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

Structures from intact myofibrils reveal mechanism of thin filament regulation through nebulin

In skeletal muscle, nebulin stabilizes and regulates the length of thin filaments, but the underlying mechanism remains nebulous. In this work, we used cryo-electron tomography and subtomogram averaging to reveal structures of native nebulin bound to thin filaments within intact sarcomeres. This in situ reconstruction provided high-resolution details of the interaction between nebulin and actin, demonstrating the stabilizing role of nebulin. Myosin bound to the thin filaments exhibited different conformations of the neck domain, highlighting its inherent structural variability in muscle. Unexpectedly, nebulin did not interact with myosin or tropomyosin, but it did interact with a troponin T linker through two potential binding motifs on nebulin, explaining its regulatory role. Our structures support the role of nebulin as a thin filament "molecular ruler" and provide a molecular basis for studying nemaline myopathies.

Expression and Purification of ZASP Subdomains and Clinically Important Isoforms: High-Affinity Binding to G-Actin

Z-disc-associated, alternatively spliced, PDZ motif-containing protein (ZASP) is a principal component of the sarcomere. The three prevalent isoforms of ZASP in skeletal muscle are generated by alternative splicing of exons 9 and 10. The long isoforms, either having (ZASP-L) or lacking exon 10 (ZASP-LΔex10), include an N-terminal PDZ domain, an actin-binding region (ABR) with a conserved motif (ZM), and three C-terminal LIM domains. The short isoform (ZASP-S) lacks the LIM domains. Mutations, A147T and A165V, within the ZM of ZASP-LΔex10 cause myofibrillar myopathy, but the mechanism is unknown. We have prepared these proteins, their ABR, and the respective mutant variants in recombinant form, characterized them biophysically, and analyzed their actin-binding properties by surface plasmon resonance and electron microscopy. All the proteins were physically homogeneous and monomeric and had circular dichroic spectra consistent with partially folded conformations. Comparison of the NMR HSQC spectra of ZASP-S and the PDZ domain showed that the ABR is unstructured. ZASP-S and its mutant variants and ZASP-LΔex10 all bound to immobilized G-actin with high affinity (K_d ≈ 10^-8 to 10^-9 M). Constructs of the isolated actin-binding region missing exon 10 (ABRΔ10) bound with lower affinity (K_d ≈ 10^-7 M), but those retaining exon 10 (ABR+10) did so only weakly (K_d ≈ 10^-5 M). ZASP-S, and the ABRΔ10, also induced F-actin and array formation, even in conditions of low ionic strength and in the absence of KCl and Mg²⁺ ions. Interestingly, the ZM mutations A147T and A165V did not affect any of the results described above.

Nebulette is a powerful cytolinker organizing desmin and actin in mouse hearts

Nebulette physically links desmin to sarcomeric actin in hearts. An intact desmin network is required for nebulette to function as major actin-binding protein in sarcomeres. This study provides biochemical evidence that the desmin–nebulette complex is involved in filament-forming desminopathy.

In the hearts of patients bearing nebulette mutations, a severe general disorganization in cardiomyocytes of the extrasarcomeric desmin intermediate filament system is frequently observed. However, the molecular and functional relationship between the desmin cytoskeleton and nebulette-containing sarcomeres is still unclear. Here we report a high-affinity in vitro interaction between nebulette and desmin filaments. A major interaction site has been mapped to the desmin α-helical rod domain, indicating that the filament core is directly involved in the binding of nebulette. The disease-mutant desmin variants E245D and T453I exhibited increased binding affinity for nebulette, delayed filament assembly kinetics, and caused significant weakening of networks. In isolated chick cardiomyocytes and sections from canine heart, we revealed by ground-state depletion and confocal microscopies that module 5 of nebulette extends outward from Z-disk–associated desmin filaments toward the center of the sarcomere. Accordingly, in the myocardium of Des^−/− mice, elevated levels of cardiac actin correlated with alterations in the distribution of nebulette. Our data suggest that a well-organized desmin network is required to accommodate an optimal conformation of nebulette on sarcomeres to bind and recruit cardiac α-actin. Hence we propose that nebulette acts in synergy with nebulin to reinforce and temporally fine-tune striated muscle relaxation–contraction cycles.

