Actin
![Ribbon model of actin extracted from the striated muscle tissue of a rabbit after Graceffa and Domínguez, 2003. The four subdomains can be seen, as well as the N and C termini and the position of the ATP bond. The molecule is oriented using the usual convention of placing the - end (pointed end) in the upper part and the + end (barbed end) in the lower part.[1]](/uploads/202501/14/G-actin_subdomains5241.png)
![Ribbon model obtained using the PyMOL programme on crystallographs of the prefoldin proteins found in the archaean Pyrococcus horikoshii. The six supersecondary structures are present in a coiled helix “hanging” from the central beta barrels. These are often compared in the literature to the tentacles of a jellyfish. As far as is visible using electron microscopy, eukariotic prefoldin has a similar structure.[33]](/uploads/202501/14/Prefoldin5241.png)
Actin is a globular multi-functional protein that forms microfilaments. It is found in essentially all eukaryotic cells (the only known exception being nematode sperm), where it may be present at concentrations of over 100 μM. An actin protein's mass is roughly 42-kDa and it 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. It can be present as either a free monomer called G-actin (globular) or as part of a linear polymer microfilament called F-actin (filamentous), both of which are essential for such important cellular functions as the mobility and contraction of cells during cell division.