In vitro expressed dystrophin fragments do not associate with each other

Structure and assembly of the dystrophin glycoprotein complex

The dystrophin glycoprotein complex (DGC) has a crucial role in maintaining cell membrane stability and integrity by connecting the intracellular cytoskeleton with the surrounding extracellular matrix1-3. Dysfunction of dystrophin and its associated proteins results in muscular dystrophy, a disorder characterized by progressive muscle weakness and degeneration4,5. Despite the important roles of the DGC in physiology and pathology, its structural details remain largely unknown, hindering a comprehensive understanding of its assembly and function. Here we isolated the native DGC from mouse skeletal muscle and obtained its high-resolution structure. Our findings unveil a markedly divergent structure from the previous model of DGC assembly. Specifically, on the extracellular side, β-, γ- and δ-sarcoglycans co-fold to form a specialized, extracellular tower-like structure, which has a central role in complex assembly by providing binding sites for α-sarcoglycan and dystroglycan. In the transmembrane region, sarcoglycans and sarcospan flank and stabilize the single transmembrane helix of dystroglycan, rather than forming a subcomplex as previously proposed6-8. On the intracellular side, sarcoglycans and dystroglycan engage in assembly with the dystrophin-dystrobrevin subcomplex through extensive interaction with the ZZ domain of dystrophin. Collectively, these findings enhance our understanding of the structural linkage across the cell membrane and provide a foundation for the molecular interpretation of many muscular dystrophy-related mutations.

Biophysical characterization of the dystrophin C-terminal domain: Dystrophin interacts differentially with dystrobrevin isoforms

Duchenne muscular dystrophy (DMD) gene encodes dystrophin, a large multidomain protein. Its nonfunctionality leads to dystrophinopathies like DMD and Becker muscular dystrophy, for which no cure is yet available. A few therapies targeted towards specific mutations can extend the lifespan of patients, although with limited efficacy and high costs, emphasizing the need for more general treatments. Dystrophin's complex structure with poorly understood domains and the presence of multiple isoforms with varied expression patterns in different tissues pose challenges in therapeutic development. The C-terminal (CT) domain of dystrophin is less understood in terms of its structure-function, although it has been shown to perform important functional roles by interacting with another cytosolic protein, dystrobrevin. Dystrophin and dystrobrevin stabilize the sarcolemma membrane by forming a multiprotein complex called dystrophin-associated glycoprotein complex that is destabilized in DMD. Dystrobrevin has two major isoforms, alpha and beta, with tissue-specific expression patterns. Here, we characterize the CT domain of dystrophin and its interactions with the two dystrobrevin isoforms. We show that the CT domain is nonglobular and shows reversible urea denaturation as well as thermal denaturation. It interacts with dystrobrevin isoforms differentially, with differences in binding affinity and the mode of interaction. We further show that the amino acid differences in the CT region of dystrobrevin isoforms contribute to these differences. These results have implications for the stability of dystrophin-associated glycoprotein complex in different tissues and explain the differing symptoms associated with DMD patients affecting organs beyond the skeletal muscles.

Dystrophin: more than just the sum of its parts

Dystrophin is one of a number of large cytoskeleton associated proteins that connect between various cytoskeletal elements and often are tethered to the membrane through other transmembrane protein complexes. These cytolinker proteins often provide structure and support to the cells where they are expressed, and mutations in genes encoding these proteins frequently gives rise to disease. Dystrophin is no exception in any of these respects, providing connections between a transmembrane complex known as the dystrophin-glycoprotein complex and the underlying cytoskeleton. The most established connection and possibly the most important is that to F-actin, but more recently evidence has been forthcoming of connections to membrane phospholipids, intermediate filaments and microtubules. Moreover it is becoming increasingly clear that the multiple spectrin-like repeats in the centre of the molecule, that had hitherto been thought to be largely redundant, harbour binding activities that have a significant impact on dystrophin functionality. This functionality is particularly apparent when assessed by the ability to rescue the dystrophic phenotype in mdx mice. This review will focus on the relatively neglected but functionally vital coiled-coil region of dystrophin, highlighting the structural relationships and interactions of the coiled-coil region and providing new insights into the functional role of this region.

