Tropomodulin: a cytoskeletal protein that binds to the end of erythrocyte tropomyosin and inhibits tropomyosin binding to actin

Proteome analysis of vinca alkaloid response and resistance in acute lymphoblastic leukemia reveals novel cytoskeletal alterations

Vinca alkaloids are used widely in the treatment of both childhood and adult cancers. Their cellular target is the beta-tubulin subunit of alpha/beta-tubulin heterodimers, and they act to inhibit cell division by disrupting microtubule dynamics. Despite the effectiveness of these agents, drug resistance is a major clinical problem. To identify the underlying mechanisms behind vinca alkaloid resistance, we have performed high resolution differential proteome analysis. Treatment of drug-sensitive human leukemia cells (CCRF-CEM) with vincristine identified numerous proteins involved in the cellular response to vincristine. In addition, differential protein expression was analyzed in leukemia cell lines selected for resistance to vincristine (CEM/VCR R) and vinblastine (CEM/VLB100). This combined proteomic approach identified 10 proteins altered in both vinca alkaloid response and resistance: beta-tubulin, alpha-tubulin, actin, heat shock protein 90beta, 14-3-3tau, 14-3-3epsilon, L-plastin, lamin B1, heterogeneous nuclear ribonuclear protein-F, and heterogeneous nuclear ribonuclear protein-K. Several of these proteins have not previously been associated with drug resistance and are thus novel targets for elucidation of resistance mechanisms. In addition, seven of these proteins are associated with the tubulin and/or actin cytoskeletons. This study provides novel insights into the interrelationship between the microtubule and microfilament systems in vinca alkaloid resistance.

Tropomodulin contains two actin filament pointed end-capping domains

Tropomodulin 1 (Tmod1) is a approximately 40-kDa tropomyosin binding and actin filament pointed end-capping protein that regulates pointed end dynamics and controls thin filament length in striated muscle. In vitro, the capping affinity of Tmod1 for tropomyosin-actin filaments (Kd approximately 50 pm) is several thousand-fold greater than for capping of pure actin filaments (Kd approximately 0.1 microM). The tropomyosin-binding region of Tmod1 has been localized to the amino-terminal portion between residues 1 and 130, but the location of the actin-capping domain is not known. We have now identified two distinct actin-capping regions on Tmod1 by testing a series of recombinant Tmod1 fragments for their ability to inhibit actin elongation from gelsolin-actin seeds using pyrene-actin polymerization assays. The carboxyl-terminal portion of Tmod1 (residues 160-359) contains the principal actin-capping activity (Kd approximately 0.4 microM), requiring residues between 323 and 359 for full activity, whereas the amino-terminal portion of Tmod1 (residues 1-130) contains a second, weaker actin-capping activity (Kd approximately 1.8 microM). Interestingly, 160-359 but not 1-130 enhances spontaneous actin nucleation, suggesting that the carboxyl-terminal domain may bind to two actin subunits across the actin helix at the pointed end, whereas the amino-terminal domain may bind to only one actin subunit. On the other hand, the actin-capping activity of the amino-terminal but not the carboxyl-terminal portion of Tmod1 is enhanced several thousand-fold in the presence of skeletal muscle tropomyosin. We conclude that the carboxyl-terminal capping domain of Tmod1 contains a TM-independent actin pointed end-capping activity, whereas the amino-terminal domain contains a TM-regulated pointed end actin-capping activity.

The interaction of tropomodulin with tropomyosin stabilizes thin filaments in cardiac myocytes

Actin (thin) filament length regulation and stability are essential for striated muscle function. To determine the role of the actin filament pointed end capping protein, tropomodulin1 (Tmod1), with tropomyosin, we generated monoclonal antibodies (mAb17 and mAb8) against Tmod1 that specifically disrupted its interaction with tropomyosin in vitro. Microinjection of mAb17 or mAb8 into chick cardiac myocytes caused a dramatic loss of the thin filaments, as revealed by immunofluorescence deconvolution microscopy. Real-time imaging of live myocytes expressing green fluorescent protein-alpha-tropomyosin and microinjected with mAb17 revealed that the thin filaments depolymerized from their pointed ends. In a thin filament reconstitution assay, stabilization of the filaments before the addition of mAb17 prevented the loss of thin filaments. These studies indicate that the interaction of Tmod1 with tropomyosin is critical for thin filament stability. These data, together with previous studies, indicate that Tmod1 is a multifunctional protein: its actin filament capping activity prevents thin filament elongation, whereas its interaction with tropomyosin prevents thin filament depolymerization.

E-Tmod capping of actin filaments at the slow-growing end is required to establish mouse embryonic circulation

Tropomodulins are a family of proteins that cap the slow-growing end of actin filaments. Erythrocyte tropomodulin (E-Tmod) stabilizes short actin protofilaments in erythrocytes and caps longer sarcomeric actin filaments in striated muscles. We report the knockin of the beta-galactosidase gene (LacZ) under the control of the endogenous E-Tmod promoter and the knockout of E-Tmod in mouse embryonic stem cells. E-Tmod(-/-) embryos die around embryonic day 10 and exhibit a noncontractile heart tube with disorganized myofibrils and underdevelopment of the right ventricle, accumulation of mechanically weakened primitive erythroid cells in the yolk sac, and failure of primary capillary plexuses to remodel into vitelline vessels, all required to establish blood circulation between the yolk sac and the embryo proper. We propose a hemodynamic "plexus channel selection" mechanism as the basis for vitelline vascular remodeling. The defects in cardiac contractility, vitelline circulation, and hematopoiesis reflect an essential role for E-Tmod capping of the actin filaments in both assembly of cardiac sarcomeres and of the membrane skeleton in erythroid cells that is not compensated for by other proteins.

Mice lacking Tropomodulin-2 show enhanced long-term potentiation, hyperactivity, and deficits in learning and memory

Actin filaments control cell morphology and are essential to the growth of dendritic spines and the plasticity of hippocampal long-term potentiation (LTP). The length of these filaments is regulated in muscle and nonmuscle cell types by tropomodulins 1-4 (Tmod1-4), a family of proteins that cap the pointed ends of actin filaments. To investigate whether tropomodulins could play a role in synaptic plasticity, learning, memory, or behavior, we created mice lacking Tropomodulin-2 (Tmod2), which is highly expressed in neuronal structures. Tmod2(lacZ-/-) mice are viable and fertile and exhibit no gross morphological or anatomical abnormalities, but behavioral analysis found hyperactivity, reduced sensorimotor gating, and impaired learning and memory. Electrophysiological analysis revealed enhanced LTP in Tmod2(lacZ-/-) mice. These studies suggest that Tmod2 plays a role in behavior, learning, memory, and synaptic plasticity.

Crystal structure of the C-terminal half of tropomodulin and structural basis of actin filament pointed-end capping

Tropomodulin is the unique pointed-end capping protein of the actin-tropomyosin filament. By blocking elongation and depolymerization, tropomodulin regulates the architecture and the dynamics of the filament. Here we report the crystal structure at 1.45-A resolution of the C-terminal half of tropomodulin (C20), the actin-binding moiety of tropomodulin. C20 is a leucine-rich repeat domain, and this is the first actin-associated protein with a leucine-rich repeat. Binding assays suggested that C20 also interacts with the N-terminal fragment, M1-M2-M3, of nebulin. Based on the crystal structure, we propose a model for C20 docking to the actin subunit at the pointed end. Although speculative, the model is consistent with the idea that a tropomodulin molecule competes with an actin subunit for a pointed end. The model also suggests that interactions with tropomyosin, actin, and nebulin are all possible sources of influences on the dynamic properties of pointed-end capping by tropomodulin.

