The structure of myosin has also been studied by breaking it down into smaller pieces with enzymatic digestion. For example, the enzyme papain splits off the head groups and a small portion of the rod from the rest of the myosin molecule. The portion with the head groups is called heavy meromyosin, whereas the rod portion is called light meromyosin. With further digestion, the two head groups can be separated from each other. As far as is known, the head groups are identical, each weighing about 120,000 daltons. In the muscle, the myosin molecules are arranged with the head groups slanting away from the middle of the thick filament. In the middle of the thick filament, the tails of the myosin molecules overlap one another end to end, creating a region devoid of head groups and with a smooth appearance on electron microscopy. A structural protein called titin acts as a central scaffold for properly arranging myosin molecules into thick filaments and provides anchoring points for the thick filaments at each opposing Z band within a sarcomere. It extends from the Z line of the sarcomere to the M band and its coiled domains provide for elastic deformation during sarcomere contraction. It functions as a sarcomeric ruler and as a template for sarcomere assembly.

Thin filaments consist chiefly of a protein called fibrous actin, or F-actin, which is in the form of a double helix. In very dilute salt solutions, F-actin breaks down into globular protein molecules called globular actin, or G-actin. These molecules are much smaller than myosin, with molecular weights of about 42,000 daltons. If the concentration of salt in the solution is increased, the G-actin molecules repolymerize end to end into their normal chainlike configuration. Thus, the actin filament is like a double string of G-actin “pearls” wound around each other. One turn of the helix contains 13.5 molecules of G-actin.

Although G-actin is the largest constituent of thin filaments, three other proteins form part of the structure and play important roles in muscle contraction. Along the notches between the two strands of actin subunits lie molecules of a globular protein, troponin. (Actually, this is a complex of three polypeptide subunits—troponin I, troponin C, and troponin T—each of which plays an important role in muscle contraction.) Attached to each troponin (at the T subunit) is a molecule of a thin, fibrous protein, tropomyosin, which lies along the grooves in the double helix. The precise disposition of tropomyosin along the F-actin chain probably varies importantly during the contraction-relaxation cycle. A third structural protein called nebulin extends along the length of thin filaments and the entire I band. Analogous to titin’s role in the thick filaments, nebulin acts as a templating scaffold for thin filament assembly.