Our trails and trials in the subsarcolemmal cytoskeleton network and muscular dystrophy researches in the dystrophin era

In 1987, about 150 years after the discovery of Duchenne muscular dystrophy (DMD), its responsible gene, the dystrophin gene, was cloned by Kunkel. This was a new substance. During these 20 odd years after the cloning, our understanding on dystrophin as a component of the subsarcolemmal cytoskeleton networks and on the pathomechanisms of and experimental therapeutics for DMD has been greatly enhanced. During this paradigm change, I was fortunately able to work as an active researcher on its frontiers for 12 years. After we discovered that dystrophin is located on the cell membrane in 1988, we studied the architecture of dystrophin and dystrophin-associated proteins (DAPs) complex in order to investigate the function of dystrophin and pathomechanism of DMD. During the conduct of these studies, we came to consider that the dystrophin-DAP complex serves to transmembranously connect the subsarcolemmal cytoskeleton networks and basal lamina to protect the lipid bilayer. It then became our working hypothesis that injury of the lipid bilayer upon muscle contraction is the cause of DMD. During this process, we predicted that subunits of the sarcoglycan (SG) complex are responsible for respective types of DMD-like muscular dystrophy with autosomal recessive inheritance. Our prediction was confirmed to be true by many researchers including ourselves. In this review, I will try to explain what we observed and how we considered concerning the architecture and function of the dystrophin-DAP complex, and the pathomechanisms of DMD and related muscular dystrophies.

The endocytic recycling protein EHD2 interacts with myoferlin to regulate myoblast fusion

Skeletal muscle is a multinucleated syncytium that develops and is maintained by the fusion of myoblasts to the syncytium. Myoblast fusion involves the regulated coalescence of two apposed membranes. Myoferlin is a membrane-anchored, multiple C2 domain-containing protein that is highly expressed in fusing myoblasts and required for efficient myoblast fusion to myotubes. We found that myoferlin binds directly to the eps15 homology domain protein, EHD2. Members of the EHD family have been previously implicated in endocytosis as well as endocytic recycling, a process where membrane proteins internalized by endocytosis are returned to the plasma membrane. EHD2 binds directly to the second C2 domain of myoferlin, and EHD2 is reduced in myoferlin null myoblasts. In contrast to normal myoblasts, myoferlin null myoblasts accumulate labeled transferrin and have delayed recycling. Introduction of dominant negative EHD2 into myoblasts leads to the sequestration of myoferlin and inhibition of myoblast fusion. The interaction of myoferlin with EHD2 identifies molecular overlap between the endocytic recycling pathway and the machinery that regulates myoblast membrane fusion.

High-resolution en-face visualization of the cardiomyocyte plasma membrane reveals distinctive distributions of spectrin and dystrophin

The actin-binding proteins, spectrin and dystrophin, are key components of the plasma membrane-associated cytoskeleton of the cardiac muscle cell. From confocal immunofluorescence studies, the distribution of spectrin appears to overlap with that of dystrophin, but the precise functional differentiation, molecular distributions and spatial relationship of these two cytoskeletal systems remain unclear. Freeze-fracture replica immuno-electron microscopy, in parallel with immunofluorescence/confocal microscopy, were applied to examine at high resolution the spatial relationships between the spectrin and dystrophin membrane-associated cytoskeleton systems in cardiac muscle. Application of freeze-fracture replica cytochemistry, with single and double immunogold labeling, permitted simultaneous examination of the organization of spectrin and dystrophin in en-face views of the plasma membrane at high resolution. In contrast to the close spatial relationship previously demonstrated for dystrophin and beta-dystroglycan, no association between the gold label marking dystrophin and that marking spectrin was observed. Our freeze-fracture cytochemical results suggest that the two membrane skeletal networks formed by dystrophin and spectrin in cardiac muscle are independently organized, implying that whatever overlap of function (e.g., in structural support to the plasma membrane) may exist between them, the two systems may each have additional distinctive roles.

Nesprin-1alpha self-associates and binds directly to emerin and lamin A in vitro

Nesprin-1alpha is a spectrin repeat (SR)-containing, transmembrane protein of the inner nuclear membrane, and is highly expressed in muscle cells. A yeast two-hybrid screen for nesprin-1alpha-interacting proteins showed that nesprin-1alpha interacted with itself. Blot overlay experiments revealed that nesprin-1alpha's third SR binds the fifth SR. The carboxy-terminal half of nesprin-1alpha directly bound lamin A, a nuclear intermediate filament protein. Biochemical analysis demonstrated that nesprin-1alpha dimers bind directly to the nucleoplasmic domain of emerin, an inner nuclear membrane protein, with an affinity of 4 nM. Binding was optimal for full nucleoplasmic dimers of nesprin-1alpha, since nesprin fragments SR1-5 and SR5-7 bound emerin as monomers with affinities of 53 nM and 250 mM, respectively. We propose that membrane-anchored nesprin-1alpha antiparallel dimers interact with both emerin and lamin A to provide scaffolding at the inner nuclear membrane.

