Nebulin as a length regulator of thin filaments of vertebrate skeletal muscles: correlation of thin filament length, nebulin size, and epitope profile

Molecular dissection of the interaction of desmin with the C-terminal region of nebulin

In vertebrate skeletal muscle, ultrastructural studies have suggested that the Z-line and extracellular intermediate filaments are linked, although a structural basis for this has remained elusive. We searched for potential novel ligands of the Z-line portion of nebulin by a yeast two-hybrid (Y2H) approach. This identified that the nebulin modules M160 to M170 interact with desmin. In desmin, deletion series experiments assigned a 19-kDa central coiled-coil domain as the nebulin-binding site. The specific interactions of nebulin and desmin were confirmed in vitro by GST pull-down experiments. In situ, the nebulin modules M176 to M181 colocalize with desmin in a Z-line-associated, striated pattern as shown by immunofluorescence studies. Our data are consistent with a model that desmin attaches directly to the Z-line through its interaction with the nebulin repeats M163-M170. This interaction may link myofibrillar Z-discs to the intermediate filament system, thereby forming a lateral linkage system which contributes to maintain adjacent Z-lines in register.

Ultrastructural and biochemical localization of N-RAP at the interface between myofibrils and intercalated disks in the mouse heart

N-RAP is a recently discovered muscle-specific protein found at cardiac intercalated disks. Double immunogold labeling of mouse cardiac muscle reveals that vinculin is located immediately adjacent to the fascia adherens region of the intercalated disk membrane, while N-RAP extends approximately 100 nm further toward the interior of the cell. We partially purified cardiac intercalated disks using low- and high-salt extractions followed by density gradient centrifugation. Immunoblots show that this preparation is highly enriched in desmin and junctional proteins, including N-RAP, talin, vinculin, beta1-integrin, N-cadherin, and connexin 43. Electron microscopy and immunolabeling demonstrate that N-RAP and vinculin are associated with the large fragments of intercalated disks that are present in this preparation, which also contains numerous membrane vesicles. Detergent treatment of the partially purified intercalated disks removed the membrane vesicles and extracted vinculin and beta1-integrin. Further separation on a sucrose gradient removed residual actin and myosin and yielded a fraction morphologically similar to fasciae adherentes that was highly enriched in N-RAP, N-cadherin, connexin 43, talin, desmin, and alpha-actinin. The finding that N-RAP copurifies with detergent-extracted intercalated disk fragments even though beta-integrin and vinculin have been completely removed suggests that N-RAP association with the adherens junction region is mediated by the cadherin system. Consistent with this hypothesis, we found that recombinant N-RAP fragments bind alpha-actinin in a gel overlay assay. In addition, immunofluorescence shows that N-RAP remains bound at the ends of isolated, detergent-treated cardiac myofibrils. These results demonstrate that N-RAP remains tightly bound to myofibrils and fasciae adherentes during biochemical purification and may be a key constituent in the mechanical link between these two structures.

Mapping protein interfaces with a fluorogenic cross-linker and mass spectrometry: application to nebulin-calmodulin complexes

Nebulin is a giant multifunctional protein that is thought to serve as both a length-regulating protein ruler and calcium/CaM-mediated regulatory protein on the thin filaments of the skeletal muscle sarcomere. To define molecular interfaces between nebulin and CaM, we thiolated lysines of CaM and ND66, a four-module cloned fragment from the C-terminus of nebulin, with 2-iminothiolane and cross-linked the complex with dibromobimane, which alkylates thiol pairs within approximately 6 A of each other to form a fluorescent adduct. Such a two-stage cross-linking generated mainly 1:1 complexes of ND66 and CaM, with a limited extent of intramolecular cross-linking. In-gel chymotryptic digestion of the dibromobimane-cross-linked complexes yielded peptides that were first screened by HPLC with fluorescence detection and then scored for cross-linking with mass spectrometry. Several inter- and intramolecular sites were identified and confirmed further by ESI-MS/MS experiments, defining molecular interfaces and patterns of protein folding. In particular, five intermolecular cross-linking products of sequences within the region of amino acids 83-99 (YKENMGKGTPLPVTPEM) in ND66 and several sequences of CaM indicate that the nebulin-CaM interface is close to, and may overlap with, the nebulin-actin interface. This proximity suggests a potential competition between CaM and actin for this nebulin interface. Intramolecular cross-linking of amino acids 13-16 (KEAF) and 13-18 (KEAFSL) with amino acids 145-148 (MTAK) and 146-148 (TAK) in CaM suggests the interaction of two lobes across the central helix. The cross-linking of amino acids 1-6 (MKTPEM) with amino acids 114-129 (YKENVGKATATPVTPE) and 115-129 (KENVGKATATPVTPE) in ND66 hints at an association of noncontiguous nebulin modules in solution.

The ultrastructure of skeletal and smooth muscle in experimental protein malnutrition in rats fed a low protein diet

Light microscopy of the pectoralis muscle of rats on a low protein diet did not show such morphological alterations as atrophy, degeneration, or sarcoplasmic edema, but electron microscopy occasionally demonstrated ultrastructural changes only in the sarcomeres of myofibrils. In the affected sarcomeres, the Z-line was disrupted and often showed a jagged structure. The Z-substance with electron opacity was frequently present flowing along the long axis of myofibrils, here referred to as the streaming of Z-lines. In addition, regular striations formed by the reciprocal arrangement of thick and thin filaments disappeared from the affected sarcomeres, though these filaments were still discernible. Two or more consecutive sarcomeres in a single myofibril were occasionally involved in these changes. A further two or more neighboring sarcomeres at the same level of myofibrils were affected transversely by these structural alterations. On the other hand, the ultrastructure of the intestinal smooth muscle was not affected by protein deficiency. The study suggests that the ultrastructural damage induced by a low protein diet is attributed to the activation of endogenous protease by the excess leaking of Ca2+ into the cytosol as a result of lipid peroxidation of cell membrane by raised free radicals, owing to the depletion of glutathione production by protein deficiency. It also suggests that the smooth muscle cells differ in their susceptibility to protein deficiency from the skeletal muscle cells.

Modular motif, structural folds and affinity profiles of the PEVK segment of human fetal skeletal muscle titin

The extension of the PEVK segment of the giant elastic protein titin is a key event in the elastic response of striated muscle to passive stretch. PEVK behaves mechanically as an entropic spring and is thought to be a random coil. cDNA sequencing of human fetal skeletal PEVK reveals a modular motif with tandem repeats of modules averaging 28 residues and with superrepeats of seven modules. Conformational studies of bacterially expressed 53-kDa fragment (TP1) by circular dichroism suggest that this soluble protein contains substantial polyproline II (PPII) type left-handed helices. Urea and thermal titrations cause gradual and reversible decrease in PPII content. The absence of sharp melting in urea and thermal titrations suggests that there is no long range cooperativity among the PPII helices. Studies with solid phase and surface plasmon resonance assays indicate that TP1 interacts with actin and some but not all cloned nebulin fragments with high affinity. Interestingly, Ca(2+)/calmodulin and Ca(2+)/S100 abolish nebulin/PEVK interaction. We suggest that in aqueous solution, PEVK is an open and flexible chain of relatively stable structural folds of the polyproline II type. PEVK region of titin may be involved in interfilament association with thin filaments in a calcium/calmodulin-sensitive manner. This adhesion may modulate titin extensibility and elasticity.