Dynamic regulation of sarcomeric actin filaments in striated muscle

In striated muscle, the actin cytoskeleton is differentiated into myofibrils. Actin and myosin filaments are organized in sarcomeres and specialized for producing contractile forces. Regular arrangement of actin filaments with uniform length and polarity is critical for the contractile function. However, the mechanisms of assembly and maintenance of sarcomeric actin filaments in striated muscle are not completely understood. Live imaging of actin in striated muscle has revealed that actin subunits within sarcomeric actin filaments are dynamically exchanged without altering overall sarcomeric structures. A number of regulators for actin dynamics have been identified, and malfunction of these regulators often result in disorganization of myofibril structures or muscle diseases. Therefore, proper regulation of actin dynamics in striated muscle is critical for assembly and maintenance of functional myofibrils. Recent studies have suggested that both enhancers of actin dynamics and stabilizers of actin filaments are important for sarcomeric actin organization. Further investigation of the regulatory mechanism of actin dynamics in striated muscle should be a key to understanding how myofibrils develop and operate.

Isolation of nebulin from rabbit skeletal muscle and its interaction with actin

Nebulin is about 800 kDa filamentous protein that binds the entire thin filament of vertebrate skeletal muscle sarcomeres. Nebulin cannot be isolated from muscle except in a completely denatured form by direct solubilization of myofibrils with SDS because nebulin is hardly soluble under salt conditions. In the present study, nebulin was solubilized by a salt solution containing 1 M urea and purified by DEAE-Toyopearl column chromatography via 4 M urea elution. Rotary-shadowed images of nebulin showed entangled knit-like particles, about 20 nm in diameter. The purified nebulin bound to actin filaments to form loose bundles. Nebulin was confirmed to bind actin, α-actinin, β-actinin, and tropomodulin, but not troponin or tropomyosin. The data shows that full-length nebulin can be also obtained in a functional and presumably native form, verified by data from experiments using recombinant subfragments.

Muscle giants: molecular scaffolds in sarcomerogenesis

Myofibrillogenesis in striated muscles is a highly complex process that depends on the coordinated assembly and integration of a large number of contractile, cytoskeletal, and signaling proteins into regular arrays, the sarcomeres. It is also associated with the stereotypical assembly of the sarcoplasmic reticulum and the transverse tubules around each sarcomere. Three giant, muscle-specific proteins, titin (3-4 MDa), nebulin (600-800 kDa), and obscurin (approximately 720-900 kDa), have been proposed to play important roles in the assembly and stabilization of sarcomeres. There is a large amount of data showing that each of these molecules interacts with several to many different protein ligands, regulating their activity and localizing them to particular sites within or surrounding sarcomeres. Consistent with this, mutations in each of these proteins have been linked to skeletal and cardiac myopathies or to muscular dystrophies. The evidence that any of them plays a role as a "molecular template," "molecular blueprint," or "molecular ruler" is less definitive, however. Here we review the structure and function of titin, nebulin, and obscurin, with the literature supporting a role for them as scaffolding molecules and the contradictory evidence regarding their roles as molecular guides in sarcomerogenesis.

The intercalated disk protein, mXinalpha, is capable of interacting with beta-catenin and bundling actin filaments [corrected]