Towards a complete atomic structure of spectrin family proteins

The spectrin family of proteins represents a discrete group of cytoskeletal proteins comprising principally alpha-actinin, spectrin, dystrophin, and homologues and isoforms. They all share three main structural and functional motifs, namely, the spectrin repeat, EF-hands, and a CH domain-containing actin-binding domain. These proteins are variously involved in organisation of the actin cytoskeleton, membrane cytoskeleton architecture, cell adhesion, and contractile apparatus. The highly modular nature of these molecules has been a hindrance to the determination of their complete structures due to the inherent flexibility imparted on the proteins, but has also been an asset, inasmuch as the individual modules were of a size amenable to structural analysis by both crystallographic and NMR approaches. Representative structures of all the major domains shared by spectrin family proteins have now been solved at atomic resolution, including in some cases multiple domains from several family members. High-resolution structures, coupled with lower resolution methods to determine the overall molecular shape of these proteins, allow us for the first time to build complete atomic structures of the spectrin family of proteins.

Dystrophin and muscular dystrophy: past, present, and future

Duchenne muscular dystrophy was described in the medical literature in the early 1850s but the molecular basis of the disease was not determined until the late 1980s. The cloning of dystrophin led to the identification of a large complex of proteins that plays an important, although not yet well understood, role in muscle biology. Concomitant with the elucidation of the function of dystrophin and its associated proteins has been the pursuit of therapeutic options for muscular dystrophy. Although there is still no cure for this disorder, great advances are being made in the areas of gene introduction and cell transplant therapy.

Dystroglycan contributes to the formation of multiple dystrophin-like complexes in brain

In muscle, dystrophin anchors a complex of proteins at the cell surface which includes alpha-dystroglycan, beta-dystroglycan, syntrophins and dystrobrevins. Mutations in the dystrophin gene lead to muscular dystrophy and mental retardation. In contrast to muscle, little is known about the localization and the molecular interactions of dystrophin and dystrophin associated proteins (DAPs) in brain. In the present study, we show that alpha-dystroglycan and dystrophin are localized to large neurones in cerebral cortex, hippocampus, cerebellum and spinal cord. Furthermore, we show that dystroglycan is a member of three distinct dystrophin-containing complexes. Two of these complexes contain syntrophin and both dystrophin and syntrophin are enriched in post-synaptic densities. These data suggest that dystrophin and DAPs may have a role in the organization of CNS synapses. Interestingly, the enrichment for syntrophin in post-synaptic densities is not affected in mice mutant for all dystrophin isoforms. Thus in the brain, unlike in muscle, the association of syntrophin with dystrophin is not crucial for the DAP complex which suggests that it may be associated with other proteins.

Binding of dystrophin's tandem calponin homology domain to F-actin is modulated by actin's structure

Dystrophin has been shown to be associated in cells with actin bundles. Dys-246, an N-terminal recombinant protein encoding the first 246 residues of dystrophin, includes two calponin-homology (CH) domains, and is similar to a large class of F-actin cross-linking proteins including alpha-actinin, fimbrin, and spectrin. It has been shown that expression or microinjection of amino-terminal fragments of dystrophin or the closely related utrophin resulted in the localization of these protein domains to actin bundles. However, in vitro studies have failed to detect any bundling of actin by either intact dystrophin or Dys-246. We show here that the structure of F-actin can be modulated so that there are two modes of Dys-246 binding, from bundling actin filaments to only binding to single filaments. The changes in F-actin structure that allow Dys-246 to bundle filaments are induced by covalent modification of Cys-374, proteolytic cleavage of F-actin's C-terminus, mutation of yeast actin's N-terminus, and different buffers. The present results suggest that F-actin's structural state can have a large influence on the nature of actin's interaction with other proteins, and these different states need to be considered when conducting in vitro assays.