Tropomyosin requires an intact N-terminal coiled coil to interact with tropomodulin

Tropomodulins (Tmods) are tropomyosin (TM) binding proteins that bind to the pointed end of actin filaments and modulate thin filament dynamics. They bind to the N termini of both "long" TMs (with the N terminus encoded by exon 1a of the alpha-TM gene) and "short" nonmuscle TMs (with the N terminus encoded by exon 1b). In this present study, circular dichroism was used to study the interaction of two designed chimeric proteins, AcTM1aZip and AcTM1bZip, containing the N terminus of a long or a short TM, respectively, with protein fragments containing residues 1 to 130 of erythrocyte or skeletal muscle Tmod. The binding of either TMZip causes similar conformational changes in both Tmod fragments promoting increases in both alpha-helix and beta-structure, although they differ in binding affinity. The circular dichroism changes in the Tmod upon binding and modeling of the Tmod sequences suggest that the interface between TM and Tmod includes a three- or four-stranded coiled coil. An intact coiled coil at the N terminus of the TMs is essential for Tmod binding, as modifications that disrupt the N-terminal helix, such as removal of the N-terminal acetyl group from AcTM1aZip or striated muscle alpha-TM, or introduction of a mutation that causes nemaline myopathy, Met-8-Arg, into AcTM1aZip destroyed Tmod binding.

Vertebrate tropomyosin: distribution, properties and function

Tropomyosin (TM) is widely distributed in all cell types associated with actin as a fibrous molecule composed of two alpha-helical chains arranged as a coiled-coil. It is localised, polymerised end to end, along each of the two grooves of the F-actin filament providing structural stability and modulating the filament function. To accommodate the wide range of functions associated with actin filaments that occur in eucaryote cells TM exists in a large number isoforms, over 20 of which have been identified. These isoforms which are expressed by alternative promoters and alternative RNA processing of four genes, TPM1, 2, 3 and 4, all conform to a general pattern of structure. Their amino acid sequences consist of an integral number, six or seven in vertebrates, of quasiequivalent regions of about 40 residues that are considered to represent the actin-binding regions of the molecule. In addition to the variable regions a large part of the polypeptide chains of the TM isoforms, mainly centrally located and expressed by five exons, is invariant. Many of the isoforms are tissue and filament specific in their distribution implying that the exons expressed in them and the regions of the molecule they represent are of significance for the function of the filament system with which they are associated. In the case of muscle there is clear evidence that the TM moves its position on the F-actin filament during contraction and it is therefore considered to play an important part in the regulation of the process. It is uncertain how the role of TM in muscle compares to that in non-muscle systems and if its function in the former tissue is unique to muscle.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

Folding properties of functional domains of tropomodulin

Tropomodulin (Tmod) stabilizes the actin-tropomyosin filament by capping the slow-growing end (P-end). The N- and C-terminal halves play distinct roles; the N-terminal half interacts with the N-terminal region of tropomyosin, whereas the C-terminal half interacts with actin. Our previous study (A. Kostyukova, K. Maeda, E. Yamauchi, I. Krieger, and Y. Maéda Y., 2000, Eur. J. Biochem. 267:6470-6475) suggested that the two halves are also structurally distinct from each other. We have now studied the folding properties of the two halves, by circular dichroism spectroscopy and by differential scanning calorimetry of the expressed chicken E-type tropomodulin and its large fragments. The results showed that the C-terminal half represents a single, independently folded unit that melts cooperatively through a two-state transition. In contrast, the N-terminal half lacks a definite tertiary structure in solution. The binding of N11, a fragment that corresponds to the first 91 amino acids of the tropomodulin, to tropomyosin substantially stabilized the tropomyosin. This may indicate that the flexible structure of the N-terminal half of tropomodulin in solution is required for binding to tropomyosin and that the N-terminal half acquires its tertiary structure upon binding to tropomyosin.

Caspase remodeling of the spectrin membrane skeleton during lens development and aging

Terminal differentiation of lens fiber cells resembles the apoptotic process in that organelles are lost, DNA is fragmented, and changes in membrane morphology occur. However, unlike classically apoptotic cells, which are disintegrated by membrane blebbing and vesiculation, aging lens fiber cells are compressed into the center of the lens, where they undergo cell-cell fusion and the formation of specialized membrane interdigitations. In classically apoptotic cells, caspase cleavage of the cytoskeletal protein alpha-spectrin to approximately 150-kDa fragments is believed to be important for membrane blebbing. We report that caspase(s) cleave alpha-spectrin to approximately 150-kDa fragments and beta-spectrin to approximately 120- and approximately 80-kDa fragments during late embryonic chick lens development. These fragments continue to accumulate with age so that in the oldest fiber cells of the adult lens, most, if not all, of the spectrin is cleaved to discrete fragments. Thus, unlike classical apoptosis, where caspase-cleaved spectrin is short lived, lens fiber cells contain spectrin fragments that appear to be stable for the lifetime of the organism. Moreover, fragmentation of spectrin results in reduced membrane association and thus may lead to permanent remodeling of the membrane skeleton. Partial and specific proteolysis of membrane skeleton components by caspases may be important for age-related membrane changes in the lens.

Leiomodin and tropomodulin in smooth muscle

Evidence is accumulating to suggest that actin filament remodeling is critical for smooth muscle contraction, which implicates actin filament ends as important sites for regulation of contraction. Tropomodulin (Tmod) and smooth muscle leiomodin (SM-Lmod) have been found in many tissues containing smooth muscle by protein immunoblot and immunofluorescence microscopy. Both proteins cofractionate with tropomyosin in the Triton-insoluble cytoskeleton of rabbit stomach smooth muscle and are solubilized by high salt. SM-Lmod binds muscle tropomyosin, a biochemical activity characteristic of Tmod proteins. SM-Lmod staining is present along the length of actin filaments in rat intestinal smooth muscle, while Tmod stains in a punctate pattern distinct from that of actin filaments or the dense body marker α-actinin. After smooth muscle is hypercontracted by treatment with 10 mM Ca2+, both SM-Lmod and Tmod are found near α-actinin at the periphery of actin-rich contraction bands. These data suggest that SM-Lmod is a novel component of the smooth muscle actin cytoskeleton and, furthermore, that the pointed ends of actin filaments in smooth muscle may be capped by Tmod in localized clusters.

The shapes and sizes of two domains of tropomodulin, the P-end-capping protein of actin-tropomyosin

Tropomodulin, the P-end (slow-growing end)-capping protein of the actin-tropomyosin filament, and its fragment (C20) of the C-terminal half were studied by synchrotron small-angle X-ray scattering, restoring low-resolution shapes using an ab initio shape-determining procedure. Tropomodulin is elongated (115 A long) and consists of two domains, one of 65 A in length and the other being similar to C20 in shape and size if the long axes of the two are tilted by about 40 degrees relative to each other. We propose a model for tropomodulin in association with tropomyosin and actin: the N-terminal half of tropomodulin, a rod, binds to the N-terminus of tropomyosin and the C-terminal triangle domain protrudes from the P-end being slightly bent towards the actin subunit at the end, thereby blocking the P-end.