Human skeletal muscle fibres: molecular and functional diversity

Contractile and energetic properties of human skeletal muscle have been studied for many years in vivo in the body. It has been, however, difficult to identify the specific role of muscle fibres in modulating muscle performance. Recently it has become possible to dissect short segments of single human muscle fibres from biopsy samples and make them work in nearly physiologic conditions in vitro. At the same time, the development of molecular biology has provided a wealth of information on muscle proteins and their genes and new techniques have allowed analysis of the protein isoform composition of the same fibre segments used for functional studies. In this way the histological identification of three main human muscle fibre types (I, IIA and IIX, previously called IIB) has been followed by a precise description of molecular composition and functional and biochemical properties. It has become apparent that the expression of different protein isoforms and therefore the existence of distinct muscle fibre phenotypes is one of the main determinants of the muscle performance in vivo. The present review will first describe the mechanisms through which molecular diversity is generated and how fibre types can be identified on the basis of structural and functional characteristics. Then the molecular and functional diversity will be examined with regard to (1) the myofibrillar apparatus; (2) the sarcolemma and the sarcoplasmic reticulum; and (3) the metabolic systems devoted to producing ATP. The last section of the review will discuss the advantage that fibre diversity can offer in optimizing muscle contractile performance.

High-affinity actin-binding nebulin fragments influence the actoS1 complex

Human nebulin fragments, NA3 and NA4, corresponding to individual superrepeats display high-affinity interactions with individual actin protomers in cosedimentation and solid-phase binding assays. Stoichiometric analysis of nebulin fragment-induced actin polymerization and inhibition of actin-activated S1 ATPase indicate that one superrepeat influences multiple actin molecules along the F-actin filament, consistent with a combination of strong and weak interactions of nebulin over the length of the actin filament. The mechanisms by which human nebulin fragments affect the interaction between actin and myosin S1 are studied by fluorescence quenching, polarization, and resonance energy transfer. We show that, under strong binding conditions, premixing actin with the NA3 prior to adding myosin subfragment 1 (S1) inhibits the rate of actoS1 association. The nebulin fragments, NA3 and NA4, caused little effect on the extent of actoS1 binding at equilibrium but did alter the nature of the complex as evidenced by an increase in the resonance energy transfer efficiencies between S1 and actin in the absence of ATP. The addition of low concentrations of ATP rapidly dissociates the strong-binding actoS1 irrespective of the presence or absence of nebulin fragment. Interestingly, the strongly bound state reforms rapidly after S1 hydrolyzes all available ATP. These observations are consistent with the notion that nebulin might contribute to optimizing the alignment of actomyosin interactions and inhibit suboptimal actomyosin contacts.

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.

Myofibrillogenesis and formation of cell contacts mediate the localization of N-RAP in cultured chick cardiomyocytes

The expression of N-RAP was investigated in immuofluorescently stained embryonic chick cardiomyocyte cultures. After 1 day in culture, the cardiomyocytes were spherical and N-RAP, titin, alpha-actinin, and vinculin were all diffusely distributed. As the cardiomyocytes spread and formed myofibrils and cell contacts, N-RAP became localized to distinct areas in the cells. During myofibrillogenesis, N-RAP was found concentrated in premyofibrils. As the premyofibrils transformed into bundles of mature myofibrils, N-RAP became concentrated at the longitundal ends of the cells, and was not found in the mature sarcomeres. At sites of cell-cell contacts, N-RAP was localized to the cell junction even in cells without any significant myofibril formation. As the cell-cell contacts became more extensive and formed structures resembling the intercalated disks found in hearts, N-RAP became even more specifically concentrated at these junctions. The results show that myofibrillogenesis and cell contact formation can each independently target N-RAP to the longitudinal ends of cardiomyocytes.

Distinct families of Z-line targeting modules in the COOH-terminal region of nebulin

To learn how nebulin functions in the assembly and maintenance of I-Z-I bands, MYC- and GFP- tagged nebulin fragments were expressed in primary cultured skeletal myotubes. Their sites of incorporation were visualized by double staining with anti-MYC, antibodies to myofibrillar proteins, and FITC- or rhodamine phalloidin. Contrary to expectations based on in vitro binding studies, none of the nebulin fragments expressed in maturing myotubes were incorporated selectively into I-band approximately 1.0-micrometer F-alpha-actin-containing thin filaments. Four of the MYC/COOH-terminal nebulin fragments were incorporated exclusively into periodic approximately 0.1-micrometer Z-bands. Whereas both anti-MYC and Rho-phalloidin stained intra-Z-band F-alpha-actin oligomers, only the latter stained the pointed ends of the polarized approximately 1.0-micrometer thin filaments. Z-band incorporation was independent of the nebulin COOH-terminal Ser or SH3 domains. In vitro cosedimentation studies also demonstrated that nebulin SH3 fragments did not bind to F-alpha-actin or alpha-actinin. The remaining six fragments were not incorporated into Z-bands, but were incorporated (a) diffusely throughout the sarcoplasm and into (b) fibrils/patches of varying lengths and widths nested among normal striated myofibrils. Over time, presumably in response to the mediation of muscle-specific homeostatic controls, many of the ectopic MYC-positive structures were resorbed. None of the tagged nebulin fragments behaved as dominant negatives; they neither blocked the assembly nor induced the disassembly of mature striated myofibrils. Moreover, they were not cytotoxic in myotubes, as they were in the fibroblasts and presumptive myoblasts in the same cultures.

Effects of thiol protease inhibitors on myoblast fusion and myofibril assembly in vitro

To investigate the roles of thiol proteases such as cathepsins and calpains in muscle differentiation, we have treated primary cultures of pectoralis muscle with a variety of protease inhibitors and examined the effects these agents have on myoblast fusion and myofibrillogenesis. We have found that a membrane-permeable inhibitor, E64D, has dramatic effects on both events of muscle differentiation. Cells treated with this inhibitor display gross morphological changes, severe delays in myofibril assembly, and reduced ability to fuse in culture. These morphological changes are correlated with a build up of beta1-integrin throughout the cytoplasm. These effects could also be produced using NH4Cl, a lysosomotrophic agent. In addition, we show that two nonpermeable inhibitors (leupeptin and E64) slightly decrease myoblast fusion, but have no effects on the ability of the cells to form mature myofibrils. These results are discussed in terms of their relevance to the inheritable disease of muscular dystrophy and I-cell disease (mucolipodosis II).

Complete cDNA sequence and tissue localization of N-RAP, a novel nebulin-related protein of striated muscle

We have cloned and sequenced the full-length cDNA of N-RAP, a novel nebulin-related protein, from mouse skeletal muscle. The N-RAP message is specifically expressed in skeletal and cardiac muscle, but is not detected by Northern blot in non-muscle tissues. The full-length N-RAP cDNA contains an open reading frame of 3,525 base pairs which is predicted to encode a protein of 133 kDa. A 587 amino acid region near the C-terminus is 45% identical to the actin binding region of human nebulin, containing more than 2 complete 245 residue nebulin super repeats. The N-terminus contains the consensus sequence of a cysteine-rich LIM domain, which may function in mediating protein-protein interactions. These data suggest that the encoded protein may link actin filaments to some other proteins or structure. We expressed full-length N-RAP in Escherichia coli, as well as the nebulin-like super repeat region of N-RAP (N-RAP-SR) and the region between the LIM domain and N-RAP-SR (N-RAP-IB). An anti-N-RAP antibody raised against a 30 amino acid peptide corresponding to sequence from N-RAP-IB detected recombinant N-RAP and N-RAP-IB, but failed to detect N-RAP-SR. This antibody specifically identified a 185 kDa band as N-RAP on immunoblots of mouse skeletal and cardiac muscle proteins. In an assay of actin binding to electrophoresed and blotted proteins, we detected significant actin binding to expressed nebulin super repeats and N-RAP-SR, but only a trace amount of binding to N-RAP-IB. In immunofluorescence experiments, N-RAP was found to be localized at the myotendinous junction in mouse skeletal muscle and at the intercalated disc in cardiac muscle. Based on its domain organization, actin binding properties, and tissue localization, we propose that N-RAP plays a role in anchoring the terminal actin filaments in the myofibril to the membrane and may be important in transmitting tension from the myofibrils to the extracellular matrix.

