The N-terminal Region of α-Dystroglycan is an Autonomous Globular Domain

The structure of the N-terminal region of mouse α-dystroglycan (DGN) was investigated by expression of two protein fragments (residues 30–180 and 30–438) in Escherichia coli cells. Trypsin susceptibility experiments show the presence of a stable α-dystroglycan N-terminal region (approximately from residue 30 to 315). In addition, guanidinium hydrochloride (Gdn/HCl) denaturation of DGN-(30–438)-peptide, monitored by means of tryptophan fluorescence, produces a cooperative transition typical of folded protein structures. These results strongly suggest that the α-dystroglycan N-terminal is an autonomous folding unit preluding a flexible mucin-like region and that its folding is not influenced by the absence of glycosylation. In order to obtain more information on the structural features of the N-terminal domain we have also used circular dichroism, analytical sedimentation and electron microscopy analysis. Circular dichroic spectra show the absence of typical secondary structure (e.g. α-helix or β-sheet) and closely resemble those recorded for loop-containing proteins. This is consistent with a sequence similarity of the α-dystroglycan domain with the loop-containing protein elastase. Analytical ultracentrifugation and electron microscopy analysis reveal that the N-terminal domain has a globular structure. DGN-(30–438)-peptide does not bind in the nanomolar range to an iodinated agrin fragment which binds with high affinity to tissue purified α-dystroglycan. No binding was detected also to laminin. This result suggests that the α-dystroglycan N-terminal domain does not contain the binding site to its extracellular matrix binding partners. It is less likely than the lack of glycosylation reduces its binding affinity, because the N-terminal globular domain only contains two glycosylation sites