Leiden Muscular Dystrophy pages

alpha- and beta-Dystroglycan (DAG1)

(last modified January 14, 2006)



Contents




Summary


alpha- and -dystroglycan (DAG1), also known as cranin (Smalheiser & Kim) are ubiquitously expressed in various tissues. They are translated from a single mRNA as a 97 kDa protein which is proteolytically cleaved into two, closely associated peptides (Ibraghimov-Beskrovnaya, Smalheiser & Schwartz). alpha-Dystroglycan is an extra-cellular protein which binds to alpha-laminin, probably through its sugar chains, and to -dystroglycan. Laminin is a component of the basal lamina (extracellular matrix), comprised of three subunits: alpha, beta and gamma. The alpha-laminin (merosin) subunit binds to alpha-dystroglycan. -dystroglycan is a transmembrane protein which, outside the cell, binds to alpha-dystroglycan and, inside the cell, to the cysteine-rich domain and the first half of the carboxyl-terminal domain of dystrophin (together designated the dystroglycan binding (D-) domain). No human disease has yet been linked to dystroglycan mutations. Mice homozygously lacking dystroglycan show embryonic lethality, thought to arise from defects in extra-embryonic structures and their association with the extra-cellular matrix (Williamson).

Dystroglycan is a heavily glycosylated, peripheral membrane protein that directly binds, probably through its sugar chains, to alpha-laminin (an extracellular matrix protein) and to -dystroglycan. The proposed function of -dystroglycan is to provide a link between the subsarcolemmal cytoskeleton and the extra-cellular matrix.

Originally, dystroglycan was described by Smalheiser & Kim as "cranin". Later, Ervasti described the same protein as 156 (alpha-) and 43 kD (-) proteins associated with dystrophin. Ibraghimov-Beskrovnaya., who suggested the name "dystroglycan" (dystrophin-associted glycoprotein), .... Smalheiser & Schwartz.... showed that both proteins are encoded by a single chromosome 3p21-derived 5.8 kb transcript. Mature alpha- and -dystroglycan are derived from this transcript by post-translational processing of the encoded 97 kD precursor protein. -Dystroglycan contains a single transmembrane domain, three potential N-linked glycosylation sites and a cytoplasmic tail. alpha-Dystroglycan contains no transmembrane domain, one potential N-linked and numerous O-linked glycosylation sites.

No human disease has yet been linked to dystroglycan mutations. Mice homozygously lacking dystroglycan show embryonic lethality, thought to arise from defects in extra-embryonic structures and their association with the extra-cellular matrix (Williamson).


The dystroglycan gene


Ibraghimov-Beskrovnaya et al. first reported the cloning of dystroglycan after screening of a rabbit skeletal muscle cDNA expression library with a polyclonal antibody raised against -dystroglycan (43 kDa DAG). The human -dystroglycan gene, designated DAG1, was cloned and characterized by Ibraghimov-Beskrovnaya et al. The coding region of the gene is split over one small and one large (2.4 kb) exon, the 5'- and 3'-untranslated regions over 2 or more exons. The chromosomal localization, determined by using somatic cell hybrids and FISH, was 3p21.1-21.31. This region is syntenic with mouse chromosome 9.

Exon Exon size (bp) Intron size (kb) 5' cDNA position Splice after Remarks
1 ? >10 ? - 5'UTR
2 >285 >6 ? 0 5'UTR / 285 bp coding
3 2,400 <3 286 - 2400 bp coding / 3'UTR; TMR, N-Glyco Asn-641, -649, -661
4.. ? ? - - 3'UTR

Legend:
Exon: numbering of exons and intron/exon boundaries are according to Ibraghimov-Beskrovnaya, with the first base of the Met-codon counted as position 1 (see Reference sequence). Exon size: size of exon indicated in basepairs. Intron size: size of intron indicated in kilobasepairs. 5' cDNA position: first base of the exon (according to cDNA sequence Ibraghimov-Beskrovnaya). Splice after: splicing occurs in between of two coding triplets (0), after the first (1) or the second (2) base of a triplet. Remarks: 5'UTR = 5' untranslated region, 3'UTR = 3' untranslated region, N-Glyco = potential N-linked glycosylation site, Phos = putative phosphorylation site, TMR = transmembrane region.

primers for DNA-amplification

None reported yet.


The dystroglycan mRNA


On Northern blots, a dystroglycan transcript of 5,8 kb could be detected in fetal and adult cardiac muscle, brain, kidney, liver and lung and in adult diaphragm, placenta, pancreas, skeletal muscle and stomach (Ibraghimov-Beskrovnaya, Ibraghimov-Beskrovnaya). Expression is most abundant in muscle and heart.

primers for RNA-amplification

None reported yet.


The dystroglycan protein


Dystroglycan, for dystrophin-associted glycoprotein (Ibraghimov-Beskrovnaya), is an integral membrane glycoprotein localized to the sarcolemma of skeletal muscle. Dystroglycan mRNA contains a 895 amino acid open reading frame encoding a precursor protein with a calculated molecular weigth of 97,552 daltons (Ibraghimov-Beskrovnaya). The first 27 amino acids of the precursor are predominantly hydrophobic and probably represent a signal peptide. Post-translational processing of the 97 kD precursor, probably through cleavage at Arg-457, ultimately yields the two mature proteins alpha-dystroglycan (C-terminal half) and -dystroglycan (N-terminal half). The exact cleavage point could not be determined because the N-terminus of alpha-dystrocglycan is blocked.

On Western blot, 43 kD beta-dystroglycan was found in membranes from skeletal and cardiac muscle, brain and lung. 156 kD alpha-Dystroglycan was found in skeletal muscle membranes, with a slightly lower Mr in cardiac muscle membranes and as a ~120 kD protein in brain and lung membranes (the size differences probably originate from differential glycosylation).

Human and rabbit dystroglycan show a 93% amino acid identity, while 90% of the amino acid changes are conservative.

dystroglycan antibodies


Dystroglycan and disease


No human disease has yet been linked to dystroglycan mutations. Mice homozygously lacking dystroglycan show embryonic lethality, thought to arise from defects in extra-embryonic structures and their association with the extra-cellular matrix (Williamson).

Although dystroglycan mRNA levels in DMD-patients and mdx-mouse muscle are normal, alpha- and -dystroglycan protein are drastically reduced. On the contrary, alpha-dystroglycan protein is normal in brain and kidney membranes of mdx-mice.


Disease-causing dystroglycan mutations

None reported yet.


dystroglycan polymorphisms


Ibraghimov-Beskrovnaya report the existence of an EcoRI polymorphism in genomic DNA hybridized with dystroglycan cDNA, i.e. hybridizing fragments of 12 kb or 9 + 3 kb.


Miscellaneous


DNA-primers:


None reported yet.


RNA-primers:


None reported yet.


dystroglycan sequences



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