Leiomodins: larger members of the tropomodulin (Tmod) gene family

The 64-kDa autoantigen D1 or 1D, first identified as a potential autoantigen in Graves' disease, is similar to the tropomodulin (Tmod) family of actin filament pointed end-capping proteins. A novel gene with significant similarity to the 64-kDa human autoantigen D1 has been cloned from both humans and mice, and the genomic sequences of both genes have been identified. These genes form a subfamily closely related to the Tmods and are here named the Leiomodins (Lmods). Both Lmod genes display a conserved intron-exon structure, as do three Tmod genes, but the intron-exon structure of the Lmods and the Tmods is divergent. mRNA expression analysis indicates that the gene formerly known as the 64-kDa autoantigen D1 is most highly expressed in a variety of human tissues that contain smooth muscle, earning it the name smooth muscle Leiomodin (SM-Lmod; HGMW-approved symbol LMOD1). Transcripts encoding the novel Lmod gene are present exclusively in fetal and adult heart and adult skeletal muscle, and it is here named cardiac Leiomodin (C-Lmod; HGMW-approved symbol LMOD2). Human C-Lmod is located near the hypertrophic cardiomyopathy locus CMH6 on human chromosome 7q3, potentially implicating it in this disease. Our data demonstrate that the Lmods are evolutionarily related and display tissue-specific patterns of expression distinct from, but overlapping with, the expression of Tmod isoforms.

The N-terminal end of nebulin interacts with tropomodulin at the pointed ends of the thin filaments

Strict regulation of actin thin filament length is critical for the proper functioning of sarcomeres, the basic contractile units of myofibrils. It has been hypothesized that a molecular template works with actin filament capping proteins to regulate thin filament lengths. Nebulin is a giant protein ( approximately 800 kDa) in skeletal muscle that has been proposed to act as a molecular ruler to specify the thin filament lengths characteristic of different muscles. Tropomodulin (Tmod), a pointed end thin filament capping protein, has been shown to maintain the final length of the thin filaments. Immunofluorescence microscopy revealed that the N-terminal end of nebulin colocalizes with Tmod at the pointed ends of thin filaments. The three extreme N-terminal modules (M1-M2-M3) of nebulin bind specifically to Tmod as demonstrated by blot overlay, bead binding, and solid phase binding assays. These data demonstrate that the N terminus of the nebulin molecule extends to the extreme end of the thin filament and also establish a novel biochemical function for this end. Two Tmod isoforms, erythrocyte Tmod (E-Tmod), expressed in embryonic and slow skeletal muscle, and skeletal Tmod (Sk-Tmod), expressed late in fast skeletal muscle differentiation, bind on overlapping sites to recombinant N-terminal nebulin fragments. Sk-Tmod binds nebulin with higher affinity than E-Tmod does, suggesting that the Tmod/nebulin interaction exhibits isoform specificity. These data provide evidence that Tmod and nebulin may work together as a linked mechanism to control thin filament lengths in skeletal muscle.

Domain structure of tropomodulin: distinct properties of the N-terminal and C-terminal halves

The structure of tropomodulin, the unique capping protein for the pointed end (the slow-growing end) of an actin filament, was studied. An improved Escherichia coli expression system for chicken E-tropomodulin was established and tropomodulin was prepared, Tmod (N39), in which 15 amino acid residues from the original C-terminus are deleted at the DNA level. This expression and purification system accidentally co-produces an 11-kDa fragment with the original N-terminus (N11). By applying limited proteolysis to Tmod (N39), a 20-kDa C-terminal fragment (C20) was obtained. The limited proteolysis data, as well as the fluorescence spectrometry and CD analyses of Tmod (N39), C20 and N11, revealed that tropomodulin is an alpha-helical protein that consists of two distinct domains. The C-terminal half (20 kDa) is resistant to proteolysis, which suggests that this domain is tightly folded. In contrast, the N-terminal half is susceptible to proteolysis, indicating that in solution this half is likely to be extended or to form a highly flexible structure. Cross-linking experiments with glutaraldehyde indicated that Tmod (N39) and N11 can form complexes with tropomyosin, whereas C20 cannot. This confirms the previous report that the site(s) of interaction with tropomyosin resides in the N-terminal 11-kDa region of tropomodulin.

Genomic organization of mouse and human erythrocyte tropomodulin genes encoding the pointed end capping protein for the actin filaments

Erythrocyte tropomodulin (E-Tmod), a globular protein of 359 residues, is highly expressed in the erythrocyte, heart and skeletal muscle. By binding to the N-terminus of tropomyosin (TM) and actin, E-Tmod blocks the elongation and depolymerization of the actin filaments at the pointed end. In erythrocytes, the E-Tmod/TM complex contributes to the formation of the short actin protofilament, which in turn defines the geometry of the membrane skeleton. In juvenile mice, over-expression of E-Tmod is associated with dilated cardiomyopathy. We have previously cloned the human E-Tmod cDNA, identified its TM-binding region, and mapped its gene to chromosome 9q22. Through genomic library screening and PCR-based genomic walking we have now cloned the mouse E-Tmod gene, whose coding region spans approximately 60kb containing nine exons and eight introns. The human E-Tmod gene obtained by PCR has an identical exon-intron organization. In sanpodo, a Tmod homologue in Drosophila, the exon boundaries are also conserved except that exons 2-5 and 6-7 are 'fused' and alternative splicing of two additional 5' exons and the 3' exons may give rise to several sanpodo isoforms. In a Tmod-like gene of C. elegans, exons 2-3 are 'fused', boundaries of exons 1, 7, 8, and 9 are conserved and exon/intron junctions of exons 4, 5 and 6 are shifted by a few residues. Analyses of 15 Tmod members from six species show no insertions or deletions of residues in the region of exons 6 and 7. A 5' rapid amplification of cDNA ends reveals that mouse E-Tmod transcripts obtained from embryonic stem cells, skeletal muscle and heart, but not smooth muscle, contain an additional 86bp untranslated cDNA sequence further upstream from exon 1. Thus, alternative promoters may provide a possible mechanism for tissue-specific expression and regulation of E-Tmod. This study is the first to report the exon organization of E-Tmod genes, which allows their regulation, manipulation, and disease relevance to be further investigated.

Protein targets of monocrotaline pyrrole in pulmonary artery endothelial cells

A single administration of monocrotaline to rats results in pathologic alterations in the lung and heart similar to human pulmonary hypertension. In order to produce these lesions, monocrotaline is oxidized to monocrotaline pyrrole in the liver followed by hematogenous transport to the lung where it injures pulmonary endothelium. In this study, we determined specific endothelial targets for (14)C-monocrotaline pyrrole using two-dimensional gel electrophoresis and autoradiographic detection of protein metabolite adducts. Selective labeling of specific proteins was observed. Labeled proteins were digested with trypsin, and the resulting peptides were analyzed using matrix-assisted laser desorption ionization mass spectrometry. The results were searched against sequence data bases to identify the adducted proteins. Five abundant adducted proteins were identified as galectin-1, protein-disulfide isomerase, probable protein-disulfide isomerase (ER60), beta- or gamma-cytoplasmic actin, and cytoskeletal tropomyosin (TM30-NM). With the exception of actin, the proteins identified in this study have never been identified as potential targets for pyrroles, and the majority of these proteins have either received no or minimal attention as targets for other electrophilic compounds. The known functions of these proteins are discussed in terms of their potential for explaining the pulmonary toxicity of monocrotaline.

