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Mechanistic target of rapamycin
The mechanistic target of rapamycin, also known as mammalian target of rapamycin (mTOR) or FK506-binding protein 12-rapamycin-associated protein 1 (FRAP1), is a protein that in humans is encoded by the ''MTOR'' gene. MTOR is a serine/threonine protein kinase that regulates cell growth, cell proliferation, cell motility, cell survival, protein synthesis, autophagy, and transcription. MTOR belongs to the phosphatidylinositol 3-kinase-related kinase protein family.
==Discovery==
MTOR was first named as the mammalian target of rapamycin. Rapamycin was discovered in a soil sample from Easter Island, known locally as Rapa Nui, in the 1970s. The bacterium ''Streptomyces hygroscopicus'', isolated from that sample, produces an antifungal that researchers named rapamycin after the island.
Rapamycin arrests fungal activity at the G1 phase of the cell cycle. In mammals, it suppresses the immune system by blocking the G1 to S phase transition in T-lymphocytes. Thus, it is used as an immunosuppressant following organ transplantation.
Molecular genetic studies in yeast (published in 1991) first identified FKBP12, TOR1, and TOR2 as the targets of rapamycin; these studies were conducted at the Biozentrum in Basel, Switzerland and Sandoz Pharmaceuticals (now Novartis) by Joseph Heitman, Rao Movva, and Michael N. Hall. They isolated rapamycin-resistant mutants of ''Saccharomyces cerevisiae'' and discovered that mutations in any of three genes can confer rapamycin resistance. Two of the genes were named TOR1 and TOR2 for targets of rapamycin (TOR) and in honor of the Spalentor, a gate to the city of Basel where TOR was first discovered. The third gene is FPR1, which encodes the yeast ortholog of FKBP12 binding protein in the TOR complexes. Loss of function mutations in FPR1 confer resistance to rapamycin, and also to FK506, providing genetic evidence the FKBP12-drug complexes are the active intracellular agents. Mutations in TOR1 or TOR2 that confer FKBP12-rapamycin resistance are dominant gain of function mutations that alter single amino acid residues within the FRB domain and thereby block FKBP12-rapamycin binding. Several years later, in 1994 the mammalian target of rapamycin (mTOR) was identified and found to be the ortholog of the yeast Tor1/2 proteins and defined as the rapamycin target in mammals by David M. Sabatini and Solomon H. Snyder (Johns Hopkins University) and also by Robert Abraham (who first named it mTOR) and Stuart L. Schreiber (Harvard University).〔 mTOR stands for mammalian Target Of Rapamycin and was named based on the precedent that TOR was first discovered via genetic and molecular studies of rapamycin-resistant mutants of ''Saccharomyces cerevisiae'' that identified Tor1 and Tor2 as the targets of rapamycin.
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