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.