The lens membrane skeleton contains structures preferentially enriched in spectrin-actin or tropomodulin-actin complexes

The spectrin-based membrane skeleton plays an important role in determining the distributions and densities of receptors, ion channels, and pumps, thus influencing cell shape and deformability, cell polarity, and adhesion. In the paradigmatic human erythrocyte, short tropomodulin-capped actin filaments are cross-linked by spectrin into a hexagonal network, yet the extent to which this type of actin filament organization is utilized in the membrane skeletons of nonerythroid cells is not known. Here, we show that associations of tropomodulin and spectrin with actin in bovine lens fiber cells are distinct from that of the erythrocyte and imply a very different molecular organization. Mechanical disruption of the lens fiber cell membrane skeleton releases tropomodulin and actin-containing oligomeric complexes that can be isolated by gel filtration column chromatography, sucrose gradient centrifugation and immunoadsorption. These tropomodulin-actin complexes do not contain spectrin. Instead, spectrin is associated with actin in different complexes that do not contain tropomodulin. Immunofluorescence staining of isolated fiber cells further demonstrates that tropomodulin does not precisely colocalize with spectrin along the lateral membranes of lens fiber cells. Taken together, our data suggest that tropomodulin-capped actin filaments and spectrin-cross-linked actin filaments are assembled in distinct structures in the lens fiber cell membrane skeleton, indicating that it is organized quite differently from that of the erythrocyte membrane skeleton.

Distinct localizations of tropomyosin isoforms in LLC-PK1 epithelial cells suggests specialized function at cell-cell adhesions

At least eight nonmuscle, nonbrain tropomyosin isoforms have been described. We used antibodies, microinjection, and transfection to characterize their expression and localization in LLC-PK1 kidney epithelial cells and compared them with other cells. Similar to primary enterocytes, LLC-PK1 cells exhibited predominantly TM-1 and TM-3 of the high-molecular-weight (HMW) isoforms; TM-5 and TM-5b of the low-molecular-weight (LMW) isoforms. Neither TM-4 nor TM-5a was detectable in the LLC-PKI cells. Immunofluorescence studies revealed that HMW isoforms were localized only on stress fibers, not adhesion belts, whereas the adhesion belts were stained by LMW isoform antibodies. When exogenous proteins are introduced either by transfection or microinjection, the HMW isoforms do not incorporate into the adhesion belt, whereas the LMW isoforms can incorporate into the stress fibers, thus indicating there are different mechanisms at work for the selective localization. Temporal changes in the microfilament system of the LLC-PK1 cells were studied during differentiation in culture as defined by spectrin expression and F-actin architecture. Western blot analysis indicated that TM-5b is only expressed in the LLC-PK1 cells after a certain degree of maturation in culture, which suggests isoform switching after the cell-cell contacts are developed. Collectively these results demonstrate that epithelial cells express a complex pattern of TM isoforms, which exhibit differential localizations within the cells and different patterns of expression depending on their origin and stage of differentiation. The implication of differential localization of TM isoforms on their specific functions is discussed.

Tropomodulin-binding site mapped to residues 7-14 at the N-terminal heptad repeats of tropomyosin isoform 5

Tropomodulin is a globular protein that caps the pointed end of actin filaments by complexing with the N-terminus of a tropomyosin (TM) molecule. TM consists of coiled coils except for the N-terminus, which may be globular. Here we report that human TM isoform 5 (hTM5) lacking the N-terminal 18 residues lost its binding activity toward tropomodulin. We further characterized the tropomodulin-binding site by creating a series of deletion and missense mutations within this region, followed by a solid-phase binding assay. I7, V10, and I14, hydrophobic residues located at the a and d positions of N-terminal heptad repeats involving intertwine, are essential for tropomodulin binding. R12, a positively charged residue at the f position, is also involved in recognition. In contrast, A2R and G3Y mutations, each creating a bulky N-terminus, did not alter the binding. In addition, rat TM5b, which differs from hTM5 in residues 4-6, exhibits a similar binding affinity. The tropomodulin-binding site, therefore, is mapped to residues 7-14 at the beginning of the long heptad repeats. Column chromatography revealed that hTM5 mutants remained capable of dimerization. Results also suggest tropomodulin has a groove-type, rather than a cavity-type, binding site for hTM5. We also mapped the epitope of monoclonal antibody LC1 to residues 4-10 of hTM5 and showed the competition between mAb LC1 and tropomodulin in hTM5 binding. Since the N-terminal residues need to overlap with the C-terminus of TM in their head-to-tail association, this investigation elucidates the mechanisms by which the tropomodulin-hTM5 complex is formed and functions in regulating the actin filaments.

Stabilization and remodeling of the membrane skeleton during lens fiber cell differentiation and maturation

Actin filaments are integral components of the plasma membrane-associated cytoskeleton (membrane skeleton) and are believed to play important roles in the determination of cell polarity, shape, and membrane mechanical properties, however the roles of actin regulatory proteins in controlling the assembly, stability, and organization of actin filaments in the membrane skeleton are not well understood. Tropomodulin is a tropomyosin and actin-binding protein that stabilizes tropomyosin-actin filaments by capping their pointed ends and is associated with the spectrin-actin membrane skeleton in erythrocytes, skeletal muscle cells, and lens fiber cells, a specialized epithelial cell type. In this study, we have investigated the role of tropomodulin and other membrane skeleton components in lens fiber cell differentiation and maturation. Our results demonstrate that tropomodulin is expressed concomitantly with lens fiber cell differentiation and assembles onto the plasma membrane only after fiber cells have begun to elongate and form apical-apical contacts with the undifferentiated epithelium. In contrast, other membrane skeleton components, spectrin, actin, and tropomyosin, are constitutively expressed and assembled on the plasma membranes of both undifferentiated and differentiated fiber cells. Tropomodulin, but not other membrane skeleton components, is also enriched at a novel structure at the apical and basal ends of newly elongated fiber cells at the fiber cell-epithelium and fiber cell-capsule interface, respectively. Once assembled, tropomodulin and its binding partners, tropomyosin and actin, remain membrane-associated and are not proteolyzed during fiber cell maturation and aging, despite proteolysis of alpha-spectrin and other cytoskeletal filament systems such as microtubules and intermediate filaments. We propose that actin filament stabilization by tropomodulin, coupled with partial proteolysis of other cytoskeletal components, represents a programmed remodeling of the lens membrane skeleton that may be essential to maintain plasma membrane integrity and transparency of the extremely elongated, long-lived cells of the lens. The unique localization of tropomodulin at fiber cell tips further suggests a new role for tropomodulin at cell-cell and cell-substratum contacts; this may be important for cell migration and/or adhesion during differentiation and morphogenesis.

