<p>The crystal structures of four growth factors; nerve growth factor, transforming growth factor-beta, platelet-derived growth factor, and human chorionic gonadotropin from four separate superfamilies revealed that these proteins are structurally related and share a common overall topology [<cite idref="PUB00000890"/>]. These proteins show very little sequence homology, but they all have an unusual arrangement of six cysteines linked to form a "cystine-knot" conformation. The active forms of these proteins are dimers, either homo- or heterodimers [<cite idref="PUB00000091"/>]. Because of their shape, there appears to be an intrinsic requirement for the cystine-knot growth factors to form dimers. This extra level of organisation increases the variety of structures built around this simple structural motif [<cite idref="PUB00001089"/>].</p> <p>Glycoprotein hormones [<cite idref="PUB00000033"/>, <cite idref="PUB00000517"/>] (or gonadotropins) are a family of proteins, which include the mammalian hormones follitropin (FSH), lutropin (LSH), thyrotropin (TSH) and chorionic gonadotropin (CG), as well as at least two forms of fish gonadotropins. All these hormones consist of two glycosylated chains (alpha and beta). In mammalian gonadotropins, the alpha chain is identical in the four types of hormones but the beta chains, while homologous, are different.</p> <p>The beta chains are proteins of about 100 to 140 amino acid residues that contain the cysteine-knot domain [<cite idref="PUB00004177"/>], as shown in the following schematic representation.</p> <p>The beta subunits of LH (also known as lupotrophin) and hCG are closely related in sequence and elicit their biological actions via the same receptor. LH is released from the anterior pituitary under the influence of gonadotrophin-releasing hormone and progesterones. CG is released by the placenta during pregnancy. In females, LH stimulates ovulation and is the major hormone involved in the regulation of progesterone secretion by the corpus luteum. In males, it stimulates Leydig cells to secrete androgens, particularly testosterone. The receptor is found in organs involved in reproductive physiology, including testicular Leydig cells, ovarian theca, granulosa, luteal and interstitial cells. The receptor activates adenylyl cyclase through G proteins and stimulates the phosphoinositide pathway through a pertussis-toxin-insensitive G-protein [<cite idref="PUB00035060"/>].</p> <p>Synonym(s): Lutropin beta.</p> Luteinizing hormone-like, beta subunit