indicated that two phosphorylated peptides were produced, with the major 32P-labeled species representing amino acid position 14-72, and a minor 32P-labeled peptide representing amino acid positions 1-13. These results demonstrate that phosphorylation of sites within the N-terminal head domain of vimentin are associated with phosphorylation induced filament disassembly.

In vitro translation of the intermediate filament proteins desmin and vimentin

Polyadenylated ribonucleic acid (RNA) was isolated from chicken skeletal and smooth muscle and translated in a cell-free rabbit reticulocyte system. Both types of muscle tissue contain messenger RNAs that code for the intermediate filament proteins desmin and vimentin, and the relative concentrations of the two translation products reflect the prevalence of the two proteins in vivo. Desmin synthesis represents a greater proportion of the total protein synthesis from smooth muscle RNA than from skeletal muscle RNA, whereas the converse is true of vimentin synthesis. Fractionation of the RNA on formamide-containing sucrose gradients before translation indicates that the desmin messenger RNA is larger than the vimentin messenger RNA and contains an extensive noncoding segment. The desmin and vimentin messages code predominantly for the non-phosphorylated forms of desmin and vimentin. However, the ratio of phosphorylated to unphosphorylated forms of the proteins could be increased by adding cyclic adenosine monophosphate-dependent kinase activity to the translation mixtures. These results suggest that desmin and vimentin are each synthesized from a single messenger RNA species and that posttranslational phosphorylation generates the additional isoelectric variants of each which are observed in vivo.

Acrylamide treatment of PtK1 cells causes dephosphorylation of keratin polypeptides

Treatment of PtKl cells with 5 mM acrylamide for 4 hr results in alterations in the distribution of keratin filaments within the cells. This effect is reversible within 18 hr. Labeling of PtKl cells with 32P demonstrates that there are four phosphorylated keratins, having Mr of 56 k, 53 k, 45 k, and 40 k. Phosphate associated with these polypeptides appears to turn over with a t1/2 of 12 hr. Incubation of labeled cells in 5 mM acrylamide results in approximately 50% dephosphorylation of keratins within 2 hr, which is 3 times faster than normal turnover. Recovery of cells from acrylamide is accompanied by rephosphorylation of keratins within 18 hr. Analysis by 31P NMR spectroscopy shows that acrylamide treatments are accompanied by a transient decrease in soluble inorganic phosphate. This is followed by a rapid increase in Pi which gradually returns to normal levels. These studies show a strong correlation between phosphorylation of PtKl cell keratins and morphological response of keratin filaments to acrylamide. These observations suggest that normal distribution of keratin filaments may be, in part, mediated by protein phosphorylation.

Isolation and characterization of tropomyosin kinase from chicken embryo

Tropomyosin kinase is partially purified from 14-day-old chicken embryos using DEAE-cellulose, cellulose phosphate and gel filtration chromatography. The purest enzyme preparation consists of two major bands of Mr = 76,000 and 43,000 on SDS-polyacrylamide gel electrophoresis. The molecular weight of the enzyme is 250,000 determined by gel filtration chromatography. It phosphorylates casein and skeletal tropomyosin equally well but histone and phosvitin at a much slower rate. Smooth muscle myosin light chain, tropomyosin from platelet, erythrocyte and smooth muscle are not phosphorylated. The apparent Km for skeletal alpha-tropomyosin and ATP is 50 microM and 200 microM, respectively. Vmax varies between 100-300 nmol/min per mg depending on the purity of the preparation. Mg2+ and dithiothreitol are essential for activity but Ca+, calmodulin and cAMP are not required. The optimum temperature is 37 degrees C and optimum pH is about 7.5. Heparin, a potent inhibitor of casein kinase II, has no inhibitory effect on the enzyme. Similar tropomyosin kinase activity is not detected in skeletal muscle in adult rabbit and chicken. The tropomyosin kinase described here represents a hitherto uncharacterized kinase responsible for phosphorylation of tropomyosin in the chicken embryo.

