Myofilament Structure

Myofilament is the term for the chains of (primarily) actin and myosin that pack a muscle fiber. These are the force generating structures.

Component Molecules

Although there are still gaps in what we know of the structure and functional significance of the myofilament lattice, some of the key proteins include:

Myosin

This peptide is responsible for force generation. It is composed of a globular head with both ATP and actin binding sites, and a long tail involved in its polymerization into myosin filaments.

Figure 1: Three-dimensional crystal structure of the S-1 portion of the myosin heavy chain molecule. (A) High resolution ribbon diagram of chicken S-1 myosin determined using X-ray crystallography. Labeled are the key features of the molecule, the ATP and actin binding sites, the cleft between them, and the a-helical region to which the myosin light chains binds. (Ribbon diagram kindly provided by Dr. Ivan Rayment, University of Wisconsin). (B) Schematic drawing of the myosin S-1 portion based on the ribbon diagram presented in (A) and used to illustrate the cross-bridge cycle.

Actin

The other major component in force production. Actin, when polymerized into filaments, forms the "ladder" along which the myosin filaments "climb" to generate motion.

Troponin

Apparently the major regulator of force production. Its three subunits lie in the groove of each actin filament blocking the myosin binding site, in the absence of ionic calcium.

Titin

An enormous (2500 kD) peptide that appears to be involve in maintaining the neatly ordered striation pattern. Closely associated with the myosin molecule, it appears to anchor the myosin network to the actin network.

Nebulin

Another actin associated molcule, nebulin appears to act as a molecular ruler regulating the length of actin filaments.

Interlocking Mesh Structure Seen in longitudinal section (with an electron microscope), a myofilament shows several distict bands, each of which has been given a special letter. The lightest (least electron dense) band is known as the I band and consists mostly of actin. The wide, dark band, known as the A band, is composed primarily of myosin. In the center of the I band is an electron dense line, known as the Z-line. In the middle of the A band is another dense line known as the M line.

In cross section, under very high magnification, both A and I bands can be seen to be hexagonal networks. These networks are apparently ordered and fixed at the M- and Z-lines. In the region where the A and I bands overlap (sometimes known as the H band) the two hexagonal networks intermesh so that each myosin filament is surrounded by six actin filaments.

These networks appear to be anchored to (and through) the cell membrane in two ways. At the ends of fibrils, special structures anchor the terminal actin filaments to the membrane. There also appear to be connections between the Z and M lines and the cell membrane.