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Cadherin-like domains in α-dystroglycan, α/ε-sarcoglycan and yeast and bacterial proteins

2002; Elsevier BV; Volume: 12; Issue: 6 Linguagem: Inglês

10.1016/s0960-9822(02)00748-0

1879-0445

Nicholas J. Dickens, Scott A. Beatson, Chris P. Ponting,

RNA Research and Splicing

Dystrophin, a gene product that is mutated in individuals with Duchenne muscular dystrophy, is tethered to the extracellular matrix via membrane-associated multimolecular complexes. In striated muscle cells this complex contains two glycoprotein subcomplexes, the sarcoglycan (SG) and dystroglycan (DG) complexes. Disruption of these large transmembrane complexes has been shown to result in muscle disease. Altered glycosylation of α-DG is associated with two types of congenital muscular dystrophy [1.Hayashi Y.K. Ogawa M. Tagawa K. Noguchi S. Ishihara T. Nonaka I. Arahata K. Selective deficiency of α-dystroglycan in Fukuyama-type congenital muscular dystrophy.Neurology. 2001; 57: 115-121Crossref PubMed Scopus (206) Google Scholar, 2.Brockington M. Blake D.J. Prandini P. Brown S.C. Torelli S. Benson M.A. Ponting C.P. Estournet B. Romero N.B. et al.Mutations in the fukutin-related protein gene (FKRP) cause a form of congenital muscular dystrophy with secondary laminin α2 deficiency and abnormal glycosylation of α-dystroglycan.Am. J. Hum. Genet. 2001; 69: 1198-1209Abstract Full Text Full Text PDF PubMed Scopus (492) Google Scholar] and mutations in the α-DG–binding laminin α2 gene product is linked to a third congenital muscular dystrophy [3.Helbling-Leclerc A. Zhang X. Topaloglu H. Cruaud C. Tesson F. Weissenbach J. Tome F.M. Schwartz K. Fardeau M. Tryggvason K. et al.Mutations in the laminin α 2-chain gene (LAMA2) cause merosin-deficient congenital muscular dystrophy.Nat. Genet. 1995; 11: 216-218Crossref PubMed Scopus (543) Google Scholar]. Mutations in α-SG (adhalin) and ε-SG result in type 2D limb-girdle muscular dystrophy (LGMD2D) and myoclonus-dystonia syndrome (a CNS disorder), respectively [4.Roberds S.L. Leturcq F. Allamand V. Piccolo F. Jeanpierre M. Anderson R.D. Lim L.E. Lee J.C. Tome F.M. Romero N.B. et al.Missense mutations in the adhalin gene linked to autosomal recessive muscular dystrophy.Cell. 1994; 78: 625-633Abstract Full Text PDF PubMed Scopus (414) Google Scholar, 5.Zimprich A. Grabowski M. Asmus F. Naumann M. Berg D. Bertram M. Scheidtmann K. Kern P. Winkelmann J. Muller-Myhsok B. et al.Mutations in the gene encoding ε-sarcoglycan cause myoclonus-dystonia syndrome.Nat. Genet. 2001; 29: 66-69Crossref PubMed Scopus (380) Google Scholar]. The dystroglycan gene product is cleaved post-translationally to yield two associated glycoproteins [6.Winder S.J. The complexities of dystroglycan.Trends Biochem. Sci. 2001; 26: 118-124Abstract Full Text Full Text PDF PubMed Scopus (212) Google Scholar]. α-DG represents the highly glycosylated amino-terminal portion, which binds several extracellular molecules, whereas the β-DG carboxy-terminal portion spans the membrane and links to the actin cytoskeleton via dystrophin or its paralogue utrophin. DG and α/ε-SG homologues are known in other vertebrates and invertebrates yet their domain contents and evolutionary heritages have not been reported. Here we reveal that DG and α/ε-SG sequences contain cadherin domain homologues (see legend to Fig. 1). In animals, cadherin domain-containing proteins are adhesion molecules that modulate a wide variety of processes including cell polarization and migration [7.Tepass U. Truong K. Godt D. Ikura M. Peifer M. Cadherins in embryonic and neural morphogenesis.Nat. Rev. Mol. Cell Biol. 2000; 1: 91-100Crossref PubMed Scopus (384) Google Scholar]. Our study also identified cadherin domains in Saccharomyces cerevisiae Axl2p (also known as Sro4p and Bud10p) and several very large proteins from magnetotactic bacteria. The use of a predicted protein from Magnetococcus MC1 was critical to the discovery of the αDG, α/ε-SG and Axl2p cadherin domains. This sequence is unusual in two respects. First, it contains 11699 amino acids, when the mean length of a bacterial sequence is approximately 300 amino acids. Second, it also contains integrin-α-like β-propellers and laminin G-like domains that are more typical of eukaryotic extracellular proteins. Axl2p, αDG, α/εSG and cadherins are similar not only in containing homologous domains, but also in their cellular localizations, functions and posttranslational modifications. Axl2p, DG, α/εSG and cadherins are all type I transmembrane proteins with their homologous cadherin domains located in the extracellular environment. Axl2p and cadherins also appear to be involved in establishing cell