1. Cytoskeletal protein (microtubules, microfilaments, and larger filaments, functioning as a structural support and transport mechanism); a subunit of microfilaments or the smallest of the cytoskeletal filaments.
2. A protein which is abundantly present in many cells; especially, muscle cells, that significantly contributes to the cell's structure and motility.

Actin can very quickly assemble into long polymer rods called microfilaments. These microfilaments have a variety of roles in that they form part of the cell's cytoskeleton, they interact with myosin to permit movement of the cell, and they pinch the cell into two during cell division.

In muscle contraction, filaments of actin and myosin alternately unlink and chemically link in a sliding action.

The energy for this reaction is supplied by adenosine triphosphate; which is, the major energy source within a cell to drive a number of biological processes; such as, photosynthesis, muscle contraction, and the synthesis of proteins.