Growth hormone/prolactin family

Growth hormone (GH), prolactin (PRL or PRL1), prolactin 2 (PRL2), somatolactin (SL), and a mammalian placental hormone, placental lactogen (PL), form a family that shares a common tertiary structure. They produce their biological effects by interacting with single transmembrane-domain receptors that belong to the class I cytokine receptor family. The binding of these hormones to their receptors causes receptor dimerization, and activates Janus kinase 2 (JAK2), a tyrosine kinase that initiates the JAK/STAT pathway. The principal biological role of GH, the control of postnatal growth, has remained quite consistent throughout vertebrate evolution and is largely mediated by insulin-like growth factors (IGFs). PRL1 has many diverse roles including control of water and salt balance, metabolism, and regulation of the immune system. The function of PRL2, a recently discovered molecule, is unknown. SL has a wide range of biological activities including body-color regulation, plasma ion metabolism, and acid-base regulation. PL modifies the metabolic state of the mother during pregnancy to facilitate the energy supply of the fetus.