25.17 : Actin and Myosin in Muscle Contraction

Actin and myosin are contractile proteins that form the sarcomere found in skeletal muscle tissues for regulating muscle contraction. Actin, a globular contractile protein, interacts with myosin for muscle contraction. The skeletal tissue appears striped or striated under a microscope due to the repeated arrangement of contractile proteins actin and myosin along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes the entire cell to appear striated or banded. Each I band has a dense line running vertically through the middle called a Z disc or Z line. The Z discs mark the border of units called sarcomeres, the functional skeletal muscle units.

Sarcomere comprises of two main types of filaments: thick filaments and thin filaments; each has different compositions and locations. The tail of a myosin connects with other myosin molecules to form the central region of a thick filament near the M line. In contrast, the heads align on either side of the thick filament where the thin filaments overlap. The thin filaments consist of actin and two other accessory proteins, tropomyosin and troponin. The actin protein has myosin-binding sites for attachment. Strands of tropomyosin block the binding sites and prevent actin-myosin interactions when the muscles are at rest. Troponin consists of three globular subunits. One subunit binds to tropomyosin, one subunit binds to actin, and one subunit binds to Ca²⁺ ions. The motion of muscle shortening occurs as myosin heads bind to actin and pull the actin inwards. This action requires energy, which ATP provides. Myosin binds to actin at a binding site on the globular actin protein. Myosin has another binding site for ATP at which enzymatic activity hydrolyzes ATP to ADP, releasing an inorganic phosphate molecule and energy.

This text is adapted from Openstax, Biology 2e, Section 38.4: Muscle Contraction and Locomotion