Tropomyosin isoform 5b is expressed in human erythrocytes: implications of tropomodulin-TM5 or tropomodulin-TM5b complexes in the protofilament and hexagonal organization of membrane skeletons

The human erythrocyte membrane skeleton consists of hexagonal lattices with junctional complexes containing F-actin protofilaments of approximately 33-37 nm in length. We hypothesize that complexes formed by tropomodulin, a globular capping protein at the pointed end of actin filaments, and tropomyosin (TM), a rod-like molecule of approximately 33-35 nm, may contribute to the formation of protofilaments. We have previously cloned the human tropomodulin complementary DNA and identified human TM isoform 5 (hTM5), a product of theγ-TM gene, as one of the major TM isoforms in erythrocytes. We now identify TM5b, a product of the -TM gene, to be the second major TM isoform. TM5a, the alternatively spliced isoform of the-TM gene, which differs by 1 exon and has a weaker actin-binding affinity, however, is not present. TM4, encoded by the δ-TM gene, is not present either. In sodium dodecyl sulfate–polyacrylamide gel electrophoresis, hTM5 comigrated with the slower TM major species in erythrocyte membranes, and hTM5b comigrated with the faster TM major species. TM5b, like TM5, binds strongly to tropomodulin, more so than other TM isoforms. The 2 major TM isoforms, therefore, share several common features: They have 248 residues, are approximately 33-35 nm long, and have high affinities toward F-actin and tropomodulin. These common features may be the key to the mechanism by which protofilaments are formed. Tropomodulin-TM5 or tropomodulin-TM5b complexes may stabilize F-actin in segments of approximately 33-37 nm during erythroid terminal differentiation and may, therefore, function as a molecular ruler. TM5 and TM5b further define the hexagonal geometry of the skeletal network and allow actin-regulatory functions of TMs to be modulated by tropomodulin.

Sequencing, expression analysis, and mapping of three unique human tropomodulin genes and their mouse orthologs

Tropomodulin (TMOD) is the actin-capping protein for the slow-growing end of filamentous actin, and a neuronal-specific isoform, neuronal tropomodulin (NTMOD), is the major binding protein to brain tropomyosin in rat. The Drosophila TMOD homolog, Sanpodo, alters sibling cell fate determination, so we used a cross-species approach to identify additional TMOD family members that may play a critical role in this process. We characterized the human and mouse orthologs to rat NTMOD (TMOD2 and Tmod2, respectively) as well as two novel tropomodulin family members (TMOD3, Tmod3 and TMOD4, Tmod4). Their expression patterns vary extensively, from ubiquitous (TMOD3 and Tmod3) to muscle (TMOD4) or neuronal tissues only (TMOD2 and Tmod2). TMOD2 and TMOD3 map next to one another on chromosome 15q21.1-q21.2, and their mouse orthologs map to a homologous region on mouse chromosome 9; TMOD4 maps to the telomeric end of 1q12 and Tmod4 to a homologous region of mouse chromosome 3. Their location and expression patterns make TMOD2 and TMOD3 candidate genes for amyotrophic lateral sclerosis 5 (ALS5) and dyslexia-1 (DYX1) and TMOD4 a candidate gene for limb girdle muscular dystrophy 1B (LGMD1B). Our mapping efforts revealed new regions of paralogy among chromosomes 1q, 9q, 15q, and 19p.

Tropomodulin increases the critical concentration of barbed end-capped actin filaments by converting ADP.P(i)-actin to ADP-actin at all pointed filament ends

The pointed end capping protein, tropomodulin, increases the critical concentration of barbed end capped actin, i.e. it lowers the apparent affinity of pointed ends for actin monomers. We show here that this is due to the conversion of pointed end ADP. P(i)-actin (low critical concentration) to ADP-actin (high critical concentration) when 70-98% of the ends are capped by tropomodulin. We propose that this is due to the low affinity of tropomodulin for pointed ends (K(d) approximately 0.3 microM), which allows tropomodulin to rapidly exchange binding sites and transiently block access of actin monomers to all pointed ends. This leaves time for ATP hydrolysis and phosphate release to go to completion between successive monomer additions to the pointed end. When the affinity of tropomodulin for pointed ends was increased about 1000-fold by the presence of tropomyosin (K(d) < 0.05 nM), capping of 95% of the ends by tropomodulin did not alter the critical concentration. However, the critical concentration did increase when the tropomodulin concentration was raised to the high values effective in the absence of tropomyosin. This may reflect transient tropomodulin binding to tropomyosin-free actin molecules at the pointed ends of the tropomyosin-actin filaments without a high affinity tropomodulin cap, i.e. the ends that determine the value of the actin critical concentration.

What do mouse gene knockouts tell us about the structure and function of the red cell membrane?

Recent development of knockout mice with targeted deletion of specific genes encoding various red cell membrane proteins has added valuable armamentarium to red cell membrane structure-function studies. In this chapter we will summarize the various recent developments regarding the structure and function of the red cell membrane derived from studies using knockout mice. In addition to being expressed in red cells, all major red cell membrane proteins are also expressed in cells of various tissues. The potential use of knockout mice to decipher the biological functions of red cell membrane proteins in non-erythroid cells is also explored.

Pathogenesis of dilated cardiomyopathy: molecular, structural, and population analyses in tropomodulin-overexpressing transgenic mice

Dilated cardiomyopathy is characterized by decreased contractile function and loss of myofibril organization. Previously unexplored structural and molecular events that precede and initiate dilation can now be studied in tropomodulin-overexpressing transgenic (TOT) mice exhibiting progressive dilated cardiomyopathy. Onset of dilation did not correspond to a change in transgene expression levels, which were more than threefold above normal at birth and remained elevated throughout postnatal life. Similarly, mitogen-activated protein kinase activation (p38, ERK1/ERK2, JNK1/JNK2) was not associated with dilation. In contrast, calcineurin was activated before dilation, presumably due to doubling of intracellular diastolic calcium levels in TOT cardiomyocytes. Amplitude of systolic calcium transients was greatly increased as well, demonstrating the novel and unique calcium handling profile of TOT cardiomyocytes. Loss of myofibril organization was not apparent by confocal microscopy until over 1 week after birth, although neonatal sarcomeric abnormalities were revealed by ultrastructural analysis. Rapid postnatal increases in heart:body weight ratio at 1.5 weeks were followed by two waves of mortality between 2 and 3 weeks after birth coincident with maturational stress. Ultimately, TOT pathogenesis is a compensatory response to altered sarcomeric structure driven by calcineurin activation within days after birth, making TOTs an excellent paradigm for studying the role of calcium overload in dilated cardiomyopathy.

Identification of a novel tropomodulin isoform, skeletal tropomodulin, that caps actin filament pointed ends in fast skeletal muscle

Tropomodulin (E-Tmod) is an actin filament pointed end capping protein that maintains the length of the sarcomeric actin filaments in striated muscle. Here, we describe the identification and characterization of a novel tropomodulin isoform, skeletal tropomodulin (Sk-Tmod) from chickens. Sk-Tmod is 62% identical in amino acid sequence to the previously described chicken E-Tmod and is the product of a different gene. Sk-Tmod isoform sequences are highly conserved across vertebrates and constitute an independent group in the tropomodulin family. In vitro, chicken Sk-Tmod caps actin and tropomyosin-actin filament pointed ends to the same extent as does chicken E-Tmod. However, E- and Sk-Tmods differ in their tissue distribution; Sk-Tmod predominates in fast skeletal muscle fibers, lens, and erythrocytes, while E-Tmod is found in heart and slow skeletal muscle fibers. Additionally, their expression is developmentally regulated during chicken breast muscle differentiation with Sk-Tmod replacing E-Tmod after hatching. Finally, in skeletal muscle fibers that coexpress both Sk- and E-Tmod, they are recruited to different actin filament-containing cytoskeletal structures within the cell: myofibrils and costameres, respectively. All together, these observations support the hypothesis that vertebrates have acquired different tropomodulin isoforms that play distinct roles in vivo.