Targeted deletion of mXinalpha results in cardiac hypertrophy and cardiomyopathy with conduction defects (Gustafson-Wagner, E., Sinn, H. W., Chen, Y.-L., Wang, D.-Z., Reiter, R. S., Lin, J. L.-C., Yang, B., Williamson, R. A., Chen, J. N., Lin, C.-I., and Lin, J. J.-C. (2007) Am. J. Physiol. 293, H2680-H2692). To understand the underlying mechanisms leading to such cardiac defects, the functional domains of mXinalpha and its interacting proteins were investigated. Interaction studies using co-immunoprecipitation, pull-down, and yeast two-hybrid assays revealed that mXinalpha directly interacts with beta-catenin. The beta-catenin-binding site on mXinalpha was mapped to amino acids 535-636, which overlaps with the known actin-binding domains composed of the Xin repeats. The overlapping nature of these domains provides insight into the molecular mechanism for mXinalpha localization and function. Purified recombinant glutathione S-transferase- or His-tagged mXinalpha proteins are capable of binding and bundling actin filaments, as determined by co-sedimentation and electron microscopic studies. The binding to actin was saturated at an approximate stoichiometry of nine actin monomers to one mXinalpha. A stronger interaction was observed between mXinalpha C-terminal deletion and actin as compared with the interaction between full-length mXinalpha and actin. Furthermore, force expression of green fluorescent protein fused to an mXinalpha C-terminal deletion in cultured cells showed greater stress fiber localization compared with force-expressed GFP-mXinalpha. These results suggest a model whereby the C terminus of mXinalpha may prevent the full-length molecule from binding to actin, until the beta-catenin-binding domain is occupied by beta-catenin. The binding of mXinalpha to beta-catenin at the adherens junction would then facilitate actin binding. In support of this model, we found that the actin binding and bundling activity of mXinalpha was enhanced in the presence of beta-catenin.

Nebulin regulates the assembly and lengths of the thin filaments in striated muscle

In many tissues, actin monomers polymerize into actin (thin) filaments of precise lengths. Although the exact mechanisms involved remain unresolved, it is proposed that "molecular rulers" dictate the lengths of the actin filaments. The giant nebulin molecule is a prime candidate for specifying thin filament lengths in striated muscle, but this idea has never been proven. To test this hypothesis, we used RNA interference technology in rat cardiac myocytes. Live cell imaging and triple staining revealed a dramatic elongation of the preexisting thin filaments from their pointed ends upon nebulin knockdown, demonstrating its role in length maintenance; the barbed ends were unaffected. When the thin filaments were depolymerized with latrunculin B, myocytes with decreased nebulin levels reassembled them to unrestricted lengths, demonstrating its importance in length specification. Finally, knockdown of nebulin in skeletal myotubes revealed its involvement in myofibrillogenesis. These data are consistent with nebulin functioning as a thin filament ruler and provide insight into mechanisms dictating macromolecular assembly.

N-RAP scaffolds I-Z-I assembly during myofibrillogenesis in cultured chick cardiomyocytes

N-RAP is a muscle-specific protein with an N-terminal LIM domain (LIM), C-terminal actin-binding super repeats homologous to nebulin (SR) and nebulin-related simple repeats (IB) in between the two. Based on biochemical data, immunofluorescence analysis of cultured embryonic chick cardiomyocytes and the targeting and phenotypic effects of these individual GFP-tagged regions of N-RAP, we proposed a novel model for the initiation of myofibril assembly in which N-RAP organizes alpha-actinin and actin into the premyofibril I-Z-I complexes. We tested the proposed model by expressing deletion mutants of N-RAP (i.e. constructs containing two of the three regions of N-RAP) in chick cardiomyocytes and observing the effects on alpha-actinin and actin organization into mature sarcomeres. Although individually expressing either the LIM, IB, or SR regions of N-RAP inhibited alpha-actinin assembly into Z-lines, expression of either the LIM-IB fusion or the IB-SR fusion permitted normal alpha-actinin organization. In contrast, the LIM-SR fusion (LIM-SR) inhibited alpha-actinin organization into Z-lines, indicating that the IB region is critical for Z-line assembly. While permitting normal Z-line assembly, LIM-IB and IB-SR decreased sarcomeric actin staining intensity; however, the effects of LIM-IB on actin assembly were significantly more severe, as estimated both by morphological assessment and by quantitative measurement of actin staining intensity. In addition, LIM-IB was consistently retained in mature Z-lines, while mature Z-lines without significant IB-SR incorporation were often observed. We conclude that the N-RAP super repeats are essential for organizing actin filaments during myofibril assembly in cultured embryonic chick cardiomyocytes, and that they also play an important role in removal of the N-RAP scaffold from the completed myofibrillar structure. This work strongly supports the N-RAP scaffolding model of premyofibril assembly.