A polymer-dependent increase in phosphorylation of beta-tubulin accompanies differentiation of a mouse neuroblastoma cell line

We have examined the phosphorylation of cellular microtubule proteins during differentiation and neurite outgrowth in N115 mouse neuroblastoma cells. N115 differentiation, induced by serum withdrawal, is accompanied by a fourfold increase in phosphorylation of a 54,000-mol-wt protein identified as a specific isoform of beta-tubulin by SDS PAGE, two-dimensional isoelectric focusing/SDS PAGE, and immunoprecipitation with a specific monoclonal antiserum. Isoelectric focusing/SDS PAGE of [35S]methionine-labeled cell extracts revealed that the phosphorylated isoform of beta-tubulin, termed beta 2, is one of three isoforms detected in differentiated N115 cells, and is diminished in amounts in the undifferentiated cells. Taxol, a drug which promotes microtubule assembly, stimulates phosphorylation of beta-tubulin in both differentiated and undifferentiated N115 cells. In contrast, treatment of differentiated cells with either colcemid or nocodazole causes a rapid decrease in beta-tubulin phosphorylation. Thus, the phosphorylation of beta-tubulin in N115 cells is coupled to the levels of cellular microtubules. The observed increase in beta-tubulin phosphorylation during differentiation then reflects developmental regulation of microtubule assembly during neurite outgrowth, rather than developmental regulation of a tubulin kinase activity.

A 300,000-mol-wt intermediate filament-associated protein in baby hamster kidney (BHK-21) cells

Native intermediate filament (IF) preparations from the baby hamster kidney fibroblastic cell line (BHK-21) contain a number of minor polypeptides in addition to the IF structural subunit proteins desmin, a 54,000-mol-wt protein, and vimentin, a 55,000-mol-wt protein. A monoclonal antibody was produced that reached exclusively with a high molecular weight (300,000) protein representative of these minor proteins. Immunological methods and comparative peptide mapping techniques demonstrated that the 300,000-mol-wt species was biochemically distinct from the 54,000- and 55,000-mol-wt proteins. Double-label immunofluorescence observations on spread BHK cells using this monoclonal antibody and a rabbit polyclonal antibody directed against the 54,000- and 55,000-mol-wt proteins showed that the 300,000-mol-wt species co-distributed with IF in a fibrous pattern. In cells treated with colchicine or those in the early stages of spreading, double-labeling with these antibodies revealed the co-existence of the respective antigens in the juxtanuclear cap of IF that is characteristic of cells in these physiological states. After colchicine removal, or in the late stages of cell spreading, the 300,00-mol-wt species and the IF subunits redistributed to their normal, highly coincident cytoplasmic patterns. Ultrastructural localization by the immunogold technique using the monoclonal antibody supported the light microscopic findings in that the 300,000-mol-wt species was associated with IF in the several physiological and morphological cell states investigated. The gold particle pattern was less intimately associated with IF than that defined by anti-54/55 and was one of non-uniform distribution along IF, being clustered primarily at points of proximity between IF, where an amorphous, proteinaceous material was often the labeled element. Occasionally, "bridges" of label were seen extending outward from such clusters on IF. Gold particles were infrequently bound to microtubules, microfilaments, or other cellular organelles, and when so, IF were usually contiguous. During multiple cycles of in vitro disassembly/assembly of the IF from native preparations, the 300,000-mol-wt protein remained in the fraction containing the 54,000- and 55,000-mol-wt structural subunits, whether the latter were in the soluble state or pelleted as formed filaments. In keeping with the nomenclature developed for the microtubule-associated proteins (MAPs), the acronym IFAP-300K (intermediate filament associated protein) is proposed for this molecule.

Expression of the intermediate-filament-associated protein synemin in chicken lens cells

Synemin, a 230-kilodalton polypeptide component of avian muscle and erythrocyte intermediate filaments, is also found in association with the vimentin filaments of lens tissue. In chicken lens cells, synemin is bound to the core vimentin polymer with the same 180-nm periodicity that it exhibits in erythrocytes. Its solubility properties are characteristic of those of intermediate filaments in general and similar to those of synemin in muscle cells and erythrocytes. Synemin appears at an early stage of lens development and undergoes a dramatic accumulation as the epithelial cells elongate and differentiate into fiber cells. In contrast to synemin in cultured skeletal muscle, lens synemin is not confined to postmitotic, terminally differentiating cells but is present in proliferative cells as well. It is lost from the fibers near the center of the lens, as are many other cellular structures including intermediate filaments. These findings provide new information about the occurrence and expression of avian synemin and new insight regarding its presumptive role as a modulator of intermediate-filament function.