Localization of the human 64kD autoantigen D1 to myofibrils in a subset of extraocular muscle fibers

PURPOSE

To evaluate the tissue-specific expression pattern of the 64kD human autoantigen D1, a tropomodulin-related protein that may be involved in thyroid-associated ophthalmopathy.

METHODS

Recombinant 64kD human autoantigen D1 was generated in a bacterial expression system and used to immunize rabbits. Specific antibodies were affinity-purified and used for Western blots on normal and hyperthyroid rat and rabbit tissue, and immunofluorescence localization on cryosections of rat tissue.

RESULTS

Anti-64kD human autoantigen D1 antibodies recognize specifically a approximately 70kD polypeptide in western blots of extraocular muscle, sternothyroid muscle, and smooth muscle. Immunofluorescence staining demonstrates that the 64kD human autoantigen D1 localizes to myofibrils in slow fibers from rat extraocular and sternothyroid muscle. The level of this protein is not altered in extraocular muscles from hyperthyroid rabbits.

CONCLUSIONS

The 64kD human autoantigen D1 is expressed in slow fibers of extraocular and sternothyroid muscles as a component of myofibrils, and is not upregulated in conditions of hyperthyroidism.

Tropomodulin isolated from rabbit skeletal muscle inhibits filament formation of actin in the presence of tropomyosin and troponin

Tropomodulin is a tropomyosin-binding protein, originally isolated from human erythrocytes. Tropomodulin is currently regarded as the sole actin pointed-end capping protein [Weber, A., Pennise, C.R., Babcock, G.G. & Fowler, V.M. (1994) J. Cell Biol. 127, 1627-1635]. This work first describes a procedure for the purification of tropomodulin from rabbit skeletal muscle. Tropomodulin almost completely inhibited filament formation of actin in the presence of tropomyosin and troponin. For the maximal inhibition of actin polymerization, approximately 0.10, 0.12 and 0.003 mol of tropomyosin, troponin and tropomodulin per mol of actin were required, respectively. Fluorescence-intensity measurements, electron-microscopy and sedimentation experiments revealed that only very short fragments and amorphous aggregates, but not filaments, were formed when actin was copolymerized with tropomyosin, troponin and tropomodulin by the addition of 50 mM KCl at pH 8.0. The effects of tropomyosin, troponin and tropomodulin were more remarkable on Ca-actin than on Mg-actin. It appears that tropomodulin caps both the pointed and barbed ends of tropomyosin- and troponin-bound actin filaments.

Tropomodulin assembles early in myofibrillogenesis in chick skeletal muscle: evidence that thin filaments rearrange to form striated myofibrils

Actin filament lengths in muscle and nonmuscle cells are believed to depend on the regulated activity of capping proteins at both the fast growing (barbed) and slow growing (pointed) filament ends. In striated muscle, the pointed end capping protein, tropomodulin, has been shown to maintain the lengths of thin filaments in mature myofibrils. To determine whether tropomodulin might also be involved in thin filament assembly, we investigated the assembly of tropomodulin into myofibrils during differentiation of primary cultures of chick skeletal muscle cells. Our results show that tropomodulin is expressed early in differentiation and is associated with the earliest premyofibrils which contain overlapping and misaligned actin filaments. In addition, tropomodulin can be found in actin filament bundles at the distal tips of growing myotubes, where sarcomeric alpha-actinin is not always detected, suggesting that tropomodulin caps actin filament pointed ends even before the filaments are cross-linked into Z bodies by alpha-actinin. Tropomodulin staining exhibits an irregular punctate pattern along the length of premyofibrils that demonstrate a smooth phalloidin staining pattern for F-actin. Strikingly, the tropomodulin dots often appear to be located between the closely spaced, dot-like Z bodies that are stained for (α)-actinin. Thus, in the earliest premyofibrils, the pointed ends of the thin filaments are clustered and partially aligned with respect to the Z bodies (the location of the barbed filament ends). At later stages of differentiation, the tropomodulin dots become aligned into regular periodic striations concurrently with the appearance of striated phalloidin staining for F-actin and alignment of Z bodies into Z lines. Tropomodulin, together with the barbed end capping protein, CapZ, may function from the earliest stages of myofibrillogenesis to restrict the lengths of newly assembled thin filaments by capping their ends; thus, transitions from nonstriated to striated myofibrils in skeletal muscle are likely due principally to filament rearrangements rather than to filament polymerization or depolymerization. Rearrangements of actin filaments capped at their pointed and barbed ends may be a general mechanism by which cells restructure their actin cytoskeletal networks during cell growth and differentiation.

Defining actin filament length in striated muscle: rulers and caps or dynamic stability?

Actin filaments (thin filaments) are polymerized to strikingly uniform lengths in striated muscle sarcomeres. Yet, actin monomers can exchange dynamically into thin filaments in vivo, indicating that actin monomer association and dissociation at filament ends must be highly regulated to maintain the uniformity of filament lengths. We propose several hypothetical mechanisms that could generate uniform actin filament length distributions and discuss their application to the determination of thin filament length in vivo. At the Z line, titin may determine the minimum extent and tropomyosin the maximum extent of thin filament overlap by regulating alpha-actinin binding to actin, while a unique Z filament may bind to capZ and regulate barbed end capping. For the free portion of the thin filament, we evaluate possibilities that thin filament components (e.g. nebulin or the tropomyosin/troponin polymer) determine thin filament lengths by binding directly to tropomodulin and regulating pointed end capping, or alternatively, that myosin thick filaments, together with titin, determine filament length by indirectly regulating tropomodulin's capping activity.

The biochemical modification of the erythrocyte membranes from women with ovarian cancer

The aim of our work was quantitative evaluation of the protein and phospholipid fractions of mature erythrocyte membranes separated from women with ovarian cancer. Blood was sampled from 30 women with ovarian cancer, aged 24-79 years, in the third stage of clinical progression of the disease. Phospholipids were separated from membranes by Müller's acidic extraction method and analysed in thin-layer two-dimensional chromatography. On the silica gel plates nine fractions of phospholipids were separated: sphingomyelin (SPH), phosphatidylethanolamine (PE), phosphatidlyserine (PS), phosphatidylcholine (PC), lysophosphatidylcholine (LPC), phosphatidic acid (PA), phosphatidylinositol (Ptd Ins), phosphatidylinositol-4-phosphate (Ptd Ins-4-P), phosphatidylinositol-4,5-diphosphate (Ptd Ins-4,5-P2). The activity of phospholipase C in erythrocyte membranes was determined by Akhrem's spectrophotometric method. Membrane proteins were separated by polyacrylamide gel electrophoresis, SDS-PAGE. It was shown that PS, SPH, LPC and PA fractions were significantly diminished. The concentration of Ptd Ins-4-P and Ptd Ins-4,5-P2 was significantly increased with simultaneous reduction in Ptd Ins level. The inhibition of phospholipase C reached 80%. The quantitative protein evaluation showed a statistically significant decrease in spectrin and a significant increase in 4.1 protein. The quantitative changes, observed in phospholipid and protein fractions, led to the restructuring of the erythrocyte membrane cytoskeleton, which may be connected to increased susceptibility to haemolysis of red blood cells.