Inhibition of nebulin and connectin (titin) for assembly of actin filaments during myofibrillogenesis

In order to examine the role of cytoskeletal scaffolding proteins, nebulin and connectin (titin), in actin dynamics during myofibrillogenesis, rhodamine (rh)-labeled actin was microinjected into cultured skeletal muscle cells in which the function of these proteins had been inhibited with their respective antibodies. In the nebulin function-inhibited cells, exogenously introduced actin formed irregularly distributed amorphous patches or bright foci inside the cells, but it was not incorporated into myofibrillar structures at any stage. Thus, the blockage of actin binding sites of nebulin seems to inhibit the association of actin monomers to the preexisting nebulin scaffold. In the cells inhibited with anti-connectin antibody, incorporation of rh-actin was similar to that in antibody-uninjected cells. These results support the idea that nebulin is related to the accessibility/exchangeability of actin into nascent myofibrils, but connectin does not have such a role in actin assembly. Since all antibodies recognizing different domains of nebulin filaments blocked actin incorporation along the entire length of actin filaments, inhibition of any domains of nebulin filaments seems to affect actin dynamics.

Terminal regions of mouse nebulin: sequence analysis and complementary localization with N-RAP

The regions of mouse nebulin extending from the ends of the super repeats to the C-terminus and N-terminus were cloned and sequenced. Comparison of the mouse sequence with the previously published human sequence shows that the terminal regions of nebulin are highly conserved. The four phosphorylation motifs and SH3 domain found at the C-terminus of mouse nebulin are identical to those found in human nebulin, with the exception of four conservative substitutions. The modules linking this C-terminal region to the super repeats have deletions relative to both fetal and adult human nebulins that correspond to integral numbers of modules, making the mouse C-terminal simple repeat region among the shortest observed to date. The N-terminal region and the C-terminal modules were expressed in Escherichia coli and used for antibody production. Immunofluorescent labeling of these regions of nebulin in isolated myofibrils demonstrates that they are located near the center of the sarcomere and near the Z-line, respectively. Immunogold labeling with antibodies raised against the N-terminal nebulin sequence localizes this region in the A-band near the tips of the thin filaments. Nebulin localization is complementary to that of N-RAP, another muscle-specific protein containing nebulin-like super repeats; nebulin is exclusively found in the sarcomeres, while N-RAP is confined to the terminal bundles of actin filaments at the myotendinous junction. Cell Motil. Cytoskeleton 3:211-222, 2000 Published 2000 Wiley-Liss, Inc.

Decreased thin filament density and length in human atrophic soleus muscle fibers after spaceflight

Soleus muscle fibers were examined electron microscopically from pre- and postflight biopsies of four astronauts orbited for 17 days during the Life and Microgravity Sciences Spacelab Mission (June 1996). Myofilament density and spacing were normalized to a 2. 4-microm sarcomere length. Thick filament density ( approximately 1, 062 filaments/microm(2)) and spacing ( approximately 32.5 nm) were unchanged by spaceflight. Preflight thin filament density (2, 976/microm(2)) decreased significantly (P < 0.01) to 2,215/microm(2) in the overlap A band region as a result of a 17% filament loss and a 9% increase in short filaments. Normal fibers had 13% short thin filaments. The 26% decrease in thin filaments is consistent with preliminary findings of a 14% increase in the myosin-to-actin ratio. Lower thin filament density was calculated to increase thick-to-thin filament spacing in vivo from 17 to 23 nm. Decreased density is postulated to promote earlier cross-bridge detachment and faster contraction velocity. Atrophic fibers may be more susceptible to sarcomere reloading damage, because force per thin filament is estimated to increase by 23%.

Architecture of the thin filament-Z-line junction: lessons from nebulette and nebulin homologies

Nebulette and nebulin are homologous proteins associated with the Z-lines of cardiac and skeletal muscle sarcomeres. Although these proteins share 70% sequence homology and an identical domain layout, nebulette is one-tenth the size of nebulin. To define structurally important features of these proteins in terms of the Z-line architecture, we have analyzed the primary structure of nebulette and nebulin from a variety of species and developmental stages. Alignment of the 35 residue nebulin-like modules from both proteins demonstrates that the individual modules share 30-90% homology across the six proteins analyzed. In addition, this analysis demonstrates a number of areas in which the identity across the six proteins is as high as 75%. These areas may be important signals for Z-line assembly and function in the striated muscles. Significantly, most of the areas of high identity also coincide with consensus phosphorylation sites. To evaluate if nebulette, like nebulin, exhibits tissue-specific and developmental specific polymorphism, a series of immunoblot assays were performed. These data demonstrate that nebulettes from different portions of the heart are the same size. Comparison of nebulette from embryonic and adult cardiac muscle also demonstrates that this protein does not appear to vary in size with developmental stage. Consistent with the large number of consensus phosphorylation sites identified in the nebulette primary structure, we find that nebulette is phosphorylated in the cardiac muscle at serine and threonine residues. These data and sequence analyses are discussed in terms of current models for Z-line architecture.

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.

Functional dissection of nebulette demonstrates actin binding of nebulin-like repeats and Z-line targeting of SH3 and linker domains

Nebulette, a 107 kDa protein associated with the I-Z-I complex of cardiac myofibrils, may play an important role in the assembly of the Z-line. Determination of the complete primary structure of 1011 residue human fetal nebulette reveals a four-domain layout similar to skeletal muscle nebulin: a short N-terminal domain, followed by 22 nebulin-like repeats that are linked to a C-terminal Src homology 3 (SH3) domain via a short linker domain. To elucidate the mechanisms of assembly for nebulette in the Z-line, the complete coding sequence or fusions of nebulette domains with green fluorescent protein (GFP) were expressed in cardiomyocytes and fibroblasts. The complete protein localized to Z-lines in cardiac cells and to dense bodies in nonmuscle cells. The GFP-repeat domain forms bundles that are associated with actin filaments in both cell types and disrupts the microfilament network. In contrast, the GFP-repeat plus linker shows limited interaction with dense bodies in nonmuscle cells and the Z-lines of cardiomyocytes. Interestingly, the tagged linker or SH3 is diffusely distributed in nonmuscle cells, but localizes to the Z-lines in cardiomyocytes. Supporting the cellular localization work, recombinant nebulette fragments bind to actin, tropomyosin, and alpha-actinin in in vitro binding assays. These results suggest the repeat domain contains actin binding functions and that the linker domain may target this interaction to Z-lines and dense bodies. Our data also indicate that the linker and SH3 domains can distinguish between dense bodies and Z-lines, suggesting that the ligands for their interactions are specific to these muscular substructures.

