Resisting sarcolemmal rupture: dystrophin repeats increase membrane-actin stiffness

Dystrophin is an essential part of a membrane protein complex that provides flexible support to muscle fiber membranes. Loss of dystrophin function leads to membrane fragility and muscle-wasting disease. Given the importance of cytoskeletal interactions in strengthening the sarcolemma, we have focused on actin-binding domain 2 of human dystrophin, constituted by repeats 11 to 15 of the central domain (DYS R11-15). We previously showed that DYS R11-15 also interacts with membrane lipids. We investigated the shear elastic constant (μ) and the surface viscosity (η(s)) of Langmuir phospholipid monolayers mimicking the inner leaflet of the sarcolemma in the presence of DYS R11-15 and actin. The initial interaction of 100 nM DYS R11-15 with the monolayers slightly modifies their rheological properties. Injection of 0.125 μM filamentous actin leads to a strong increase of μ and η(s,) from 0 to 5.5 mN/m and 2.4 × 10(-4) N · s/m, respectively. These effects are specific to DYS R11-15, require filamentous actin, and depend on phospholipid nature and lateral surface pressure. These findings suggest that the central domain of dystrophin contributes significantly to the stiffness and the stability of the sarcolemma through its simultaneous interactions with the cytoskeleton and lipid membrane. This mechanical link is likely to be a major contributing factor to the shock absorber function of dystrophin and muscle sarcolemmal integrity on mechanical stress.