Obscurin Acts as a Semi-Flexible Chain in Solution

Abstract

Obscurin, a giant modular cytoskeletal protein, is comprised mostly of tandem Ig-like domains. This architecture allows obscurin to connect distal targets within the cell. The linkers connecting the Ig domains are usually short (3-4 residues). The physical effect arising from these short linkers is not known; such linkers may lead to a stiff elongated molecule or, conversely, may lead to a more compact and dynamic structure. In an effort to better understand how linker length affects obscurin flexibility, and to better understand the physical underpinnings of this flexibility, here we study the structure and dynamics of four representative sets of dual obscurin Ig domains using experimental and computational techniques. We find that in all cases tested, tandem obscurin Ig domains interact at the poles of each domain and tend to stay relatively extended in solution. NMR, SAXS and MD simulations reveal that while tandem domains are elongated, they also bend and flex significantly. By applying this behavior to a simplified model, it becomes apparent that obscurin can link targets up to 200 nm away. However, as targets get further than 200 nm apart, obscurin begins acting as a spring, and requires progressively more energy to elongate further.