Molecular interactions of N-RAP, a nebulin-related protein of striated muscle myotendon junctions and intercalated disks

N-RAP is a recently discovered muscle-specific protein that is concentrated at the myotendon junctions in skeletal muscle and at the intercalated disks in cardiac muscle. The C-terminal half of N-RAP contains a region with sequence homology to nebulin, while a LIM domain is found at its N-terminus. N-RAP is hypothesized to perform an anchoring function, linking the terminal actin filaments of myofibrils to protein complexes located beneath the sarcolemma. We used a solid-phase assay to screen myofibrillar and junctional proteins for binding to several recombinant fragments of N-RAP, including the nebulin-like super repeat region (N-RAP-SR), the N-terminal half including the LIM domain (N-RAP-NH), and the region of N-RAP between the super repeat region and the LIM domain (N-RAP-IB). Actin is the only myofibrillar protein tested that exhibits specific binding to N-RAP, with high-affinity binding to N-RAP super repeats, and 10-fold weaker binding to N-RAP-IB. In contrast, myosin, isolated myosin heads, tropomyosin, and troponin exhibited no specific interaction with N-RAP domains. A recombinant fragment corresponding to the C-terminal one-fourth of vinculin also binds specifically to N-RAP super repeats, while no specific N-RAP binding activity was observed for other regions of the vinculin molecule. Finally, talin binds with high affinity to the LIM domain of N-RAP. These results support our hypothesis that N-RAP is part of a complex of proteins that anchors the terminal actin filaments of the myofibril to the membrane, and functions in transmitting tension from the myofibrils to the extracellular matrix.

Characterization of nebulette and nebulin and emerging concepts of their roles for vertebrate Z-discs

Nebulin is an 800 kDa large actin-binding protein specific to skeletal muscle and thought to act as a molecular template that regulates the length of thin filaments. Recently, a 100 kDa nebulin-like protein has been described in the avian cardiac muscle and referred to as nebulette. We have determined the full-length (8 kb) cDNA sequence of the human nebulette. Its open reading frame (3044 bp) encodes a 109 kDa protein that shares extensive similarity with the C-terminal region of human nebulin. The C-terminal regions of nebulin and nebulette are identical in domain organization and share a family of highly related C-terminal repeats, a serine-rich domain with potential phosphorylation sites, and an SH3 domain. Immunoelectron-microscopy suggests that the C-terminal 30 kDa of nebulin and nebulette filaments integrate into the Z-disc lattice, whereas their N termini appear to project into the I-band. Gene mapping studies assign the human nebulette gene to chromosome 10p12, whereas the nebulin gene has been previously assigned to 2q21. Evolutionary constraints appear to have maintained identical modular arrangements in these two independent genes. Comparison of nebulin and nebulette cDNAs demonstrates that a subgroup of repeats within the C-terminal regions is regulated tissue-specifically and stage-dependently during development of both molecules. This leads to a substantial diversity of nebulin and nebulette isoforms. Their further study is likely to provide insights into how they contribute to the molecular diversity of Z-discs from different muscle tissues and fiber types.

A six-module human nebulin fragment bundles actin filaments and induces actin polymerization

We have investigated the interaction of a 6-repeat recombinant human nebulin fragment (S6R2R7) with F-actin, with Mg2+-induced actin paracrystals, and G-actin, respectively. This fragment corresponds to super-repeat 6, repeat 2 to 7 of human nebulin, and is located in the N-terminal part of the super-repeat region of the nebulin molecule. The S6R2R7 fragment included an immuno-tag of three amino-acid residues (EEF) at one end which was detectable by a monoclonal anti-tubulin YL1/2. By a cosedimentation assay, interaction between F-actin and S6R2R7 was observed. Electron microscopy revealed the formation of large bundle-like aggregates containing highly parallelized actin filaments, apparently caused by actin bundling of the nebulin fragment. Compared with Mg2+-induced actin paracrystals where the helices of the actin filaments are arranged in register, the filaments in the actin-nebulin bundles seem to be packed in a different way and show no obvious periodicity. The bundles were also visible in the light microscope, and immunofluorescence microscopy revealed binding of the nebulin fragment S6R2R7 to both preformed Mg2+ paracrystals and to F-actin. We also analyzed the effect of S6R2R7 on actin under non-polymerizing conditions by cosedimentation assays and pyrene actin fluorimetry, as well as fluorescence microscopy and electron microscopy. Nebulin-induced actin polymerization was observed with an enhancement of the nucleation step indicating a stabilization of actin nuclei by S6R2R7. Light and electron microscopy revealed bundle-like actin-nebulin aggregates similar to those formed by pre-assembled F-actin and S6R2R7. Thus, even in the absence of salt, S6R2R7 promotes actin polymerization and induces formation of tightly packed actin filament bundles. We assume that the actin filaments are crosslinked by the nebulin fragments, indicating a rather low cooperativity of binding to a single filament.

SH3 in muscles: solution structure of the SH3 domain from nebulin

The huge modular protein nebulin is located in the thin filament of striated muscle in vertebrates and is thought to bind and stabilize F-actin. The C-terminal part of human nebulin is anchored in the sarcomeric Z-disk and contains an SH3 domain, the first of such motifs to be identified in a myofibrillar protein. We have determined the nebulin SH3 sequence from several species and found it strikingly conserved. We have also shown that the SH3 transcripts are constitutively expressed in skeletal muscle tissues. As the first step towards a molecular understanding of nebulin's cellular role we have determined the three-dimensional structure of the human nebulin SH3 domain in solution by nuclear magnetic resonance (NMR) spectroscopy and compared it with other known SH3 structures. The nebulin SH3 domain has a well-defined structure in solution with a typical SH3 topology, consisting of a beta-sandwich of two triple-stranded, antiparallel beta-sheets arranged at right angles to each other and of a single turn of a 310-helix. An additional double-stranded antiparallel beta-sheet in the RT loop bends over the beta-sandwich. The derived structure reveals a remarkable similarity with a distinct subset of SH3 domains, especially in the structural features of the exposed hydrophobic patch that is thought to be the site of interaction with polyproline ligands. On the basis of this similarity, we have modelled the interaction with an appropriate polyproline ligand and attempted to delineate the characteristics of the physiological SH3-binding partner in the Z-disk. Our results represent the first step in reconstructing the structure of nebulin and are expected to contribute to our understanding of nebulin's functional role in myofibrillar assembly.

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.

Expression and purification of large nebulin fragments and their interaction with actin

cDNA clones encoding mouse skeletal muscle nebulin were expressed in Escherichia coli as thioredoxin fusion proteins and purified in the presence of 6 M urea. These fragments, called 7a and 8c, contain 28 and 19 of the weakly repeating approximately 35-residue nebulin modules, respectively. The nebulin fragments are soluble at extremely high pH, but aggregate when dialyzed to neutral pH, as assayed by centrifugation at 16,000 x g. However, when mixed with varying amounts of G-actin at pH 12 and then dialyzed to neutral pH, the nebulin fragments are solubilized in a concentration-dependent manner, remaining in the supernatant along with the monomeric actin. These results show that interaction with G-actin allows the separation of insoluble nebulin aggregates from soluble actin-nebulin complexes by centrifugation. We used this property to assay the incorporation of nebulin fragments into preformed actin filaments. Varying amounts of aggregated nebulin were mixed with a constant amount of F-actin at pH 7.0. The nebulin aggregates were pelleted by centrifugation at 5200 x g, whereas the actin filaments, including incorporated nebulin fragments, remained in the supernatant. Using this assay, we found that nebulin fragments 7a and 8c bound to actin filaments with high affinity. Immunofluorescence and electron microscopy of the actin-nebulin complexes verified that the nebulin fragments were reorganized from punctate aggregates to a filamentous form upon interaction with F-actin. In addition, we found that fragment 7a binds to F-actin with a stoichiometry of one nebulin module per actin monomer, the same stoichiometry we found in vivo. In contrast, 8c binds to F-actin with a stoichiometry of one module per two actin monomers. These data indicate that 7a can be incorporated into actin filaments to the same extent found in vivo, and suggest that shorter fragments may not bind actin filaments in the same way as the native nebulin molecule.

