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.