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.