Phosphorylation of a high molecular weight (approximately 600 kDa) protein regulates catch in invertebrate smooth muscle

A unique property of smooth muscle is its ability to maintain force with a very low expenditure of energy. This characteristic is highly expressed in molluscan smooth muscles, such as the anterior byssus retractor muscle (ABRM) of Mytilus edulis, during a contractile state called 'catch'. Catch occurs following the initial activation of the muscle, and is characterized by prolonged force maintenance in the face of a low [Ca2+]i, high instantaneous stiffness, a very slow cross-bridge cycling rate, and low ATP usage. In the intact muscle, rapid relaxation (release of catch) is initiated by serotonin, and mediated by an increase in cAMP and activation of protein kinase A. We sought to determine which proteins undergo a change in phosphorylation on a time-course that corresponds to the release of catch in permeabilized ABRM. Only one protein consistently satisfied this criterion. This protein, having a molecular weight of approximately 600 kDa and a molar concentration about 30 times lower than the myosin heavy chain, showed an increase in phosphorylation during the release of catch. Under the mechanical conditions studied (rest, activation, catch, and release of catch), changes in phosphorylation of all other proteins, including myosin light chains, myosin heavy chain and paramyosin, are minimal compared with the cAMP-induced phosphorylation of the approximately 600 kDa protein. Under these conditions, somewhat less than one mole of phosphate is incorporated per mole of approximately 600 kDa protein. Inhibition of A kinase blocked both the cAMP-induced increase in phosphorylation of the protein and the release of catch. In addition, irreversible thiophosphorylation of the protein prevented the development of catch. In intact muscle, the degree of phosphorylation of the protein increases significantly when catch is released with serotonin. In muscles pre-treated with serotonin, a net dephosphorylation of the protein occurs when the muscle is subsequently put into catch. We conclude that the phosphorylation state of the approximately 600 kDa protein regulates catch.

Titin elasticity and mechanism of passive force development in rat cardiac myocytes probed by thin-filament extraction

Titin (also known as connectin) is a giant filamentous protein whose elastic properties greatly contribute to the passive force in muscle. In the sarcomere, the elastic I-band segment of titin may interact with the thin filaments, possibly affecting the molecule's elastic behavior. Indeed, several studies have indicated that interactions between titin and actin occur in vitro and may occur in the sarcomere as well. To explore the properties of titin alone, one must first eliminate the modulating effect of the thin filaments by selectively removing them. In the present work, thin filaments were selectively removed from the cardiac myocyte by using a gelsolin fragment. Partial extraction left behind approximately 100-nm-long thin filaments protruding from the Z-line, whereas the rest of the I-band became devoid of thin filaments, exposing titin. By applying a much more extensive gelsolin treatment, we also removed the remaining short thin filaments near the Z-line. After extraction, the extensibility of titin was studied by using immunoelectron microscopy, and the passive force-sarcomere length relation was determined by using mechanical techniques. Titin's regional extensibility was not detectably affected by partial thin-filament extraction. Passive force, on the other hand, was reduced at sarcomere lengths longer than approximately 2.1 microm, with a 33 +/- 9% reduction at 2.6 microm. After a complete extraction, the slack sarcomere length was reduced to approximately 1.7 microm. The segment of titin near the Z-line, which is otherwise inextensible, collapsed toward the Z-line in sarcomeres shorter than approximately 2.0 microm, but it was extended in sarcomeres longer than approximately 2.3 microm. Passive force became elevated at sarcomere lengths between approximately 1.7 and approximately 2.1 microm, but was reduced at sarcomere lengths of >2.3 microm. These changes can be accounted for by modeling titin as two wormlike chains in series, one of which increases its contour length by recruitment of the titin segment near the Z-line into the elastic pool.

Effects of aging on shortening velocity and myosin isoform composition in single human skeletal muscle cells

Maximum unloaded shortening velocity (V0) and maximum force normalized to cross-sectional fiber area (specific tension) were determined in 400 single quadriceps muscle cells of young (n = 4, 25-31 yr) and old (n = 4, 73-81 yr) men. Two of the old men were physically very active, and the subjects were divided into young control, old control, and old physically active groups. The expressions of types I, IIa, and IIb myosin heavy chains (MHC) and essential and regulatory myosin light chains were determined by 6 and 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis. V0 was highly dependent on the MHC isoform composition, and a gradient from slow to fast was observed in the sequence I, I-IIa, IIa, IIab, and IIb MHC cells. The V0 values and specific tensions of types I and IIa MHC fibers in the young control group were significantly higher than those in the old control and old physically active groups. Thus the present results provide evidence of qualitative changes in contractile properties of human skeletal muscle in old age, which probably play an important role in the age-related impairment of skeletal muscle function.

Is nebulin truly a component of the thin filament?

Thin filaments were prepared from rabbit and beef skeletal muscle with three different procedures, both at high and low ionic strength. Nebulin was always found to be associated with the myosin fraction and was always absent from the thin filament fraction.

cDNA cloning of mouse nebulin. Evidence that the nebulin-coding sequence is highly conserved among vertebrates

Nebulin is a family of giant myofibrillar proteins with molecular masses ranging over 700-900 kDa. Using a human nebulin cDNA probe, we isolated three nebulin cDNA clones from a mouse skeletal muscle cDNA library. These three clones, labeled 8c. 7a and 4b. carry inserts of 2.0, 3.0 and 3.5 kb, respectively. In Northern blots, each insert detected the same approximately = 25 kb message from skeletal muscle as the human nebulin probe, while detecting no messages from cardiac muscle. Sequence data in combination with reverse-transcriptase PCR indicates that clones 7a and 8c overlap to form 4076 bp contiguous sequence. Alignment with the published full-length human nebulin sequence indicates that clone 4b overlaps with clone 7a over 1596 bp. However, after the first 798-bp overlap, the sequence of these two mouse nebulin clones diverge, suggesting that they derive from distinct transcripts encoding isoforms of mouse nebulin. The mouse nebulin clones encode a series of = 245-residue super repeats, each of which can be subdivided into seven = 35-residue, weakly repeating modules centered around a conserved tyrosine residue, consistent with the human nebulin sequence. The mouse nebulin clones align along the central third of the full-length human sequence, corresponding to super repeats 8-16 of the 22 super repeats found in human nebulin. The translated sequence is greater than 90% identical to the human sequence, with the exception of a 200-amino-acid region at the C-terminus of clone 4b, which is less than 60% identical. In genomic Southern blots, a mouse nebulin probe detected a homologous sequence in a wide variety of vertebrate species under stringent conditions. However, no significant hybridization was observed to genomic DNA from invertebrates and microorganisms, even under very low stringency. The sequence and Southern-blot data suggest that the nebulin sequence is highly conserved among vertebrate species.