Myozenin: An -actinin- and -filamin-binding protein of skeletal muscle Z lines

To better understand the structure and function of Z lines, we used sarcomeric isoforms of alpha-actinin and gamma-filamin to screen a human skeletal muscle cDNA library for interacting proteins by using the yeast two-hybrid system. Here we describe myozenin (MYOZ), an alpha-actinin- and gamma-filamin-binding Z line protein expressed predominantly in skeletal muscle. Myozenin is predicted to be a 32-kDa, globular protein with a central glycine-rich domain flanked by alpha-helical regions with no strong homologies to any known genes. The MYOZ gene has six exons and maps to human chromosome 10q22.1-q22.2. Northern blot analysis demonstrated that this transcript is expressed primarily in skeletal muscle with significantly lower levels of expression in several other tissues. Antimyozenin antisera stain skeletal muscle in a sarcomeric pattern indistinguishable from that seen by using antibodies for alpha-actinin, and immunogold electron microscopy confirms localization specifically to Z lines. Thus, myozenin is a skeletal muscle Z line protein that may be a good candidate gene for limb-girdle muscular dystrophy or other neuromuscular disorders.

Myozenin: an alpha-actinin- and gamma-filamin-binding protein of skeletal muscle Z lines

To better understand the structure and function of Z lines, we used sarcomeric isoforms of alpha-actinin and gamma-filamin to screen a human skeletal muscle cDNA library for interacting proteins by using the yeast two-hybrid system. Here we describe myozenin (MYOZ), an alpha-actinin- and gamma-filamin-binding Z line protein expressed predominantly in skeletal muscle. Myozenin is predicted to be a 32-kDa, globular protein with a central glycine-rich domain flanked by alpha-helical regions with no strong homologies to any known genes. The MYOZ gene has six exons and maps to human chromosome 10q22.1-q22.2. Northern blot analysis demonstrated that this transcript is expressed primarily in skeletal muscle with significantly lower levels of expression in several other tissues. Antimyozenin antisera stain skeletal muscle in a sarcomeric pattern indistinguishable from that seen by using antibodies for alpha-actinin, and immunogold electron microscopy confirms localization specifically to Z lines. Thus, myozenin is a skeletal muscle Z line protein that may be a good candidate gene for limb-girdle muscular dystrophy or other neuromuscular disorders.

Dystrophins and dystrobrevins

SUMMARY

A unique arrangement of domains makes up the common region of two otherwise very different proteins - long, elegant dystrophin, and its rather dumpy distant cousin, dystrobrevin. The two work in concert, forming the core of a large membrane-bound complex in all metazoa. Like many proteins, dystrophin and dystrobrevin have diversified in the vertebrate clade, as have their binding partners, yielding specialized complexes tailored to different cellular and subcellular locations. Disruption of several components of the complex is known to result in syndromes that include progressive myopathy, sometimes combined with cognitive defects; the best known of these is Duchenne muscular dystrophy. Despite a wealth of biochemical, cell biological and genetic information, the precise role of dystrophins, dystrobrevins and their collaborators remains unclear.

The structure of the N-terminal actin-binding domain of human dystrophin and how mutations in this domain may cause Duchenne or Becker muscular dystrophy

BACKGROUND

Dystrophin is an essential component of skeletal muscle cells. Its N-terminal domain binds to F-actin and its C terminus binds to the dystrophin-associated glycoprotein (DAG) complex in the membrane. Dystrophin is therefore thought to serve as a link from the actin-based cytoskeleton of the muscle cell through the plasma membrane to the extracellular matrix. Pathogenic mutations in dystrophin result in Duchenne or Becker muscular dystrophy.

RESULTS

The crystal structure of the dystrophin actin-binding domain (ABD) has been determined at 2.6 A resolution. The structure is an antiparallel dimer of two ABDs each comprising two calponin homology domains (CH1 and CH2) that are linked by a central alpha helix. The CH domains are both alpha-helical globular folds. Comparisons with the structures of utrophin and fimbrin ABDs reveal that the conformations of the individual CH domains are very similar to those of dystrophin but that the arrangement of the two CH domains within the ABD is altered. The dystrophin dimer reveals a change of 72 degrees in the orientation of one pair of CH1 and CH2 domains (from different monomers) relative to the other pair when compared with the utrophin dimer. The dystrophin monomer is more elongated than the fimbrin ABD.

