The cytoskeleton in muscle cells in relation to function.

The number of proteins identified to be involved in the cytoskeleton is steadily increasing. For insight into their assembly in situ, muscle fibres offer special advantages related to their regular organization of myofibrils, composed of interdigitating thick and thin filaments. The thick filaments, mainly composed of myosin molecules, form the A-band, whereas the thin filaments composed of actin, tropomysin and troponin project from either face of Z-discs in the I-band and interdigitate with the thick filaments in the A-band. The repeating unit, the sarcomere, extends from a Z-disc to a Z-disc. The arrangement of the filamentous muscle cytoskeleton is illustrated schematically in Fig. 1. It shows the myofibrils and two non-contractile filamentous systems, the intermediate filament lattice, which is an exosarcomeric lattice surrounding the myofibrils, which links them to the sarcolemma, to the nucelus and the titin lattice, which is an endosarcomeric lattice present within the myofibrils (for review, see [I, 21). In muscle cells and especially in working myocardial cells an extensive lattice of microtubules is also present (for review, see [ 31).