Identification of two of the major phosphorylated polypeptides of the bovine lens utilizing a lens cAMP-dependent protein kinase system

Two of the major in vitro phosphorylated polypeptides of the bovine lens have been identified. Analysis by means of two-dimensional gel electrophoresis (IEF) has demonstrated that the lens phosphorylated 57,000 and 43,000 dalton polypeptides correspond in mobility to purified phosphorylated bovine lens vimentin and chicken gizzard actin, respectively. Purified actin and vimentin were phosphorylated by a partially purified cAMP-dependent protein kinase isolated from the outer cortex water soluble fraction. All detectable bovine lens vimentin isoelectric variants were phosphorylated. In both the lens fiber cell and chicken gizzard actin preparations, the phosphorylated actin isoelectric variants did not correspond in mobility to the major actin isoelectric variant, but were more acidic. Phosphorylation in all preparations occurred at serine residues.

Expression of the intermediate-filament-associated protein synemin in chicken lens cells

Synemin, a 230-kilodalton polypeptide component of avian muscle and erythrocyte intermediate filaments, is also found in association with the vimentin filaments of lens tissue. In chicken lens cells, synemin is bound to the core vimentin polymer with the same 180-nm periodicity that it exhibits in erythrocytes. Its solubility properties are characteristic of those of intermediate filaments in general and similar to those of synemin in muscle cells and erythrocytes. Synemin appears at an early stage of lens development and undergoes a dramatic accumulation as the epithelial cells elongate and differentiate into fiber cells. In contrast to synemin in cultured skeletal muscle, lens synemin is not confined to postmitotic, terminally differentiating cells but is present in proliferative cells as well. It is lost from the fibers near the center of the lens, as are many other cellular structures including intermediate filaments. These findings provide new information about the occurrence and expression of avian synemin and new insight regarding its presumptive role as a modulator of intermediate-filament function.

Immunocytochemical analysis of intermediate filaments in embryonic heart cells with monoclonal antibodies to desmin

Monoclonal antibodies ( McAbs ) have been generated against a preparation of intermediate filament proteins (IFP) from adult chicken gizzard. Two antibodies, D3 and D76 , have been characterized in detail. They bind specifically to desmin but recognize different epitopes. In the adult chicken, both McAbs produced equivalent immunofluorescent staining patterns, reacting in frozen sections with all forms of muscle tissue, including vascular smooth muscle, but with no other tissue types. In isolated skeletal myofibrils and in longitudinal frozen sections of cardiac and skeletal muscle, desmin was detected with both McAbs at the Z-band and in longitudinally-oriented filament bundles between myofibrils. In contrast to these results in the adult, the intermediate filaments (IF) of embryonic cardiac myocytes in primary cultures were decorated only with McAb D3, whereas McAb D76 was completely unreactive with these cells. Similarly, frozen sections through the heart at early stages of embryonic chick development (Hamburger-Hamilton stages 17-18) revealed regions of myocytes, identified by double immunofluorescence with myosin-specific McAbs , that were unstained with McAb D76 even though similar regions were stained by McAb D3. That McAb D76 reacted with desmin in all adult cardiac myocytes but not with all embryonic heart cells indicates that embryonic and adult cardiac IF are immunologically distinct and implies a conversion in IF immunoreactivity during cardiac development.

Glial fibrillary acidic protein: norepinephrine stimulated phosphorylation in intact C-6 glioma cells

Coelectrophoresis in two-dimensional gels of rat glial fibrillary acidic protein (GFA) and 32P-labeled whole cell extracts of rat C-6 glioma cells showed that the GFA migrated in close proximity to a previously noted phosphoprotein, 50K-6.1, of these cells. GFA electrophoresed as a 50K polypeptide with at least four charge variants, the most acidic of which coelectrophoresed with 50K-6.1. Exposure of the C-6 cultures to dibutyryl cyclic AMP (dbcAMP) for 48 h increased the relative abundance of the endogenous polypeptide associated with 50K-6.1 by threefold, consistent with the hypothesis that 50K-6.1 was GFA. Norepinephrine stimulated 50K-6.1 phosphorylation 3.2-fold in dbcAMP-induced cultures. Peptide mapping with V8 protease and subtilisin was used to test the hypothesis that GFA and 50K-6.1 were identical polypeptides. With V8 protease, the peptides generated from the [35S]methionine labeled putative GFA spot of the C-6 cells were indistinguishable from the stained bands derived from authentic GFA in mixed samples of the two proteins. Likewise, the 35S-labeled acidic satellite to the putative GFA spot also yielded a peptide map that matched that of the authentic GFA. 32P-labeled peptides derived from the 50K-6.1 protein were a subset of those from authentic GFA. With three subtilisin concentrations, 32P-labeled 50K-6.1 was degraded to peptides which were again a subset of the stained GFA peptides. A cytoskeletal fraction from 32P-labeled C-6 cells contained a 50K phosphoprotein.(ABSTRACT TRUNCATED AT 250 WORDS)