polarity, albeit in different unicellular and multicellular contexts [7.Tepass U. Truong K. Godt D. Ikura M. Peifer M. Cadherins in embryonic and neural morphogenesis.Nat. Rev. Mol. Cell Biol. 2000; 1: 91-100Crossref PubMed Scopus (384) Google Scholar, 8.Roemer T. Madden K. Chang J. Snyder M. Selection of axial growth sites in yeast requires Axl2p, a novel plasma membrane glycoprotein.Genes Dev. 1996; 10: 777-793Crossref PubMed Scopus (112) Google Scholar]. Furthermore, disruption of glycosylation of Axl2p and αDG causes dysfunction [6.Winder S.J. The complexities of dystroglycan.Trends Biochem. Sci. 2001; 26: 118-124Abstract Full Text Full Text PDF PubMed Scopus (212) Google Scholar, 9.Sanders S.L. Gentzsch M. Tanner W. Herskowitz I. O-glycosylation of Axl2/Bud10p by Pmt4p is required for its stability, localization, and function in daughter cells.J. Cell Biol. 1999; 145: 1177-1188Crossref PubMed Scopus (60) Google Scholar] which, for αDG, results in muscle disease. Crystal structures have revealed that multiple cadherin domains form Ca2+-dependent rod-like structures with a conserved Ca2+-binding pocket at the domain–domain interface [10.Nagar B. Overduin M. Ikura M. Rini J.M. Structural basis of calcium-induced E-cadherin rigidification and dimerization.Nature. 1996; 380: 360-364Crossref PubMed Scopus (543) Google Scholar]. Multiple alignment of αDG, α/εSG, Axl2p and bacterial cadherin domain sequences with cadherin domains of known structure (Fig. 1) show that most of the Ca2+-binding residues in the crystal structures are conserved. This implies that the newly identified cadherin domains may associate via a Ca2+-dependent association mechanism, by forming homotypic or heterotypic multimers. These predictions may assist the rational design of DG- or α/ε-SG-binding drugs. αDG represents an important therapeutic target not only for its role in muscle disease, but also for its role as a cellular receptor for Mycobacterium leprae, the causative organism of leprosy, and for Old World arenaviruses [6.Winder S.J. The complexities of dystroglycan.Trends Biochem. Sci. 2001; 26: 118-124Abstract Full Text Full Text PDF PubMed Scopus (212) Google Scholar]. The amino-terminal region (amino acids 80–180) of αDG had previously been suggested to belong to the immunoglobulin K family [11.Bozic D. Engel J. Brancaccio A. Sequence analysis suggests the presence of an IG-like domain in the N-terminal region of α-dystroglycan which was crystallized after mutation of a protease susceptible site (Arg168→His).Matrix Biol. 1998; 17: 495-500Crossref PubMed Scopus (16) Google Scholar]. This region overlaps with the cadherin homology domain (amino acids 58–158, approximately) identified here. The carboxy-terminal cadherin domain of αDG (amino acids 493–597, approximately) has a protein-binding function. It coincides with the αDG regions (amino acids 485–651 and 494–653, respectively) that bind αDG and the leucine-rich repeat proteoglycan biglycan [12.Sciandra F. Schneider M. Giardina B. Baumgartner S. Petrucci T.C. Brancaccio A. Identification of the β-dystroglycan binding epitope within the C-terminal region of α-dystroglycan.Eur. J. Biochem. 2001; 268: 4590-4597Crossref PubMed Scopus (38) Google Scholar, 13.Bowe M.A. Mendis D.B. Fallon J.R. The small leucine-rich repeat proteoglycan biglycan binds to α-dystroglycan and is upregulated in dystrophic muscle.J. Cell Biol. 2000; 148: 801-810Crossref PubMed Scopus (129) Google Scholar]. The cadherin domain in αSG is the location of 16 out of 28 known mis-sense mutations that result in type 2D limb-girdle muscular dystrophy (Fig. 1). As cadherin domains often are involved in protein–protein interactions it is likely that the α/εSG cadherin domains function by associating with other molecules of the dystrophin–glycoprotein complexes. Mapping the 16 amino acid substitutions onto cadherin crystal structures shows that they all lie within, or at the ends of, β-strands (Fig. 1 and Fig. 2). This implies that these mutations are likely to disrupt the structure or folding of this domain. The exceptions to this are replacements of arginines with both histidine and cysteine at two distinct positions (R34→His, R34→Cys; R98→His, R98→Cys) which map onto Ca2+-binding sites in cadherin structures (Fig. 2). It is possible therefore, that these substitutions abrogate an α-sarcoglycan Ca2+-binding function. These findings are likely to assist in the understanding of DG and α/ε-SG functions, and their roles in muscle and CNS disease. Preliminary sequence data were obtained from the DOE Joint Genome Institute (JGI) at www.jgi.doe.gov/JGI_microbial/html/index.html. We thank Kay Davies and Derek Blake for valuable discussions.