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Actin is a spherical 43 kilo-Dalton protein that is found in many cell types. It belongs to the class of proteins known as [http://en.wikipedia.org/wiki/Motor_proteins motor proteins]. Motor proteins function in the movement of substances across a substrate by utilizing [http://en.wikipedia.org/wiki/Adenosine_triphosphate ATP] as an energy source. It is one of the most highly conserved proteins differing by no more than 20% in species ranging from yeast to humans. ("Mediation, Modulation and Consequences of Membrane-Cytoskeleton Interactions".) In an alignment of the 81 unique actin protein sequences, scientists found that 17.4% of positions in the consensus sequence are invariant in all actins, 35.6 % of positions are invariant or have a difference in only one of the 81 actins, and 40.4% of positions are invariant or have what are considered conservative changes (Review Article Molecular genetics of actin function)   

Actin is the monomeric subunit of microfilaments, one of the three key components of the [http://en.wikipedia.org/wiki/Cytoskeleton cytoskeleton] and thin filaments, which is part of the contractile unit in muscle cells. The cytoskeleton is a three-dimensional complex of filamentous protein that creates the shape and structure of cells, as well as fills in the gaps between organelles. This is what provides the cells with motility. There are three main proteins that make up the cytoskeleton. They are microfilaments, intermediate filaments and microtubules.  (http://www.bms.ed.ac.uk/research/others/smaciver/lectures/Cs1.htm

In 1942, [http://en.wikipedia.org/wiki/Brun%C3%B3_Ferenc_Straub Bruno Ferenc Straub] isolated actin as a water soluble component of muscle acetone powder. He found that at increased ionic strength the G-actin molecules would come together to form the F-actin filament. Actin was later found in non-muscle cells and its structure was seen through the use of [http://en.wikipedia.org/wiki/Immunofluorescence_microscopy immunofluorescence microscopy]. The next major accomplishment came with the discovery of the amino acid sequence. This allowed researchers to locate chemical and enzymatic alterations and to study how they affected the properties of the molecule. The high conservation of actin’s structure throughout various species was discovered through [http://en.wikipedia.org/wiki/Immunofluorescence_microscopy comparative sequence analysis]. In 1950, Straub reported that actin contained bound ATP and that during the polymerization of the monomers into microfilaments, the ATP was hydrolyzed to ADP and inorganic phosphate. (Straub FB, Feuer G (1989). "Adenosinetriphosphate. The functional group of actin. 1950". Biochim. Biophys. Acta 1000: 180–95.PMID 2673365.) His discovery led to his suggestion that ATP-bound actin transforming into ADP-bound actin was what created muscle contractions. In the early 1980’s the principles of actin polymerization were exposed and the first actin-binding proteins (ABPs) were characterized according to their in vivo and in vitro functions. In 1990, the crystal structure of G-actin was solved by Kabsch. (Kabsch W, Mannherz HG, Suck D, Pai EF, Holmes KC (September 1990). "Atomic structure of the actin:DNase I complex". Nature 347 (6288): 37–44. doi:10.1038/347037a0. PMID 2395459.) In this same year a model of F-actin was suggested by Holmes et al. The model was created by fitting a helix of G-actin structures according to low-resolution fiber diffraction data from the F-actin. ( Holmes KC, Popp D, Gebhard W, Kabsch W (September 1990). "Atomic model of the actin filament". Nature 347 (6288): 44–9.doi:10.1038/347044a0. PMID 2395461.) Further advances in cryoelectron microscopy allowed scientists to find the binding sites for myosin and tropomyosin. A combination of [http://en.wikipedia.org/wiki/X-ray_crystallography X-ray crystallography] with fiber diffraction led to an atomic model of F-actin. (Structure and Function of Actin by Wolfgang Kabsch) After some debate actin also became generally accepted as an important structural and functional component of the cell nucleus (Conformational Dynamics of Actin: Effectors and Implications for Biological Function) Since this pioneering work by Straub, interest in actin has increased tremendously and so has the realization of how complex its structure is.