1. (Science: cell biology)
A filamentous proteins (42 kD) involved in muscle contraction in both smooth and striated muscle and also serves as an important structural molecule for the cytoskeleton of many eukaryotic cells. (biology-online.org)
2. A globular, roughly 42-kDa multi-functional protein found in all eukaryotic cells (the only known exception being nematode sperm), where it may be present at concentrations of over 100 μM. (Wikipedia.org)
3. One of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans. Actin is the monomeric subunit of two types of filaments in cells: microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the contractile apparatus in muscle cells. Thus, actin participates in many important cellular processes, including muscle contraction, cell motility, cell division and cytokinesis, vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. (Wikipedia.org)
4. Filamentous Proteins that are the main constituent of the thin filaments of Muscle fibers. The filaments (known also as filamentous or F-actin) can be dissociated into their globular subunits; each subunit is composed of a single polypeptide 375 Amino Acids long. This is known as globular or G-actin. In conjunction with Myosins, actin is responsible for the contraction and Relaxation of Muscle. (online-medical-dictionary.org)
5. A protein found in all eukaryotic cells, forming filaments that make up a main component of the cell’s supporting matrix or cytoskeleton. Actin and the protein myosin together make up the contractile units (called sarcomeres) of skeletal muscle fibers. (The American Heritage® Science Dictionary Copyright © 2005 by Houghton Mifflin Company)
Word origin: From Ancient Greek ἀκτίς (aktis, “ray”), based on the shape of the filament formed, and the English chemical suffix -in.