<P> Actin and microfilament - mediated processes have long been a subject of research . Engelmann (1879) suggested that many kinds of movement observed in plants and protozoa like cytoplasmic streaming and amoeboid movement were in fact a primitive version of the movements of muscle contraction . </P> <P> In the 1930s, Szent - Györgyi and collaborators, violating one of the canons of biochemistry, started to study the residue instead of the extract ", that is, structural proteins and not enzymes, leading to the many discoveries related to microfilaments . </P> <P> Actin filaments are assembled in two general types of structures: bundles and networks . Bundles can be composed of polar filament arrays, in which all barbed ends point to the same end of the bundle, or non-polar arrays, where the barbed ends point towards both ends . A class of actin - binding proteins, called cross-linking proteins, dictate the formation of these structures . Cross-linking proteins determine filament orientation and spacing in the bundles and networks . These structures are regulated by many other classes of actin - binding proteins, including motor proteins, branching proteins, severing proteins, polymerization promoters, and capping proteins . </P> <P> Measuring approximately 6 nm in diameter, microfilaments are the thinnest fibers of the cytoskeleton . They are polymers of actin subunits (globular actin, or G - actin), which as part of the fiber are referred to as filamentous actin, or F - actin . Each microfilament is made up of two helical, interlaced strands of subunits . Much like microtubules, actin filaments are polarized . Electron micrographs have provided evidence of their fast - growing barbed - ends and their slow - growing pointed - end . This polarity has been determined by the pattern created by the binding of myosin S1 fragments: they themselves are subunits of the larger myosin II protein complex . The pointed end is commonly referred to as the minus (−) end and the barbed end is referred to as the plus (+) end . </P>

Thin structures of cytokinesis amoeboid movement changes in the cell shape