Actin

A globular protein found in muscle tissue as thin filaments and in microfilaments that form portions of cell cytoskeletons. It links into chains, and paired chains twist helically around each other, forming microfilaments in muscle and other contractile elements in cells. Actin and myosin filaments interact to initiate muscle contraction.

Tropomyosin and troponin are two protein molecules associated with its filaments in muscle. Tropomyosin runs along the length of these fila­ments and covers the area of the these molecule that interacts with myosin when at rest. on the other hand, when a muscle is contracted, tropomyosin is replaced with troponin as it binds to calcium ions. Troponin is located at regular intervals along the actin filament and allows actin to interact with myosin.