Lasp-1 binds to non-muscle F-actin in vitro and is localized within multiple sites of dynamic actin assembly in vivo

Lasp-1 has been identified as a signaling molecule that is phosphorylated upon elevation of [cAMP]i in pancreas, intestine and gastric mucosa and is selectively expressed in cells within epithelial tissues. In the gastric parietal cell, cAMP-dependent phosphorylation induces the partial translocation of lasp-1 to the apically directed F-actin-rich canalicular membrane, which is the site of active HCl secretion. Lasp-1 is an unusual modular protein that contains an N-terminal LIM domain, a C-terminal SH3 domain and two internal nebulin repeats. Domain-based analyses have recently categorized this protein as an epithelial representative of the nebulin family, which also includes the actin binding, muscle-specific proteins, nebulin, nebulette and N-RAP. In this study, we show that lasp-1 binds to non-muscle filamentous (F) actin in vitro in a phosphorylation-dependent manner. In addition, we provide evidence that lasp-1 is concentrated within focal complexes as well as in the leading edges of lamellipodia and the tips of filopodia in non-transformed gastric fibroblasts. In actin pull-down assays, the apparent K(d) of bacterially expressed his-tagged lasp-1 binding to F-actin was 2 micro M with a saturation stoichiometry of approximately 1:7. Phosphorylation of recombinant lasp-1 with recombinant PKA increased the K(d) and decreased the B(max) for lasp-1 binding to F-actin. Microsequencing and site-directed mutagenesis localized the major in vivo and in vitro PKA-dependent phosphorylation sites in rabbit lasp-1 to S(99) and S(146). BLAST searches confirmed that both sites are conserved in human and chicken homologues. Transfection of lasp-1 cDNA encoding for alanine substitutions at S(99) and S(146), into parietal cells appeared to suppress the cAMP-dependent translocation of lasp-1 to the intracellular canalicular region. In gastric fibroblasts, exposure to the protein kinase C activator, PMA, was correlated with the translocation of lasp-1 into newly formed F-actin-rich lamellipodial extensions and nascent focal complexes. Since lasp-1 does not appear to be phosphorylated by PKC, these data suggest that other mechanisms in addition to cAMP-dependent phosphorylation can mediate the translocation of lasp-1 to regions of dynamic actin turnover. The localization of lasp-1 to these subcellular regions under a range of experimental conditions and the phosphorylation-dependent regulation of this protein in F-actin rich epithelial cells suggests an integral and possibly cell-specific role in modulating cytoskeletal/membrane-based cellular activities.

Each actin subunit has three nebulin binding sites: implications for steric blocking

Nebulin is a giant protein that spans most of the muscle thin filament. Mutations in nebulin result in myopathies and dystrophies. Nebulin contains approximately 200 copies of approximately 35 residue modules, each believed to contain an actin binding site, organized into seven-module superrepeats. The strong correlation between the number of nebulin modules and the length of skeletal muscle thin filaments in different species suggests that nebulin determines thin filament length. Little information exists about the interactions between intact nebulin and F-actin. More insight has come from working with fragments of nebulin, containing from one to hundreds of actin binding modules. However, the observed stoichiometry of binding between these fragments and actin has ranged from 0.4 to 13 modules per actin subunit. We have used electron microscopy and a novel method of helical image analysis to characterize complexes of F-actin with a nebulin fragment. The fragment binds as an extended structure spanning three actin subunits and binding to different sites on each actin. Muscle regulation involves tropomyosin movement on the surface of actin, with binding in three states. Our results suggest the intriguing possibility that intact nebulin may also be able to occupy three different sites on F-actin.

