Actin is a type of protein found in muscles. It is a globular protein that is involved in muscle contraction and cell movement. Actin is the most abundant protein in the human body, making up around 10% of total body protein. It is also found in other types of cells, such as those in the immune system and in the nervous system. Actin plays an important role in muscle contraction, as it helps to move the myosin filaments that are responsible for muscle contraction.