Cell biology. Actin is the most common protein in the cytoplasm of eukaryotic cells. It is the main constituent of thin filaments of muscle fibers. In conjunction with myosin (actomyosin), actin is responsible for the contraction and relaxation of muscles. G-actin (globular actin) is a monomer and builds by self-aggregation F-actin (filamentous actin). These microfilaments (actin filaments) serve also as important structural molecules for the cytoskeleton.