Targeted disruption of nebulette protein expression alters cardiac myofibril assembly and function

To evaluate nebulette's role in cardiac myofibrils, cardiomyocytes expressing green fluorescent protein (GFP)-nebulette constructs were monitored for their ability to contract and myofilament protein distribution was analyzed. Cells expressing full-length GFP-nebulette appear unaffected and exhibit normal beating frequencies. Expression of the GFP linker and SH3 results in loss of the endogenous nebulette and tropomyosin; however, Z-line and thick filaments are undisturbed. Cells expressing either of these domains have dramatically reduced beating frequencies, consistent with the loss of thin filament proteins. This loss was inhibited by the addition of protease inhibitors during culturing. The GFP repeat domain disrupts both myofibrillogenesis and contraction in spreading cardiomyocytes, whereas introduction of this protein into well-spread cardiomyocytes results in localization at the Z-line and a 50% reduction in beating frequency. Ultimately, these cells form bundles containing the GFP repeat and many myofilament proteins. Interestingly, butanedione monoxime inhibition of contraction inhibited the formation of these bundles. These results show that the GFP-nebulette domains have a dominant-negative effect on the distribution and function of the sarcomeric proteins. Taken together with the observation that nebulette colocalizes with alpha-actinin in the pre-, nascent, and mature myofibrils, our data demonstrate the importance of this cardiac-specific nebulin isoform in myofibril organization and function.

Targeting and functional role of N-RAP, a nebulin-related LIM protein, during myofibril assembly in cultured chick cardiomyocytes

Targeting and functional effects of N-RAP domains were studied by expression as GFP-tagged fusion proteins in cultured embryonic chick cardiomyocytes. GFP-tagged N-RAP was targeted to myofibril precursors, myofibril ends and cell contacts, expression patterns that are similar to endogenous N-RAP. The GFP-tagged N-RAP LIM domain (GFP-N-RAP-LIM) was targeted to the membrane in cells with myofibril precursors and cell-cell contacts. The GFP-tagged super repeats (N-RAP-SR) and the GFP-tagged domain normally found in between the super repeats and the LIM domain (N-RAP-IB) were each observed at sites of myofibril assembly, incorporating into myofibril precursors in a manner similar to full length N-RAP. However, unlike full-length N-RAP, N-RAP-SR and N-RAP-IB were also found in mature myofibrils, associating with the sarcomeric actin filaments and the Z-lines, respectively. N-RAP-IB was also colocalized with α-actinin at cell contacts. Each of the N-RAP constructs could inhibit the formation of mature myofibrils in cultured cardiomyocytes, with the effects of N-RAP-SR and N-RAP-IB depending on the time of transfection. The results show that each region of N-RAP is crucial for myofibril assembly. Combining the targeting and functional effects of N-RAP domains with information in the literature, we propose a new model for initiation of myofibrillogenesis.

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.

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.

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.

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.

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.

Alpha actinin-CapZ, an anchoring complex for thin filaments in Z-line

CapZ is a widely distributed and highly conserved, heterodimeric protein, that nucleates actin polymerization and binds to the barbed ends of actin filaments, preventing the addition or loss of actin monomers. CapZ interaction with actin filaments was shown to be of high affinity and decreased in the presence of PIP2. CapZ was located in nascent Z-lines during skeletal muscle myofibrillogenesis before the striated appearance of thin filaments in sarcomers. In this study, the stabilization and the anchorage of thin filaments were explored through identification of CapZ partners in the Z-line. Fish (sea bass) striated white muscle and its related Z-line proteins were selected since they correspond to the simplest Z-line organization. We report here the interaction between purified CapZ and alpha-actinin, a major component of Z filaments and polar links in Z-discs. Affinity of CapZ for alpha-actinin, estimated by fluorescence and immunochemical assays, is in the microM range. This association was found to be independent of actin and shown to be weakened in the presence of phosphoinositides. Binding contacts on the alpha-actinin molecule lie in the 55 kDa repetitive domain. A model including CapZ/alpha-actinin/titin/actin interactions is proposed considering Luther's 3D Z-line reconstruction.