A study on the mechanism of phalloidin-induced tension changes in skinned rabbit psoas muscle fibres

The time course of phalloidin induced changes in isometric tension of partially activated skinned rabbit psoas fibres was studied as a function of both phalloidin concentration and time of pre-incubation with phalloidin. Upon addition of phalloidin to non-pretreated (control) fibres there was a fall in tension followed by an increase in tension. The latency of both parts of the response was inversely related to the phalloidin concentration in the range 40-130 microM phalloidin. By preincubating the fibres with phalloidin for varying periods of time it was possible to obtain responses which appeared to represent later portions of the control response. Thus after pre-treatment with 40 microM phalloidin in either rigor or relaxing solution for 5 min (the time corresponding to minimal tension in the control response) the tension response resembled that of the control, beginning from the vicinity of the minimum. The pattern of staining of the fibres by rhodamine-phalloidin was analysed by laser confocal microscopy to relate the mechanical response to phalloidin localization. If fibres were treated with rhodamine-phalloidin for 20-25 min there was a labelling of the I-Z-I segment with intense peaks of fluorescence at the Z-line and the ends of the I filaments. If fibres were pre-incubated for 5 min with phalloidin and then labelled with rhodamine-phalloidin the fluorescence at the Z-line and at the ends of the I filaments was suppressed and the peak of the fluorescence intensity was shifted toward the middle part of the I filament. The data indicate that the decrease in tension caused by phalloidin was associated with binding of phalloidin to the pointed ends of actin filament and the Z-line region, whereas the increase in tension occurred when phalloidin was bound along entire length of the actin filament.

Human skeletal muscle nebulin sequence encodes a blueprint for thin filament architecture. Sequence motifs and affinity profiles of tandem repeats and terminal SH3

Analysis of deduced protein sequence and structural motifs of approximately 5500 residues of human fetal skeletal muscle nebulin reveals the design principles of this giant multifunctional protein in the sarcomere. The bulk of the sequence is constructed of approximately 150 tandem copies of approximately 35-residue modules that can be classified into seven types. The majority of these modules form 20 super-repeats, with each super-repeat containing a 7-module set (one of each type in the same order). These super-repeats are further divided into eight segments: with six segments containing adjacent, highly homologous super-repeats, one single repeat segment consisting of 8 nebulin modules of the same type, and a non-repeat segment terminating with a SH3 domain at the C terminus. The interactions of actin, tropomyosin, troponin, and calmodulin with nebulin fragments consisting of either repeating modules or the SH3 domain support its role as a giant actin-binding cofilament of the composite thin filament. Such affinity profiles also suggest that nebulin may bind to tropomyosin and troponin to form a composite calcium-linked regulatory complex on the thin filament. The modular construction, super-repeat structure, and segmental organization of nebulin sequence appear to encode thin filament length, periodicity, insertion, and sarcomere proportion in the resting muscle.

Nonuniform elasticity of titin in cardiac myocytes: a study using immunoelectron microscopy and cellular mechanics

Titin (also known as connectin) is a muscle-specific giant protein found inside the sarcomere, spanning from the Z-line to the M-line. The I-band segment of titin is considered to function as a molecular spring that develops tension when sarcomeres are stretched (passive tension). Recent studies on skeletal muscle indicate that it is not the entire I-band segment of titin that behaves as a spring; some sections are inelastic and do not take part in the development of passive tension. To better understand the mechanism of passive tension development in the heart, where passive tension plays an essential role in the pumping function, we investigated titin's elastic segment in cardiac myocytes using structural and mechanical techniques. Single cardiac myocytes were stretched by various amounts and then immunolabeled and processed for electron microscopy in the stretched state. Monoclonal antibodies that recognize different titin epitopes were used, and the locations of the titin epitopes in the sarcomere were studied as a function of sarcomere length. We found that only a small region of the I-band segment of titin is elastic; its contour length is estimated at approximately 75 nm, which is only approximately 40% of the total I-band segment of titin. Passive tension measurements indicated that the fundamental determinant of how much passive tension the heart develops is the strain of titin's elastic segment. Furthermore, we found evidence that in sarcomeres that are slack (length, approximately 1.85 microns) the elastic titin segment is highly folded on top of itself. Based on the data, we propose a two-stage mechanism of passive tension development in the heart, in which, between sarcomere lengths of approximately 1.85 microns and approximately 2.0 microns, titin's elastic segment straightens and, at lengths longer than approximately 2.0 microns, the molecular domains that make up titin's elastic segment unravel. Sarcomere shortening to lengths below slack (approximately 1.85 microns) also results in straightening of the elastic titin segment, giving rise to a force that opposes shortening and that tends to bring sarcomeres back to their slack length.

Assembly of nebulin into the sarcomeres of avian skeletal muscle

In developing muscle, relatively little is known about the synthesis and incorporation of the large actin binding protein, nebulin, into the sarcomere. To determine the temporal pattern of nebulin assembly into the myofibrils of differentiating skeletal muscle cells, myofibril assembly was examined by immunofluorescence microscopy. The distribution of nebulin was compared to other myofibrillar and cytoskeletal proteins (myosin, titin, actin, desmin, tubulin). At the onset of differentiation, we observed that nebulin is first seen in a diffuse distribution throughout the cytoplasm. At this time, muscle specific myosin and titin are also distributed in this manner. Myosin and titin become associated with the nascent myofibrils prior to the addition of nebulin. The mature striated pattern of myosin and titin also preceded the development of striations with nebulin. After nebulin becomes organized into a striated pattern, actin filaments separate across the A-band and form thin filaments of uniform length. These patterns of assembly suggest that nebulin is required for restricting the lengths of the thin filaments. We have employed the strategy of using ethyl methane sulfonate and taxol to perturb myofibril assembly to examine interactions critical for the addition of nebulin to the developing sarcomeres. The same temporal pattern of assembly seen in the normal cultures was observed in the ethyl methane sulfonate treated cultures, but at a much slower rate. In cultures treated with the microtubule stabilizing drug taxol, the amount of stress fibers and nascent I-bands was greatly diminished as previously reported by others; however, nebulin was found associated with myofibrils in a mature striated distribution. In addition, our results indicate that the taxol treated cultures contain remnants of the Z-line. These results suggest that nebulin assembly into the myofibril requires interactions or anchorage at the Z-line and within the A-band.

Phalloidin unzips nebulin from thin filaments in skeletal myofibrils

Fluorescent phallotoxins such as rhodamine-phalloidin take hours to bind uniformly to thin filaments of skeletal myofibrils, after fast initial binding to both ends of thin filaments. Observation of this process in skeletal and cardiac myofibrils and of the resulting re-distribution of nebulin using anti-nebulin antibody showed that: (1) rhodamine-phalloidin binds uniformly to actin in cardiac myofibrils within minutes, in contrast to skeletal myofibrils; (2) overnight pre-incubation of skeletal myofibrils with phalloidin results in uniform initial binding of rhodamine-phalloidin and a changed nebulin localization; (3) pre-incubation of skeletal myofibrils with Ca(2+)-calmodulin results in uniform initial binding of rhodamine-phalloidin; (4) the binding of rhodamine-phalloidin to actin in skeletal myofibrils is unidirectional, i.e. the fluorescence of incorporated rhodamine-phalloidin moves from the pointed ends where it is bound initially toward the barbed end at the Z-band; (5) the unidirectional binding of rhodamine-phalloidin results in redistribution of nebulin, i.e. the initial fluorescent bands associated with the epitopes of bound nebulin antibody change to a single band located close to Z-line. These results indicate that nebulin inhibits rhodamine-phalloidin binding to actin and suggests that the unidirectional rhodamine-phalloidin binding may be due to cooperative competitive binding, i.e. phalloidin ‘unzips’ nebulin starting from the pointed ends of the thin filaments.

