Filamentous Aggregates of Native Titin and Binding of C-protein and AMP-deaminase

In solutions of high ionic strength, native titin-2, a large extractable fragment of the sarcomere matrix protein titin, appears as extremely long, flexible, and slender beaded strings. We report here that in solutions of lower ionic strength near neutral pH, titin-2 assembles into higher-order aggregates with surface projections. Solid phase binding assays show that two myosin-binding proteins, C-protein and AMP-deaminase, are also titin-binding proteins. Both proteins decorate titin aggregates, producing filaments of more uniform appearance. Numerical Fourier transforms of these decorated aggregates show ∼12-nm periodicities. The interaction of titin with myosin-associated proteins such as C-protein may take part in the anchoring mechanism that prevents the stretching and extension of titin filaments in the A band.