Vimentin gene expression during myogenesis: two functional transcripts from a single copy gene

We have previously reported that a single vimentin gene is present in the chicken genome (22). In vivo transcription of this gene yields two distinct classes of mRNA's (approximatley 2200 and-2500 nts) generated through the possible differential utilization of polyadenylylation sites. In this report we demonstrate that 1) both transcripts are functional in vitro and direct the cell-free synthesis of the vimentin polypeptide, as judged by two dimensional gel analysis; 2) three of the four possible adenylylation signals indicated in the sequence of the gene are utilized in vivo as determined by S1 analysis; 3) furthermore, these adenylylation sites are utilized identically in all tissues of the chicken examined; 4) the adenylylation site closest to the body of the message is apparently not used as judged by the S1 conditions employed in the assay. Levels of vimentin mRNA decrease in different tissues of the chicken during the embryonic to adult transition. There is no evidence for a tissue specific or developmentally regulated pattern of expression for either of the two vimentin transcripts. The same pattern of vimentin mRNA expression is seen in all tissues examined, only the level of expression is altered.

Proteolysis of vimentin and desmin by the Ca2+-activated proteinase specific for these intermediate filament proteins

The degradation of vimentin and desmin by the Ca2+-activated proteinase specific for these intermediate filament proteins proceeds in two stages in the form of a limited proteolysis. At first, the reaction is very rapid, with the stepwise and complete removal of a peptide (ca. 9,000 daltons) from the N-terminal of vimentin and desmin. This results in the production of a characteristic "staircase" of degradation products, as seen in two-dimensional polyacrylamide gel electrophoresis. The second stage of proteolysis is characterized by the accumulation of peptides which are resistant to further proteolysis; this is due not to product inhibition but to the fact that these peptides are not substrates for the proteinase and therefore do not protect the latter from inactivation (autodigestion). In vitro phosphorylation of the substrates does not affect proteinase activity, probably because the phosphorylation site is located towards the C-terminal of the molecules. The specific and limited proteolysis of vimentin and desmin results in the deletion of the nucleic acid binding and filament assembly site of these proteins, indicating that the Ca2+-activated proteinase plays a role in regulating the function(s) of these intermediate filament proteins, rather than their simple turnover during the cell cycle.

Cell configuration-related control of vimentin biosynthesis and phosphorylation in cultured mammalian cells

The cell configuration-related control of a cytoskeletal protein (vimentin) expression was examined by varying cell shape between flat and spherical. Cultivation of cells in monolayer or in a spherical configuration on poly-2-hydroxyethylmethacrylate-coated plates revealed a preferential down regulation of vimentin synthesis during suspension culture. The mechanism(s) regulating the decrease in the expression of vimentin in spherical cells appears to be at the level of translation, because mRNAs extracted from monolayer and suspension-cultured cells were equally active in directing vimentin synthesis in the rabbit reticulocyte cell-free system. When after prolonged suspension culture, the cells were allowed to reattach and spread, vimentin synthesis recovered rapidly to the control monolayer rate. The phosphorylation of vimentin was also reduced dramatically during suspension culture. However, unlike the rapid recovery of vimentin biosynthesis upon reattachment (less than 6 h), the recovery in the rate of vimentin phosphorylation was much slower (greater than 20 h) and paralleled the recovery to the monolayer growth rate. Although the control of vimentin biosynthesis in suspension culture is a cell configuration-related process, the decrease in the rate of vimentin phosphorylation in suspension culture appears to be the result of the slower growth rate and may reflect the reported correlation between the rate of vimentin phosphorylation and the accumulation of cells in mitosis.

