The Structure of the T190M Mutant of Murine alpha-Dystroglycan at High Resolution: Insight into the Molecular Basis of a Primary Dystroglycanopathy

The severe dystroglycanopathy known as a form of limb-girdle muscular dystrophy (LGMD2P) is an autosomal recessive disease caused by the point mutation T192M in alpha-dystroglycan [1]. Functional expression analysis in vitro and in vivo indicated that the mutation was responsible for a decrease in posttranslational glycosylation of dystroglycan, eventually interfering with its extracellular-matrix receptor function and laminin binding in skeletal muscle and brain. The X-ray crystal structure of the missense variant T190M of the murine N-terminal domain of alpha-dystroglycan (50-313) has been determined [2], and showed an overall topology (Ig-like domain followed by a basket-shaped domain reminiscent of the small subunit ribosomal protein S6) very similar to that of the wild-type structure [3]. The crystallographic analysis revealed a change of the conformation assumed by the highly flexible loop encompassing residues 159-180. Moreover, a solvent shell reorganization around Met190 affects the interaction between the B1-B5 anti-parallel strands forming part of the floor of the basket-shaped domain, with likely repercussions on the folding stability of the protein domain(s) and on the overall molecular flexibility. Chemical denaturation and limited proteolysis experiments point to a decreased stability of the T190M variant with respect to its wild-type counterpart. This mutation may render the entire L-shaped protein architecture less flexible. The overall reduced flexibility and stability may affect the functional properties of ?-dystroglycan via negatively influencing its binding behavior to factors needed for dystroglycan maturation, and may lay the molecular basis of the T190M-driven primary dystroglycanopathy. [1] Y. Hara, B. Balci-Hayta, T. Yoshida-Moriguchi, M. Kanagawa, et al. N Engl J Med. 2011, 364, 939. [2] D. Bozic, F. Sciandra, D. Lamba, A. Brancaccio J Biol Chem. 2004, 279, 44812. [3] M. Bozzi, A.Cassetta, S. Covaceuszach, M.G. Bigotti, et al. PLoS One. 2015, 10, e0124277.