Probing the stability of the "naked" mucin-like domain of human α-dystroglycan

Background: alpha-Dystroglycan (alpha-DG) is heavily glycosylated within its central mucin-like domain. The glycosylation shell of alpha-dystroglycan is known to largely influence its functional properties toward extracellular ligands. The structural features of this alpha-dystroglycan domain have been poorly studied so far. For the first time, we have attempted a recombinant expression approach in E. coli cells, in order to analyze by biochemical and biophysical techniques this important domain of the alpha-dystroglycan core protein. Results: We expressed the recombinant mucin-like domain of human alpha-dystroglycan in E. coli cells, and purified it as a soluble peptide of 174 aa. A cleavage event, that progressively emerges under repeated cycles of freeze/thaw, occurs at the carboxy side of Arg461, liberating a 151 aa fragment as revealed by mass spectrometry analysis. The mucin-like peptide lacks any particular fold, as confirmed by its hydrodynamic properties and its fluorescence behavior under guanidine hydrochloride denaturation. Dynamic light scattering has been used to demonstrate that this mucin-like peptide is arranged in a conformation that is prone to aggregation at room temperature, with a melting temperature of similar to 40 degrees C, which indicates a pronounced instability. Such a conclusion has been corroborated by trypsin limited proteolysis, upon which the protein has been fully degraded in less than 60 min. Conclusions: Our analysis indirectly confirms the idea that the mucin-like domain of alpha-dystroglycan needs to be extensively glycosylated in order to reach a stable conformation. The absence/reduction of glycosylation by itself may greatly reduce the stability of the dystroglycan complex. Although an altered pattern of alpha-dystroglycan O-mannosylation, that is not significantly changing its overall glycosylation fraction, represents the primary molecular clue behind currently known dystroglycanopathies, it cannot be ruled out that still unidentified forms of alpha DG-related dystrophy might originate by a more substantial reduction of alpha-dystroglycan glycosylation and by its consequent destabilization.