Phosphorylation of the myofibrillar proteins and the regulation of contractile activity in muscle

Evidence now exists for the phosphorylation of all the major proteins of the myofibril with the exception of troponin C. Although uncertainty exists in most cases about the role of phosphorylation of the myofibrillar proteins, there is substantial evidence that phosphorylation of serine 20 of rabbit cardiac troponin I leads to a lowering of the sensitivity of the actomyosin ATPase to Ca2+. This process is of special importance in the physiological response of the heart to adrenalin. A well defined enzymic system involving a specific kinase and a phosphatase is present in most muscles for the phosphorylation and dephosphorylation of the P light chain (regulatory, L2 or DTNB light chain) of myosin. Myosin light-chain kinase is very active in fast skeletal muscles, and although it is unlikely that phosphorylation followed by dephosphorylation of the P light chain occurs fast enough to be synchronous with the contractile cycle, phosphorylation may have a modulatory role in this tissue. Both post-tetanic potentiation and the reduced actomyosin ATPase turnover rate observed in fast-twitch muscle as a consequence of sustained forceful contraction have been suggested by different investigators to be consequences of P light chain phosphorylation. Nevertheless, unequivocal evidence associating either of these effects with phosphorylation is not yet available. Kinase activity is also high in vertebrate smooth muscle and it has been suggested that phosphorylation of the P light chain is the process that activates the actomyosin ATPase in this tissue. Evidence from a number of studies indicates, however, that regulation of smooth muscle actomyosin ATPase may not be a simple phosphorylation-dephosphorylation process.

Effects of Zn2+ ions on protein phosphorylation in epithelial cell membranes

The regulation of protein phosphorylation by Zn2+ ions and by other divalent cations was studied in membrane vesicles from a normal mouse epithelial cell line, MMC-E (Mus musculus castaneous). Four major phosphoacceptor polypeptides were found in these membranes. Micromolar concentrations of Zn2+ ions inhibited the phosphorylation of the epidermal growth factor (EGF) receptor and of threonine residues in a 47,000-dalton polypeptide. In contrast, two polypeptides with molecular weights of 54,000 and 57,000 showed increased phosphorylation, mainly of serine residues, in the p.esence of Zn2+ ions. These results were not obtained using similar concentrations of other divalent cations and were apparently not due to an effect of Zn2+ ions on phosphoprotein phosphatases. Thus, the effects of Zn2+ ions on protein phosphorylation in membrane vesicles are complex and are not restricted to an inhibition of a single protein phosphatase or kinase.

Proteolysis of vimentin and desmin by the Ca2+-activated proteinase specific for these intermediate filament proteins

The degradation of vimentin and desmin by the Ca2+-activated proteinase specific for these intermediate filament proteins proceeds in two stages in the form of a limited proteolysis. At first, the reaction is very rapid, with the stepwise and complete removal of a peptide (ca. 9,000 daltons) from the N-terminal of vimentin and desmin. This results in the production of a characteristic "staircase" of degradation products, as seen in two-dimensional polyacrylamide gel electrophoresis. The second stage of proteolysis is characterized by the accumulation of peptides which are resistant to further proteolysis; this is due not to product inhibition but to the fact that these peptides are not substrates for the proteinase and therefore do not protect the latter from inactivation (autodigestion). In vitro phosphorylation of the substrates does not affect proteinase activity, probably because the phosphorylation site is located towards the C-terminal of the molecules. The specific and limited proteolysis of vimentin and desmin results in the deletion of the nucleic acid binding and filament assembly site of these proteins, indicating that the Ca2+-activated proteinase plays a role in regulating the function(s) of these intermediate filament proteins, rather than their simple turnover during the cell cycle.

Vimentin filaments are assembled from a soluble precursor in avian erythroid cells

The synthesis and assembly of vimentin was studied in erythroid cells from 10-d-old chicken embryos. After various periods of [35S]methionine incorporation, cells were lysed in a Triton X-100-containing buffer and separated into a soluble and an insoluble (cytoskeletal) fraction. Analysis of these two fractions by two-dimensional gel electrophoresis shows that vimentin is almost exclusively present in the cytoskeletal fraction and that newly synthesized vimentin is rapidly incorporated into this fraction. However, after a short pulse-labeling period, a prominent labeled protein at the position of vimentin is present in the soluble fraction. By immunoautoradiography and immunoprecipitations with vimentin antibodies, this protein was identified as vimentin. The vimentin in the soluble fraction is not sedimented by high speed centrifugation, suggesting that it does not consist of short filaments. After different pulse-labeling periods, assembly of newly synthesized vimentin in the cytoskeletal fraction increases linearly, while the radioactivity in the soluble vimentin remains constant. During a 2-h pulse-chase period, the vimentin in the soluble fraction is chased into the cytoskeletal fraction, with a half-life of 7 min. These results suggest that in chicken embryo erythroid cells newly synthesized vimentin is rapidly assembled into filaments from a soluble precursor.

