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Review Increasing Complexity of the Dystrophin-Associated Protein Complex Jonathon M Proc. Nadl. Acad. Sci. USA Vol. 91, pp. 8307-8313, August 1994 Review Increasing complexity of the dystrophin-associated protein complex Jonathon M. Tinsley, Derek J. Blake, Richard A. Zuellig, and Kay E. Davies Molecular Genetics Group, Institute of Molecular Medicine, John Radcliffe Hospital, Headington, Oxford OX3 9DU, United Kingdom ABSTRACT Duchenne muscular dys- Purkinje neurons. Alternatively spliced dystrophin-1 (Dp7l) and apo-dystro- trophy is a severe X chromosome-linked, isoforms originating from the carboxyl- phin-3 are regulated by a promoter situ- muscle-wasting disease caused by lack of terminal coding region ofdystrophin have ated between exons 62 and 63 of the the protein dystrophin. The exact function also been described. The significance of dystrophin gene and are expressed in of dystrophin rem to be determined. these isoforms at the RNA and protein nonmuscle tissues, including brain, lung, However, analysis of its interaction with a level has not been elucidated. liver, and kidney. Apo-dystrophin-1 tran- large oligomeric protein complex at the Dystrophin is a 427-kDa protein local- scripts are only detectable in fetal and sarcolemma and the identicaton of a ized to the cytoplasmic face of the sar- newborn muscle. Muscle samples taken structurally related protein, utrophin, is colemma, enriched at myotendinous after 15 days postnatally have no apo- leading to the characterization ofcandidate junctions and the postsynaptic mem- dystrophin-1 transcript as determined by genes for other neuromusular disorders. brane of the neuromuscular junction reverse transcription-PCR. In rat brain, (NMJ). Dystrophin colocalizes with apo-dystrophin-1 transcripts continue in- Duchenne muscular dystrophy (DMD) is f3-spectrin and vinculin in three distinct creasing until they reach a maximum the most common muscular dystrophy, domains at the sarcolemma (overlaying after -1 mo. Thus apo-dystrophin-1 ap- affecting 1 in 3,300 boys. It leads to both I bands and M lines), lying as an pears to be developmentally regulated in severe muscle wasting and eventual array ofthick bands localizing at the sites muscle and brain (4). Dpll6 (apo- death in the late teens or early twenties of attachment of the sarcomeres to the dystrophin-2) is regulated by a promoter due to cardiac or respiratory failure. muscle plasma membrane. The carboxyl- situated between exons 55 and 56 of the Becker muscular dystrophy (BMD) is an terminal region of dystrophin is bound to dystrophin gene and is expressed in pe- allelic disorder that is less common (1 in the protoplasmic half of the plasma- ripheral nerve. In contrast to dystrophin, 30,000 boys) and less severe, with a later lemma. Thus dystrophin forms an intri- neither apo-dystrophin-1 nor apo-dystro- onset and much longer survival rate. cate part of the muscle cytoskeleton and phin-2 is expressed before organogenesis Both disorders are caused by mutations may function to link the normal contrac- in the developing mouse embryo (5). within the dystrophin gene, resulting in a tile apparatus to the sarcolemma. In the Apo-dystrophin-1 is first detected at 11.5 lack of dystrophin (usually DMD) or the brain, dystrophin is also localized at the days in the ventral midline of the hind- expression ofmutant forms ofdystrophin postsynaptic regions of some neurons. brain. Later expression of apo-dystro- (usually BMD). Dystrophin is a member The amino terminus of dystrophin has phin-1 occurs in central nervous system of a growing family of related proteins, been shown in vitro to contain a functional glia, teeth primordia, and in the liver. including the autosomally encoded pro- actin-binding domain. Deletions within Apo-dystrophin-2 appears to be a rare tein utrophin. There have been several this region ofthe dystrophin gene result in transcript during development but is de- recent reviews covering the structure and a range of phenotypes from mild to se- tected at low levels in the region of the potential functions of dystrophin and vere, suggesting that the ability to bind caudate putamen in newborn mouse utrophin, and the reader is referred to actin is an important, but not an absolute, brain (5). these for detailed references (1-3). This requirement for dystrophin function. The Utrophin is a 395-kDa protein encoded review focuses on the proteins with central rod domain consists ofa numberof by a large (1 Mb) multi-exonic gene lo- which dystrophin and utrophin interact. a-helical coiled-coil repeats that are sim- cated on chromosome 6q24(6). The amino ilar to spectrin and probably give the acid sequence shows that utrophin has a Dystrphin and Utrophin Genes: molecule a flexible rod-shaped structure. strong similarity to dystrophin. Utrophin A Brief Overview In-frame deletions within this domain usu- has an amino-terminal actin-binding do- ally give rise to intermediate or milder main (S. J. Winder, L. Hemmings, S. K. The dystrophin gene is the largest gene so BMD. The carboxyl-terminal domain has