European Molecular Biology Laboratory, Structural Biology Division, Heidelberg, Germany.
The thick filaments of vertebrate striated muscles contain with myosin a number of accessory proteins of the intracellular immunoglobulin superfamily, which are localized in a distinct pattern of stripes 43 nm apart. The specific localization of these proteins is believed to be due partly to their interaction with the giant muscle protein titin (also called connectin), which spans the entire sarcomere and may act as a molecular ruler. We have used recombinant fragments of titin covering the thick filament region to investigate their interaction with myosin-binding protein C (MyBP-C) from skeletal and cardiac muscle. The interaction of titin and MYBP-C is directed by a subset of titin immunoglobulin domains that are specific for the C-region of the thick filament, supporting the ruler hypothesis for the myosin-binding proteins. The interaction of recombinant titin with overlapping fragments of human cardiac MyBP-C maps the titin-binding site within the C-terminal region, which is deleted in patients suffering from the chromosome-11-associated form of familial hypertrophic cardiomyopathy. This disorder is therefore likely to be the result of thick-filament misassembly by abolishing the ternary interaction of titin, myosin and MyBP-C.