Biophysical characterization of the dystrophin C-terminal domain: Dystrophin interacts differentially with dystrobrevin isoforms.

Duchenne muscular dystrophy (DMD) gene encodes dystrophin, a large multi-domain protein. Its non-functionality leads to dystrophinopathies like DMD and Becker muscular dystrophy (BMD), for which no cure is yet available. A few therapies targeted towards specific mutations can extend the lifespan of patients, although with limited efficacy and high costs, emphasizing the need for more general treatments. Dystrophin's complex structure with poorly understood domains and the presence of multiple isoforms with varied expression patterns in different tissues pose challenges in therapeutic development. The C-terminal (CT) domain of dystrophin is less understood in terms of its structure-function, although it has been shown to perform important functional roles by interacting with another cytosolic protein, dystrobrevin. Dystrophin and dystrobrevin stabilize the sarcolemma membrane by forming a multi-protein complex called dystrophin-associated glycoprotein complex (DAGC) that is destabilized in DMD. Dystrobrevin has two major isoforms, alpha and beta, with tissue-specific expression patterns. Here, we characterize the CT domain of dystrophin and its interactions with the two dystrobrevin isoforms. We show that the CT domain is non-globular and shows reversible urea denaturation as well as thermal denaturation. It interacts with dystrobrevin isoforms differentially, with differences in binding affinity and the mode of interaction. We further show that the amino acid differences in the C-terminal region of dystrobrevin isoforms contribute to these differences. These results have implications for the stability of DAGC in different tissues and explain the differing symptoms associated with DMD patients affecting organs beyond the skeletal muscles.