Molecular basis of the head-to-tail assembly of giant muscle proteins obscurin-like 1 and titin.

Large filament proteins in muscle sarcomeres comprise many immunoglobulin-like domains that provide a molecular platform for self-assembly and interactions with heterologous protein partners. We have unravelled the molecular basis for the head-to-tail interaction of the carboxyl terminus of titin and the amino-terminus of obscurin-like-1 by X-ray crystallography. The binary complex is formed by a parallel intermolecular beta-sheet that presents a novel immunoglobulin-like domain-mediated assembly mechanism in muscle filament proteins. Complementary binding data show that the assembly is entropy-driven rather than dominated data by specific polar interactions. The assembly observed leads to a V-shaped zipper-like arrangement of the two filament proteins.