Actin proteins are major components of the eukaryotic cytoskeleton. At least six vertebrate actin isoforms have been identified. The cytoplasmic β-and γ-actin proteins are referred to as “non-muscle” actin proteins as they are predominantly expressed in non-muscle cells where they control cell structure and motility. The α-cardiac and α-skeletal actin proteins are expressed in striated cardiac and skeletal muscles, respectively. The smooth muscle α-actin and γ-actin proteins are found primarily in vascular smooth muscle and enteric smooth muscle, respectively. The α-smooth muscle actin (ACTA2) is also known as aortic smooth muscle actin. These actin isoforms regulate the contractile potential of muscle cells.
Activin and inhibin are two closely related protein complexes that have almost directly opposite biological effects. The activin and inhibin protein complexes are both dimeric in structure, and, in each complex, the two monomers are linked to one another by a single disulfide bond. Activin is composed of two ß subunits, ßA ßA (activin A), ßB ßB (activin B), or ßA ßB (activin AB). Inhibin is composed of an alpha and one of two ß subunits, ßA (inhibin A) or ßB (inhibin B). Activins are produced in many cell types and organs, such as gonads, pituitary gland, and placenta. In the ovarian follicle, activin increases FSH binding and FSH-induced aromatization. It participates in androgen synthesis enhancing LH action in the ovary and testis. In the male, activin enhances spermatogenesis. Also, Activin plays a role in wound repair and skin morphogenesis. Activin is strongly expressed in wounded skin, and overexpression of activin in the epidermis of transgenic mice improves wound healing and enhances scar formation. Activin also regulates the morphogenesis of branching organs such as the prostate, lung, and kidney. There is also evidence showed that lack of activin during development results in neural developmental defects.
Activin proteins that belong to the transforming growth factor-beta (TGF-β) superfamily; exert their biological actions by binding to heteromeric receptor complexes of type I and type II serine/threonine kinase receptors. On ligand binding; type I and II receptors form a stable complex; resulting in phosphorylation of type I receptors by type II receptors with constitutive kinase activity; and subsequently initiates the activation of downstream molecules including the endogenous Smads. ActRIIB; also known as ActRIIB; is a type II receptor containing an extracellular domain (ECD); a transmembrane segment; and a cytoplasmic region that includes the kinase domain. ActRIIB is a receptor for activin A; activin B and inhibin A. Multiple ActRIIB isoforms can also be generated; which bind activin isoforms with different affinities.
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