Contractile properties in single muscle fibres from chronically overused motor units in relation to motoneuron firing properties in prior polio patients

The relation between motoneuron firing rate in vivo and maximum velocity of unloaded shortening (Vmax) and myosin isoform composition in single chemically skinned muscle fibres was investigated in chronically overused motor units. Ten patients with loss of a large proportion of the motoneuron pool due to a prior polio lesion and compensatory overuse of residual neurones were studied and compared with normal individuals. The tibialis anterior muscle (TA) was chosen and prior polio patients who used all residual TA motor units at high rates during the normal step cycle were selected. In prior polio patients, all motor units fired at approximately 40 Hz when maximum voluntary force was reached. A common firing rate of 30 Hz yielded 70-90% maximum force. In normal subjects, on the other hand, maximum TA force was reached when low threshold units fired at 25-30 Hz and high threshold units at 50 Hz. Myosin heavy chain (MHC) and light chain (MLC) isoforms were resolved by 6% and 12% sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), respectively, and quantified densitometrically. In the whole biopsy cross-sections, types I, IIA and IIB MHC proportions were 97, 3 and 0% in a typical prior polio patient and 65, 25 and 10% in an age- and sex-matched control subject. Vmax differed significantly (p < 0.001) between type I fibres from the patient (0.54 +/- 0.12 ML/s) and the control subject (0.29 +/- 0.08 ML/s). The composition and relative contents of essential and regulatory MLC isoforms differed in single type I MHC fibres from the control subject and prior polio patient. 65% of the fibres co-expressed the fast and slow isoform of the regulatory light chain (MLC2) in the patient, while this combination was only observed in one of the control type I fibres. All prior polio fibres with a Vmax higher than 0.45 ML/s, except one, co-expressed MLC2s and MLC2f and the only control fibre co-expressing the slow and fast MLC2 isoform had the highest Vmax (0.50 ML/s) among control fibres. On the other hand, a high relative content of MLC3 was not associated with a high Vmax in type I MHC fibres. It is suggested that the composition of fast and slow isoforms of MLC2 has a significant modulatory influence on Vmax within type I MHC fibres. This combination of MLCs and high Vmax in type I MHC fibres is probably induced by chronic motor unit overuse and an altered motoneuron firing pattern.

Characterization of a novel 350-kilodalton nuclear phosphoprotein that is specifically involved in mitotic-phase progression

A gene assigned to human chromosome 1q32-41 encodes a novel protein of 3,113 amino acids containing an internal tandem repeat of 177 amino acids. The protein, which we have named "mitosin," was identified by direct binding to purified retinoblastoma protein in vitro with a region distantly related to the retinoblastoma protein-binding site of E2F-1. Mitosin is expressed throughout S, G2, and M phases of the cell cycle but is absent in G1. Its localization is dramatically reorganized from a rather homogeneous nuclear distribution in S phase to paired dots at the kinetochore/centromere region, to the spindle apparatus, and then to the midbody during M-phase progression. This spatial reorganization coincides closely with the temporal phosphorylation patterns of mitosin. Overexpression of N-terminally truncated mutants blocks cell cycle progression mainly at G2/M. These results suggest that mitosin may play an important role in mitotic-phase progression.

Immunocytochemistry of the muscle cell cytoskeleton

The muscle cell cytoskeleton is defined for this review as any structure or protein primarily involved in linking or connecting protein filaments to each other or to anchoring sites. In striated muscle, the M line connects thick filaments at their centers to adjacent thick filaments. Titin forms elastic filaments that extend from the M line to the Z line and may contribute to the resting tension properties of striated muscle. Nebulin forms inextensible filaments in skeletal muscle that are closely associated with thin filaments and that may provide a length template for thin filaments. Z lines anchor thin filaments from adjacent sarcomeres via the actin-binding function of alpha-actinin. Other proteins located at the Z line include Cap Z, Z-nin, Z protein, and zeugmatin. Intermediate filaments connect myofibrils to each other at the level of the Z line and to the sarcolemma at the Z- and possibly the M-line levels. Immunolocalization has identified the adhesion plaque proteins spectrin, vinculin, dystrophin, ankyrin, and talin at subsarcolemmal sites where they may be involved with filament attachment. Smooth muscle cell cytoskeletons are believed to include membrane associated dense bodies (MADBs), intermediate filaments, cytoplasmic dense bodies (CDBs), and perhaps a subset of actin filaments. MADBs contain a menu of attachment plaque proteins and anchor both thin filaments and intermediate filaments to the sarcolemma. CDBs are intracellular analogs of striated muscle Z lines and anchor thin filaments and intermediate filaments.

Changes in the molecular types of connectin and nebulin during development of chicken skeletal muscle

Changes in the molecular types of connectin and nebulin during development of chicken breast and leg muscles were determined by an improved SDS-polyacrylamide gel electrophoresis (PAGE) using 2% polyacrylamide slab gel. The adult leg-type alpha-connectin (alpha L-connectin) and nebulin (L-nebulin) appeared in embryonic breast muscle, and changed into the adult breast-type ones (alpha B-connectin, B-nebulin) specific for adult breast muscle after hatching. In leg muscle, alpha L-connectin and L-nebulin appeared in an embryonic stage, and remained unchanged in molecular types throughout the entire process of development. alpha-Connectin and nebulin seemed to be regulated by a similar mechanism during development. On the other hand, beta-connectin appeared in an earlier stage of development of the embryonic breast muscle, independently of alpha-connectin.

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.

The complete primary structure of human nebulin and its correlation to muscle structure

Nebulin is a giant filamentous protein specific for vertebrate skeletal muscles. The correlation of its size to thin filament lengths in vertebrates suggests that nebulin may function as a molecular ruler to determine thin filament length. We have isolated a full-length cDNA of 20.8 kb encoding human nebulin and determined its sequence. The cDNA's predicted peptide has a molecular weight of 773 kDa, and 97% of its mass consists of 185 copies of -35-residue module. Within the molecule, different sub-families of modules can be distinguished, and their arrangement is correlated to the structure of the thin filament. The central 154 copies are grouped into 22 seven-module super repeats corresponding to 38.5 nm thin filament repeats. In the thin filament ruler region, multiple isoforms are generated by alternative exon usage which is likely to explain the developmental and tissue-specific size variations of nebulins previously found in vertebrate skeletal muscles. We propose that different types of nebulin molecular rulers are expressed in the different types of skeletal muscles by differential splicing. Outside the super repeat region, the presence of distinct module arrangements implies functional diversity of the nebulin module family. A novel "simple repeat" family together with an SH3 domain at the C-terminus appear to anchor the nebulin filament system in the Z-disc. Nebulin's SH3 domain is highly related in sequence to the SH3 domains in yeast actin binding protein ABP-1 and to the src substrate p80/85 in chicken, both proteins which are involved in regulating actin assembly of the cytoskeleton in non-muscle cells. Study of nebulins terminal sequences is likely to reveal how integration of the nebulin filament into the sarcomere is regulated.

Passive tension in cardiac muscle: contribution of collagen, titin, microtubules, and intermediate filaments

The passive tension-sarcomere length relation of rat cardiac muscle was investigated by studying passive (or not activated) single myocytes and trabeculae. The contribution of collagen, titin, microtubules, and intermediate filaments to tension and stiffness was investigated by measuring (1) the effects of KCl/KI extraction on both trabeculae and single myocytes, (2) the effect of trypsin digestion on single myocytes, and (3) the effect of colchicine on single myocytes. It was found that over the working range of sarcomeres in the heart (lengths approximately 1.9-2.2 microns), collagen and titin are the most important contributors to passive tension with titin dominating at the shorter end of the working range and collagen at longer lengths. Microtubules made a modest contribution to passive tension in some cells, but on average their contribution was not significant. Finally, intermediate filaments contributed about 10% to passive tension of trabeculae at sarcomere lengths from approximately 1.9 to 2.1 microns, and their contribution dropped to only a few percent at longer lengths. At physiological sarcomere lengths of the heart, cardiac titin developed much higher tensions (> 20-fold) than did skeletal muscle titin at comparable lengths. This might be related to the finding that cardiac titin has a molecular mass of 2.5 MDa, 0.3-0.5 MDa smaller than titin of mammalian skeletal muscle, which is predicted to result in a much shorter extensible titin segment in the I-band of cardiac muscle. Passive stress plotted versus the strain of the extensible titin segment showed that the stress-strain relationships are similar in cardiac and skeletal muscle. The difference in passive stress between cardiac and skeletal muscle at the sarcomere level predominantly resulted from much higher strains of the I-segment of cardiac titin at a given sarcomere length. By expressing a smaller titin isoform, without changing the properties of the molecule itself, cardiac muscle is able to develop significant levels of passive tension at physiological sarcomere lengths.

