Molecular architecture in muscle contractile assemblies.

Publisher Summary This chapter discusses the major muscle components, the actin and myosin filaments; and it also presents a third set of filaments, the titin filaments, which have remarkable properties and play a central role in integrating the sarcomere structure. The chapter further describes the way these filaments are organized in the muscle repeating unit, the sarcomere, through the cross-linking M-band and Z-band structures. The muscle sarcomere contains the principal contractile proteins, myosin and actin, which on their own can produce force and movement, together with a number of cytoskeletal and regulatory proteins. In a cross-section through the A-bands of vertebrate striated muscles, the myosin filaments lie on a hexagonal lattice, and the actin filaments are at the trigonal points, midway between three mutually adjacent myosin filaments. Vertebrate striated muscle sarcomeres contain two remarkable molecular rulers. One of these is titin, which runs from the Z-band to the M-band and provides the sarcomere with mechanical continuity. The other is nebulin, which is an I-band protein anchored in the Z-band.