Maciver, S. J. Bolton, J.M.T., K.E.D., far identified in humans, covering >2.5 no homology with any other identified D. R. Critchley, and J. Kendrick-Jones, megabases and containing 79 exons. The sequences apart from the related protein personal communication), a repeated unusually high mutation rate at the DMD utrophin. Mutations in the cysteine-rich coiled-coil rod domain, and a cysteine- locus may, inpart, be accountedforby the domain and first half of the carboxyl- rich carboxyl-terminal domain. In normal large size of the gene. The corresponding terminal domain almost always result in adult muscle, utrophin is localized at the 14-kb dystrophin mRNA is expressed pre- severe DMD phenotypes, indicating their membrane of the NMJ. High-resolution dominantly in skeletal, cardiac, and importance for the normal function of analysis of rat NMJs suggests that utro- smooth muscle; lower levels appear in dystrophin. Loss of the last 200 amino phin is colocalized with the acetylcholine brain. Transcription of dystrophin in dif- acids ofdystrophin appears to produce an receptors (AChRs) at the crests of the ferent tissues is regulated from either the intermediate DMD/BMDphenotype. The junctional folds (7) and may play a role in brain promoter (predominantly active in potential functions of the dystrophin car- neuronal cells) or muscle promoter (dif- boxyl terminus are discussed later. Abbreviations: DMD, Duchenne muscular ferentiated myogenic cells and primary Three shorter transcripts, expressed dystrophy; BMD, Becker muscular dystro- glial cells) giving rise to different first from two different phy; NMJ, neuromuscular junction; DAP, promoters, encode dystrophin-associated protein; DAG, dystro- exons. A thirdpromoterbetweenthe mus- proteins that have a majority ofsequence phin-associated glycoprotein; AChR, acetyl- cle promoter and the second exon ofdys- identical to that of the cysteine-rich and choline receptor; SCARMD, severe childhood trophin regulates expression in cerebellar carboxyl terminus of dystrophin. Apo- autosomal-recessive muscular dystrophy. 8307 Downloaded by guest on September 28, 2021 8308 Review: Tinsley et al. Proc. Natl. Acad. Sci. USA 91 (1994) the clustering of AChRs (8). In fetal mus- complex and other associated proteins cle (18). The protein consists of a short cle development, utrophin is expressed localized to the sarcolemma (described signal sequence, one transmembrane do- before dystrophin. Higher levels of utro- later). main, and two potential sites for N-linked phin mRNA have been detected in human The 43DAG (A3) and 156DAG are de- glycosylation. In contrast to dystrogly- and mouse fetal tissue compared with rived from a single 97-kDa precursor pro- can, expression of the SODAG is specific adult. In normal humanfetal muscle, utro- tein translated from a 3.8-kb transcript to skeletal, cardiac, and smooth muscle phin is localized to the sarcolemma and (15). The 43-156DAG transcript is ex- (18, 19). This protein has been renamed NMJs, showing maximum expression at pressed not only in skeletal, cardiac, and adhalin (from the Arabic word for mus- 17-18 weeks of gestation (9). When dys- smooth muscle but also in other tissues- cle, adhal). trophin is expressed, utrophin is gradually namely, brain, lung, liver, and kidney. At present the cloning of the 35DAG depleted from the sarcolemma to be re- Cleavage and posttranslational modifica- (A4) and 25DAP (AS) has not been re- placed by dystrophin, leaving the NMJs tion of the carboxyl-terminal portion ported, so their potential functions have as the only site ofutrophinlocalization (9). gives rise to the 43DAG. The 43DAG has yet to be elucidated. However, 35DAG It is not known whether utrophin has a sites for glycosylation, a transmembrane expression appears restricted to striated functional role while located at the fetal domain, and a cytoplasmic tail (15). On muscle (19). A 94-kDa protein, AO, which sarcolemma-i.e., the fetal form of dys- acrylamide gels, the 43DAG (A3) is a also copurifies with the DAP complex, trophin-or is simply in the process of component of a doublet, A3a and A3b has been identified (12). Circumstantial iigrating with the developing AChR clus- (12, 16). The 43DAG is the same as A3a, evidence suggests that AO may be related ters beforeformation ofthe mature NMJs. as determined by peptide sequence anal- to the Torpedo 87-kDa postsynaptic pro- In contrast to dystrophin, utrophin is de- ysis of the purified gel band. A3b is tein (discussed later). tectable in most tissues-particularly the distinct from A3a, as it has a different Recently, the purified DAP complex lung, blood vessels, and nerves. Utrophin proteolytic cleavage pattern (14) and is purified from rabbit skeletal muscle has is enriched in the vascular regions of the unable to bind dystrophin (16). The 56- been divided into three subcomplexes brain-for instance, the choroid plexus kDa amino-terminal core protein once based upon detergent solubilization using and at the astrocytic foot processes ofthe glycosylated at many of the potential 1-octyl f-D-glucoside (14).
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