Roles of titin in the structure and elasticity of the sarcomere

Abstract

The giant protein titin is thought to play major roles in the assembly and function of muscle sarcomeres. Structural details, such as widths of Z- and M-lines and periodicities in the thick filaments, correlate with the substructure in the respective regions of the titin molecule. Sarcomere rest length, its operating range of lengths, and passive elastic properties are also directly controlled by the properties of titin. Here we review some recent titin data and discuss its implications for sarcomere architecture and elasticity.

Figure 1

Schematic representation of titin layout in the sarcomere (a) and of the domain periodicity in the thick filament bound part (b). Striated zones in the sarcomere and the titin molecule (a) show the location of the large superrepeat.

Figure 2

Electron micrographs of negatively stained titin, illustrating domain substructure and flexibility (a), and the effect of bundling on the apparent stiffness of the molecule (b). (a) Note that each of the flexibility “waves” seen in the titin contour contains about 4 domains. This number is comparable with the number of domains (3-4) expected for segments of about the persistence length of the protein. (b) This micrograph shows partially dissolved titin “end-filament”. The “wavy” contours of the molecules in the unbundled region contrast with their relatively straight shapes in the bundled part. Magnification: bar (a) 20 nm; (b) 50 nm.