Microscopic analysis of the elastic properties of nebulin in skeletal myofibrils

The elastic properties of nebulin were studied by measuring the elasticity of single skeletal myofibrils, from which the portion of the thin filament located at the I band had been selectively removed by treatment with plasma gelsolin under rigor conditions. In this myofibril model, a portion of each nebulin molecule at the I band was expected to be free of actin filaments and exposed. The length of the exposed portion of the nebulin molecule was controlled by performing the gelsolin treatment at various sarcomere lengths. The relation between the passive tension and extension of the exposed portion of the nebulin showed a convex curve starting from a slack length, apparently in a fashion similar to that of wool. The slack sarcomere length shifted depending on the length of the exposed portion of the nebulin, however, the relation being represented by a single master curve. The elastic modulus of nebulin was estimated to be two to three orders of magnitude smaller than that of an actin filament. Based on these results, we conclude that nebulin attaches to an actin filament in a side-by-side fashion and that it does not significantly contribute to the elastic modulus of thin filaments. The relation between the passive tension and extension of connectin (titin) was obtained for a myofibril from which thin filaments had been completely removed with gelsolin under contracting conditions; this showed a concave curve, consistent with the previous results obtained in single fibers.

Inhibition of CapZ during myofibrillogenesis alters assembly of actin filaments

The actin filaments of myofibrils are highly organized; they are of a uniform length and polarity and are situated in the sarcomere in an aligned array. We hypothesized that the barbed-end actin-binding protein, CapZ, directs the process of actin filament assembly during myofibrillogenesis. We tested this hypothesis by inhibiting the actin-binding activity of CapZ in developing myotubes in culture using two different methods. First, injection of a monoclonal antibody that prevents the interaction of CapZ and actin disrupts the non-striated bundles of actin filaments formed during the early stages of myofibril formation in skeletal myotubes in culture. The antibody, when injected at concentrations lower than that required for disrupting the actin filaments, binds at nascent Z-disks. Since the interaction of CapZ and the monoclonal antibody are mutually exclusive, this result indicates that CapZ binds nascent Z-disks independent of an interaction with actin filaments. In a second approach, expression in myotubes of a mutant form of CapZ that does not bind actin results in a delay in the appearance of actin in a striated pattern in myofibrils. The organization of alpha-actinin at Z-disks also is delayed, but the organization of titin and myosin in sarcomeres is not significantly altered. We conclude that the interaction of CapZ and actin is important for the organization of actin filaments of the sarcomere.

Nebulette: a 107 kD nebulin-like protein in cardiac muscle

A 107-kD protein has been identified in primary cultures of chicken embryonic cardiomyocytes by immunoprecipitations with certain anti-nebulin monoclonal antibodies (mAbs). These mAbs, prepared against a fragment of human skeletal muscle nebulin located near the carboxyl terminus, detect a 107-kD protein in extracts of adult chicken heart, adult mouse heart, and adult rabbit heart by immunoblot analysis. A partial cDNA corresponding to this protein has been isolated by immunological screening of a chicken heart cDNA expression vector library. The partial cDNA encodes a 380-amino acid open reading frame composed entirely of nebulin-like 35-residue modules marked by the highly conserved sequence motifs: SXXXYK and TPD. The open reading frame exhibits 60-85% homology with skeletal muscle nebulins from a variety of species. This cDNA recognizes an approximately 8-kb transcript in cardiac RNA and does not hybridize to skeletal muscle RNAs by northern analysis. Immunofluorescence localization of this nebulin-like protein in primary cultures of chicken cardiomyocytes and embryonic chicken cardiac myofibrils indicates that the protein is localized to the I-Z-I complex of the myofibrils, extending approximately 25% of the thin filament length. Comparisons of the distribution of this protein relative to actin, myosin, and titin in spreading cardiomyocytes suggest that the cardiac nebulin-like protein becomes aligned with the nascent myofibrils early during myofibrillogenesis. To distinguish this petite nebulin-like protein from the 600-900 kD skeletal muscle nebulin, we have named it nebulette.

Exogenous gelsolin binds to sarcomeric thin filaments without severing

We have investigated the binding of gelsolin to thin myofilaments in situ and their stability against severing. Differentiated myotubes from chicken skeletal muscle containing cross-striated myofibrils were permeabilized with Triton X-100 and incubated with gelsolin. Immunofluorescence microscopy localized both endogenous and exogenous gelsolin in the I-Z-I-regions of the sarcomers. The staining pattern suggested a binding of the exogenous gelsolin along the entire length of the thin filaments. This binding was Ca2+ dependent, but gelsolin was not removed after subsequent addition of EGTA. The fluorescence staining for actin remained unchanged after gelsolin incubation, indicating that thin filaments in cross-striated myofibrils were resistant to the severing action of gelsolin, in contrast to the microfilaments in stress fibers. After extraction of the permeabilized cells with high ionic strength to remove tropomyosin and myosin, gelsolin still bound along the entire thin filament and the actin pattern also remained unchanged. After Triton X-100 permeabilization and high ionic strength extraction, the giant protein nebulin was found to be still present as a myofibrillar component. Gelsolin treatment after high salt extraction affected neither actin nor nebulin in the thin filaments. We therefore conclude that nebulin confers the gelsolin resistance to the sarcomeric actin filaments.

Tropomodulin caps the pointed ends of actin filaments

Many proteins have been shown to cap the fast growing (barbed) ends of actin filaments, but none have been shown to block elongation and depolymerization at the slow growing (pointed) filament ends. Tropomodulin is a tropomyosin-binding protein originally isolated from red blood cells that has been localized by immunofluorescence staining to a site at or near the pointed ends of skeletal 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). Our experiments demonstrate that tropomodulin in conjunction with tropomyosin is a pointed end capping protein: it completely blocks both elongation and depolymerization at the pointed ends of tropomyosin-containing actin filaments in concentrations stoichiometric to the concentration of filament ends (Kd < or = 1 nM). In the absence of tropomyosin, tropomodulin acts as a "leaky" cap, partially inhibiting elongation and depolymerization at the pointed filament ends (Kd for inhibition of elongation = 0.1-0.4 microM). Thus, tropomodulin can bind directly to actin at the pointed filament end. Tropomodulin also doubles the critical concentration at the pointed ends of pure actin filaments without affecting either the rate of extent of polymerization at the barbed filament ends, indicating that tropomodulin does not sequester actin monomers. Our experiments provide direct biochemical evidence that tropomodulin binds to both the terminal tropomyosin and actin molecules at the pointed filament end, and is the long sought-after pointed end capping protein. We propose that tropomodulin plays a role in maintaining the narrow length distributions of the stable, tropomyosin-containing actin filaments in striated muscle and in red blood cells.