CONCLUSIONS

The dystrophin ABD structure reveals a previously uncharacterised arrangement of the CH domains within the ABD. This observation has implications for the mechanism of actin binding by dystrophin and related proteins. Examining the position of three pathogenic missense mutations within the structure suggests that they exert their effects through misfolding of the ABD, rather than through disruption of the binding to F-actin.

Characterization of PDZ-binding kinase, a mitotic kinase

hDlg, the human homologue of the Drosophila Discs-large (Dlg) tumor suppressor protein, is known to interact with the tumor suppressor protein APC and the human papillomavirus E6 transforming protein. In a two-hybrid screen, we identified a 322-aa serine/threonine kinase that binds to the PDZ2 domain of hDlg. The mRNA for this PDZ-binding kinase, or PBK, is most abundant in placenta and absent from adult brain tissue. The protein sequence of PBK has all the characteristic protein kinase subdomains and a C-terminal PDZ-binding T/SXV motif. In vitro, PBK binds specifically to PDZ2 of hDlg through its C-terminal T/SXV motif. PBK and hDlg are phosphorylated at mitosis in HeLa cells, and the mitotic phosphorylation of PBK is required for its kinase activity. In vitro, cdc2/cyclin B phosphorylates PBK. This evidence shows how PBK could link hDlg or other PDZ-containing proteins to signal transduction pathways regulating the cell cycle or cellular proliferation.

Plasma membrane cytoskeleton of muscle: a fine structural analysis

The discovery of dystrophin and its definition as the causative molecule in Duchenne Muscular Dystrophy has led to a renewed interest in the molecular structure of the muscle fiber plasma membrane and its association with the extracellular basal lamina. The original identification of dystrophin gave credence to the possibility that the plasma membrane of the muscle fiber may be highly organized and involved in maintaining appropriate homeostasis in this actively contracting cellular system. In this review, we examine the currently known members of the muscle fiber plasma membrane cytoskeleton and the interactions that occur between the different members of this complex using histological, electron microscopic, and confocal methods. From our studies and others cited in this review, it is clear that the dystrophin cytoskeletal complex is not completely understood and component molecules continue to be discovered. Perhaps equally importantly, currently defined molecules (such as alpha-actinin or neuronal nitric oxide synthase) are being recognized as being specifically associated with the complex. What is striking from all of the studies, to date, is that while we are able to identify members of the dystrophin cytoskeletal complex and while we are able to associate mutations of individual molecules with disease(s), we are still unable to truly define the roles of each of the molecules in maintaining the normal physiology of the muscle fiber.

Utrophin lacks the rod domain actin binding activity of dystrophin

We previously identified a cluster of basic spectrin-like repeats in the dystrophin rod domain that binds F-actin through electrostatic interactions (Amann, K. J., Renley, B. A., and Ervasti, J. M. (1998) J. Biol. Chem. 273, 28419-28423). Because of the importance of actin binding to the presumed physiological role of dystrophin, we sought to determine whether the autosomal homologue of dystrophin, utrophin, shared this rod domain actin binding activity. We therefore produced recombinant proteins representing the cluster of basic repeats of the dystrophin rod domain (DYSR11-17) or the homologous region of the utrophin rod domain (UTROR11-16). Although UTROR11-16 is 64% similar and 41% identical to DYSR11-17, UTROR11-16 (pI = 4. 86) lacks the basic character of the repeats found in DYSR11-17 (pI = 7.44). By circular dichroism, gel filtration, and sedimentation velocity analysis, we determined that each purified recombinant protein had adopted a stable, predominantly alpha-helical fold and existed as a highly soluble monomer. DYSR11-17 bound F-actin with an apparent K(d) of 7.3 +/- 1.3 microM and a molar stoichiometry of 1:5. Significantly, UTROR11-16 failed to bind F-actin at concentrations as high as 100 microM. We present these findings as further support for the electrostatic nature of the interaction of the dystrophin rod domain with F-actin and suggest that utrophin interacts with the cytoskeleton in a manner distinct from dystrophin.

Alpha-actinin and spectrin structures: an unfolding family story

In red blood cells, the integrity of the spectrin network is essential for normal cell shape and elasticity. To understand the molecular basis for spectrin's mechanical properties, one must determine how spectrin subunits interact with each other. The newly described crystallographic structures of two consecutive homologous repeats of human alpha-actinin, a member of the spectrin superfamily, shed new light on alpha-actinin interchain binding properties. Here I present evidence that interchain binding at the tail end of the spectrin molecule is likely to occur via a mechanism similar to that observed for alpha-actinin.