Actin binding and nucleation by Autographa california M nucleopolyhedrovirus

The budded form of Autographa californica M nucleopolyhedrovirus enters permissive cells via adsorptive endocytosis. Shortly after nucleocapsid penetration into the cytoplasm, thick actin cables form, which frequently project toward the nucleus. These actin cables are transient structures, formed in association with viral nucleocapsids prior to viral gene expression and concomitant with nucleocapsid transport to the nucleus. In this paper we report that nucleocapsids are capable of nucleating actin polymerization in vitro in a concentration-dependent manner. Two viral-encoded capsid proteins, p39 and p78/83, were found to bind actin directly and therefore could be involved in the observed acceleration of actin polymerization. When nucleocapsids were added to actin in the presence of cytochalasin D, actin polymerization was reduced to levels below those obtained with actin and cytochalasin D alone, suggesting that the nucleocapsids bound to the pointed ends of actin filaments. Finally, treatment of infected cells with the myosin inhibitor 2,3-butanedione monoxime delayed nucleocapsid transport to the nucleus. We postulate that upon entering the cytoplasm, AcMNPV nucleocapsids induce the polymerization of actin cables, which, in conjunction with a myosin-like motor, facilitate their transport to and/or into the nucleus.

Altered expression of tropomodulin in cardiomyocytes disrupts the sarcomeric structure of myofibrils

Tropomodulin is a tropomyosin-binding protein that terminates "pointed-end" actin filament polymerization. To test the hypothesis that regulation of tropomodulin:actin filament stoichiometry is critical for maintenance of actin filament length, tropomodulin levels were altered in cells by infection with recombinant adenoviral expression vectors, which produce either sense or antisense tropomodulin mRNA. Neonatal rat cardiomyocytes were infected, and sarcomeric actin filament organization was examined. Confocal microscopy indicated that overexpression of tropomodulin protein shortened actin filaments and caused myofibril degeneration. In contrast, decreased tropomodulin content resulted in the formation of abnormally long actin filament bundles. Despite changes in myofibril structure caused by altered tropomodulin expression, total protein turnover of the cardiomyocytes was unaffected. Biochemical analyses of infected cardiomyocytes indicated that changes in actin distribution, rather than altered actin content, accounted for myofibril reorganization. Ultrastructural analysis showed thin-filament disarray and revealed the presence of leptomeres after tropomodulin overexpression. Tropomodulin-mediated effects constitute a novel mechanism to control actin filaments, and our findings demonstrate that regulated tropomodulin expression is necessary to maintain stabilized actin filament structures in cardiac muscle cells.

Myofibril degeneration caused by tropomodulin overexpression leads to dilated cardiomyopathy in juvenile mice

Loss of myofibril organization is a common feature of chronic dilated and progressive cardiomyopathy. To study how the heart compensates for myofibril degeneration, transgenic mice were created that undergo progressive loss of myofibrils after birth. Myofibril degeneration was induced by overexpression of tropomodulin, a component of the thin filament complex which determines and maintains sarcomeric actin filament length. The tropomodulin cDNA was placed under control of the alpha-myosin heavy chain gene promoter to overexpress tropomodulin specifically in the myocardium. Offspring with the most severe phenotype showed cardiomyopathic changes between 2 and 4 wk after birth. Hearts from these mice present characteristics consistent with dilated cardiomyopathy and a failed hypertrophic response. Histological analysis showed widespread loss of myofibril organization. Confocal microscopy of isolated cardiomyocytes revealed intense tropomodulin immunoreactivity in transgenic mice together with abnormal coincidence of tropomodulin and alpha-actinin reactivity at Z discs. Contractile function was compromised severely as determined by echocardiographic analyses and isolated Langendorff heart preparations. This novel experimentally induced cardiomyopathy will be useful for understanding dilated cardiomyopathy and the effect of thin filament-based myofibril degeneration upon cardiac structure and function.

Purification and characterization of an alpha 1 beta 2 isoform of CapZ from human erythrocytes: cytosolic location and inability to bind to Mg2+ ghosts suggest that erythrocyte actin filaments are capped by adducin

CapZ ("capping protein") is a heterodimeric actin capping protein that blocks actin filament assembly and disassembly at the fast growing (barbed) filament ends and is proposed to function in regulating actin filament dynamics as well as in stabilizing actin filament lengths in muscle and nonmuscle cells. We show here that erythrocytes contain a nonmuscle isoform of capZ (EcapZ) that is present exclusively in the cytosol and is not associated with the short actin filaments in the erythrocyte membrane skeleton. This is unlike other cell types where capZ is associated with cytoskeletal actin filaments and suggests that cytosolic EcapZ may be inactive, or alternatively, that the barbed ends are capped by adducin, a membrane skeleton protein that was shown recently to cap actin filament barbed ends in vitro [Kuhlman, P. A., Hughes, C. A., Bennett, V., & Fowler, V. M. (1996) J. Biol. Chem. 271, 7986]. To distinguish between these possibilities, we purified EcapZ from erythrocyte cytosol and characterized its biochemical and functional properties. Two-dimensional gel electrophoresis and western blotting reveals the EcapZ subunit composition to be alpha1beta2, as described for capZ from many other nonmuscle cells, with no evidence for posttranslational modifications. Purified EcapZ is fully functional in blocking actin elongation from barbed filament ends (Kcap approximately 1-5 nM) as well as in nucleating actin polymerization. Furthermore, cytosolic EcapZ binds to actin filament barbed ends, indicating that sequestering of EcapZ by a cytosolic inhibitory factor or insufficient amounts of EcapZ in cytosol also cannot account for its absence from the membrane skeleton. To test directly whether the barbed ends of the erythrocyte actin filaments were already capped, we measured binding of purified EcapZ to isolated membranes. Purified EcapZ does not cosediment with membranes prepared by hypotonic lysis in the presence of magnesium, suggesting that the barbed ends of the erythrocyte actin filaments are capped under these conditions but not by EcapZ. In contrast, purified EcapZ stoichiometrically reassociates with all the actin filament barbed ends in membranes prepared by hypotonic lysis in 5 mM sodium phosphate, pH 8.0 (5P8), conditions in which the barbed filament ends were previously reported to be uncapped. Comparison of the amounts of adducin associated with membranes prepared in the presence and absence of magnesium reveals that 60-80% of the adducin dissociates from the membrane during hemolysis and washing in 5P8 buffer, suggesting that the barbed ends become artifactually uncapped due to loss of adducin. The erythrocyte actin filaments may thus represent a specialized class of membrane-associated actin filaments that are capped by adducin instead of capZ.

Tropomyosin isoforms in nonmuscle cells

Vertebrate nonmuscle cells, such as human and rat fibroblasts, express multiple isoforms of tropomyosin, which are generated from four different genes and a combination of alternative promoter activities and alternative splicing. The amino acid variability among these isoforms is primarily restricted to three alternatively spliced exon regions; an amino-terminal region, an internal exon, and a carboxyl-terminal exon. Recent evidence reveals that these variable exon regions encode amino acid sequences that may dictate isoform-specific functions. The differential expression of tropomyosin isoforms found in cell transformation and cell differentiation, as well as the differential localization of tropomyosin isoforms in some types of culture cells and developing neurons suggest a differential isoform function in vivo. Tropomyosin in striated muscle works together with the troponin complex to regulate muscle contraction in a Ca(2+)-dependent fashion. Both in vitro and in vivo evidence suggest that multiple isoforms of tropomyosin in nonmuscle cells may be required for regulating actin filament stability, intracellular granule movement, cell shape determination, and cytokinesis. Tropomyosin-binding proteins such as caldesmon, tropomodulin, and other unidentified proteins may be required for some of these functions. Strong evidence for the distinct functions carried out by different tropomyosin isoforms has been generated from genetic analysis of yeast and Drosophila tropomyosin mutants.