The turnover of vimentin in Ehrlich ascites tumour cells

The turnover of vimentin and vimentin-derived peptides has been examined in logarithmically growing Ehrlich ascites tumour cells. Cells were pulse-labelled with [35S]methionine for 30 min and then chased for up to 60 h. It was found that the specific radioactivity of the main isoelectric variant of vimentin decreased to half the original value in 15.3 h which was close to the division time of the cells (16 h). The protein moiety of the phosphorylated variant of vimentin also turned over very slowly, in contrast to the turnover rate of the phosphate group itself which has a half-life of 1.4 h. The role of the intermediate filament-specific, Ca2+-activated proteinase has been considered in relationship to the slow turnover of vimentin.

Cyclic AMP-modulated phosphorylation of intermediate filament proteins in cultured avian myogenic cells

The intermediate filament proteins desmin and vimentin and the muscle tropomyosins were the major protein phosphate acceptors in 8-day-old myotubes incubated for 4 h in medium containing radiolabeled phosphate. The addition of isoproterenol or 8-bromo-cyclic AMP (BrcAMP) resulted in a two- to threefold increase in incorporation of 32PO4 into both desmin and vimentin, whereas no changes in the incorporation of 32PO4 into tropomyosin or other cellular proteins were observed. The BrcAMP- or hormonally induced increase in 32PO4 incorporation into desmin and vimentin was independent of protein synthesis and was not caused by stimulation of protein phosphate turnover. In addition, BrcAMP did not induce significant changes in the specific activity of the cellular ATP pool. These data suggest that the observed increase in 32PO4 incorporation represented an actual increase in phosphorylation of the intermediate filament proteins desmin and vimentin. Two-dimensional tryptic analysis of desmin from 8-day-old myotubes revealed five phosphopeptides of which two showed a 7- to 10-fold increase in 32PO4 incorporation in BrcAMP-treated myotubes. Four of the phosphopeptides identified in desmin labeled in vivo were also observed in desmin phosphorylated in vitro by bovine heart cAMP-dependent protein kinase. Although phosphorylation of desmin and vimentin was apparent in myogenic cells at all stages of differentiation, BrcAMP- and isoproterenol-induced increases in phosphorylation of these proteins were restricted to mature myotubes. These data strongly suggest that in vivo phosphorylation of the intermediate filament proteins desmin and vimentin is catalyzed by the cAMP-dependent protein kinases and that such phosphorylation may be regulated during muscle differentiation.

Follicle-stimulating hormone-dependent phosphorylation of vimentin in cultures of rat Sertoli cells

Endogenous protein phosphorylation was investigated in cultured rat Sertoli cells after treatment with follicle-stimulating hormone (FSH) and pharmacological agents that activate cAMP-dependent protein kinases. In intact Sertoli cells, both phosphorylation and dephosphorylation of proteins occurred in response to treatment with these agents. Studies using cell-free preparations suggest that four phosphoproteins phosphorylated by cAMP or the catalytic subunit of cAMP-dependent protein kinase were also phosphorylated in a FSH-dependent manner in intact cells. These data suggest that FSH-dependent phosphorylation in Sertoli cells occurs through activation of a cAMP-dependent protein kinase. A FSH-dependent phosphoprotein with a molecular weight of 58,000 was identified as the intermediate filament protein vimentin, based on its migration in two-dimensional gels and its peptide map. The cellular distribution of vimentin was monitored by immunofluorescence in Sertoli cells after treatment with FSH. Results of this study support a role for intermediate filaments in FSH-dependent events in Sertoli cells.

Calcium/calmodulin-dependent phosphorylation of vimentin in rat sertoli cells

Ca2+-dependent protein phosphorylation and the role of calmodulin in this process was investigated in subcellular fractions of primary cultures of rat Sertoli cells. Significant Ca2+/calmodulin-dependent protein phosphorylation in Sertoli cells was restricted to the cytosol fraction. The calmodulin dependence of these effects was confirmed by using the calmodulin inhibitor trifluoperazine. One of the Ca2+/calmodulin-dependent phosphoproteins was identified as the intermediate filament protein vimentin, based on the following criteria: (i) migration pattern in two-dimensional polyacrylamide gels, (ii) Ca2+/calmodulin-dependent phosphorylation of a 58-kilodalton protein present in detergent-insoluble intermediate filament protein extract of Sertoli cells, and (iii) peptide mapping of the phosphoprotein. These data support a role for Ca2+/calmodulin-dependent protein phosphorylation in the modulation of Sertoli cell cytoskeletal components.