The 2.0 A structure of the second calponin homology domain from the actin-binding region of the dystrophin homologue utrophin

Utrophin is a close homologue of dystrophin, the protein defective in Duchenne muscular dystrophy. Like dystrophin, it is composed of three regions: an N-terminal region that binds actin filaments, a large central region with triple coiled-coil repeats, and a C-terminal region that interacts with components in the dystroglycan protein complex at the plasma membrane. The N-terminal actin-binding region consists of two calponin homology domains and is related to the actin-binding domains of a superfamily of proteins including alpha-actinin, spectrin and fimbrin. Here, we present the 2.0 A structure of the second calponin homology domain of utrophin solved by X-ray crystallography, and compare it to the other calponin homology domains previously determined from spectrin and fimbrin.

A cluster of basic repeats in the dystrophin rod domain binds F-actin through an electrostatic interaction

The dystrophin rod domain is composed of 24 spectrin-like repeats and was thought to act mainly as a flexible spacer between the amino-terminal actin binding domain and carboxyl-terminal membrane-associated domains. We previously demonstrated that a fragment of the dystrophin rod domain also binds F-actin. However, the nature and extent of rod domain association with F-actin is presently unclear. To begin addressing these questions, we characterized two recombinant proteins representing adjacent regions of the dystrophin rod. DYS1416 (amino acids 1416-1880) bound F-actin with a Kd of 14.2 +/- 5.2 microM and a stoichiometry of 1 mol:mol of actin. However, DYS1030 (amino acids 1030-1494) failed to bind F-actin, suggesting that not all rod domain repeats are capable of binding F-actin. Interestingly, DYS1416 corresponds to a unique region of the dystrophin rod rich in basic amino acids, whereas DYS1030 is composed mainly of acidic repeats. This observation suggested that DYS1416 may interact with acidic actin filaments through an electrostatic interaction. Supporting this hypothesis, actin binding by DYS1416 was dramatically inhibited by increasing ionic strength. We suggest that electrostatic interactions between basic spectrin-like repeats and actin filaments may contribute to the actin binding activity of other members of the actin cross-linking protein family.

Human skeletal muscle-specific alpha-actinin-2 and -3 isoforms form homodimers and heterodimers in vitro and in vivo

Alpha-actinins belong to a family of actin-binding and crosslinking proteins and are expressed in many different cell types. Multiple isoforms of alpha-actinin are found in humans and are encoded by at least four distinct genes. Human skeletal muscle contains two sarcomeric isoforms, alpha-actinin-2 and -3. Previous studies have shown that the alpha-actinins function as anti-parallel homodimers but the question of heterodimer formation between two different isoforms expressed in the same cell type has not been explored. To address this issue, we expressed both alpha-actinin-2 and -3 in vitro and were able to detect their interaction by both blot overlay and co-immunoprecipitation methods. We were also able to demonstrate the presence of heterodimers in vivo in human skeletal muscle and in COS-1 cells transiently transfected with both isoforms. Our results clearly demonstrate the potential for alpha-actinin isoforms to form heterodimers which might have unique functional characteristics.

Dystrobrevin and dystrophin: an interaction through coiled-coil motifs

Dystrobrevin, a dystrophin-related and -associated protein, has been proposed to be important in the formation and maintenance of the neuromuscular junction. Dystrobrevin coprecipitates with both the acetylcholine receptor complex as well as the dystrophin glycoprotein complex. Although the nature of dystrobrevin's association with the dystrophin glycoprotein complex remains unclear, it is known that dystrobrevin binds directly to the syntrophins, a heterologous group of dystrophin-associated proteins. Using the yeast two-hybrid system to identify protein-protein interactions, we present evidence for the heterodimerization of dystrobrevin directly with dystrophin. The C terminus of dystrobrevin binds specifically to the C terminus of dystrophin. We further refined this site of interaction to these proteins' homologous coiled-coil motifs that flank their respective syntrophin-binding sites. We also show that the interaction between the dystrobrevin and dystrophin coiled-coil domains is specific and is not due to a nonspecific coiled-coil domain interaction. From the accumulated evidence of protein-protein interactions presented here and elsewhere, we propose a partially revised model of the organization of the dystrophin-associated glycoprotein complex.