N-tropomodulin: a novel isoform of tropomodulin identified as the major binding protein to brain tropomyosin

We have identified and characterized two proteins in rat brain that bind to the neuron-specific tropomyosin isoform, TMBr3. The two proteins were identified by blot overlay assay, in which the proteins immobilized on the membrane were probed by epitope-tagged TMBr3, followed by detection with anti-epitope antibody. We have purified these proteins using a TMBr3 affinity column. Peptide sequencing as well as immunoblotting showed that one of the two proteins is identical to tropomodulin, a tropomyosin-binding protein originally identified in erythrocytes. The cDNA for the other protein was cloned from an adult rat brain cDNA library using degenerate oligonucleotides that we designed based on the peptide sequences. Sequence analysis of the cDNA clone revealed this protein to be a novel isoform of tropomodulin which is the product of a distinct gene, and is herein referred to as N-tropomodulin. Recombinant N-tropomodulin bound to TMBr3 as well as to other low molecular mass tropomyosins (TM5a or TM5), but not to high molecular mass tropomyosins (TM2 or TMBr1). Northern blotting and RNase protection assays as well as immunoblotting showed that N-tropomodulin is expressed predominantly in brain. Furthermore, RNase protection assays revealed no alternatively spliced regions within the coding sequence. Developmentally, N-tropomodulin was detected in rat brain as early as embryonic day 14 and reaches the adult level before birth. Immunofluorescence of primary frontal cortex cell cultures showed that N-tropomodulin is specifically expressed in neurons. The neuron-specific expression of N-tropomodulin strongly suggests specialized roles of this TM-binding protein in neurons.

The differential adhesion forces of anterior cruciate and medial collateral ligament fibroblasts: effects of tropomodulin, talin, vinculin, and alpha-actinin

We have determined the effects of tropomodulin (Tmod), talin, vinculin, and alpha-actinin on ligament fibroblast adhesion. The anterior cruciate ligament (ACL), which lacks a functional healing response, and the medial collateral ligament (MCL), a functionally healing ligament, were selected for this study. The micropipette aspiration technique was used to determine the forces needed to separate ACL and MCL cells from a fibronectin-coated surface. Delivery of exogenous tropomodulin, an actin-filament capping protein, into MCL fibroblasts significantly increased adhesion, whereas its monoclonal antibody (mAb) significantly decreased cell adhesiveness. However, for ACL fibroblasts, Tmod significantly reduced adhesion, whereas its mAb had no effect. mAbs to talin, vinculin, and alpha-actinin significantly decreased the adhesion of both ACL and MCL cells with increasing concentrations of antibody, and also reduced stress fiber formation and cell spreading rate as revealed by immunofluorescence microscopy. Disruption of actin filament and microtubule assembly with cytochalasin D and colchicine, respectively, also significantly reduced adhesion in ACL and MCL cells. In conclusion, both ACL and MCL fibroblast adhesion depends on cytoskeletal assembly; however, this dependence differs between ACL and MCL fibroblasts in many ways, especially in the role of Tmod. These results add yet another possible factor in explaining the clinical differences in healing between the ACL and the MCL.

Regulation of actin filament length in erythrocytes and striated muscle

Actin filaments polymerize in vitro to lengths which display an exponential distribution, yet in many highly differentiated cells they can be precisely maintained at uniform lengths in elaborate supramolecular structures. Recent results obtained using two classic model systems, the erythrocyte membrane cytoskeleton and the striated muscle sarcomere, reveal surprising similarities and instructive differences in the molecules and mechanisms responsible for determining and maintaining actin filament lengths in these two systems. Tropomodulin caps the slow-growing, pointed filament ends in muscle and in erythrocytes. CapZ caps the fast-growing, barbed filament ends in striated muscle, whereas a newly discovered barbed end capping protein, adducin, may cap the barbed filament ends in erythrocytes. The mechanisms responsible for specifying the characteristic filament lengths in these systems are more elusive and may include strict control of the relative amounts of actin filament capping proteins and side-binding proteins, molecular templates (e.g. tropomyosin and nebulin) and/or verniers (e.g. tropomyosin).

Requirement of pointed-end capping by tropomodulin to maintain actin filament length in embryonic chick cardiac myocytes

Control of actin filament length and dynamics is important for cell motility and architecture and is regulated in part by capping proteins that block elongation and depolymerization at both the fast-growing (barbed) and slow-growing (pointed) ends. Tropomodulin is a capping protein for the pointed end of the actin filament; it is associated with the free, pointed ends of the thin filaments in striated muscle, where it is thought to bind to both tropomyosin and actin. In embryonic chick cardiac myocytes, tropomodulin assembles after the thin, as well as the thick, filaments have become organized into periodic I and A bands, suggesting that tropomodulin might be involved in maintaining actin filament length. Here we show that microinjection of an antibody that inhibits tropomodulin's pointed-end-capping activity in vitro results in a marked elongation of actin filaments from their pointed ends and a > 80% reduction in the percentage of beating cells. This demonstrates that pointed-end capping by tropomodulin is required to maintain actin filament length in vivo and that this is essential for contractile function in embryonic chick cardiac myocytes.

Mechanisms of thin filament assembly in embryonic chick cardiac myocytes: tropomodulin requires tropomyosin for assembly

Tropomodulin is a pointed end capping protein for tropomyosin-coated actin filaments that is hypothesized to play a role in regulating the precise lengths of striated muscle thin filaments (Fowler, V. M., M. A. Sussman, P. G. Miller, B. E. Flucher, and M. P. Daniels. 1993. J. Cell Biol. 120:411-420; Weber, A., C. C. Pennise, G. G. Babcock, and V. M. Fowler. 1994, J. Cell Biol. 127:1627-1635). To gain insight into the mechanisms of thin filament assembly and the role of tropomodulin therein, we have characterized the temporal appearance, biosynthesis and mechanisms of assembly of tropomodulin onto the pointed ends of thin filaments during the formation of striated myofibrils in primary embryonic chick cardiomyocyte cultures. Our results demonstrate that tropomodulin is not assembled coordinately with other thin filament proteins. Double immunofluorescence staining and ultrastructural immunolocalization demonstrate that tropomodulin is incorporated in its characteristic sarcomeric location at the pointed ends of the thin filaments after the thin filaments have become organized into periodic I bands. In fact, tropomodulin assembles later than all other well characterized myofibrillar proteins studied including: actin, tropomyosin, alpha-actinin, titin, myosin and C-protein. Nevertheless, at steady state, a significant proportion (approximately 39%) of tropomodulin is present in a soluble pool throughout myofibril assembly. Thus, the absence of tropomodulin in some striated myofibrils is not due to limiting quantities of the protein. In addition, kinetic data obtained from [35S]methionine pulse-chase experiments indicate that tropomodulin assembles more slowly into myofibrils than does tropomyosin. This observation, together with results obtained using a novel permeabilized cell model for thin filament assembly, indicate that tropomodulin assembly is dependent on the prior association of tropomyosin with actin filaments. We conclude that tropomodulin is a late marker for the assembly of striated myofibrils in cardiomyocytes; its assembly appears to be linked to their maturity. We propose that tropomodulin is involved in maintaining and stabilizing the final lengths of thin filaments after they are assembled.