Mechanistic target of rapamycin (mTOR) FKBP12-rapamycin binding domain (FRB domain) (mTOR FRB domain)

The FKBP12-rapamycin binding (FRB) domain is a highly conserved ~100-amino acid region within the Mechanistic target of rapamycin (mTOR) protein, situated between the FAT and kinase domains [4, 12]. It serves as the specific docking site for the complex formed by the immunosuppressant drug rapamycin (sirolimus) and the intracellular protein FKBP12 [1, 7]. Binding of the FKBP12-rapamycin complex to the FRB domain induces allosteric changes and steric hindrance that selectively inhibit the activity of mTOR Complex 1 (mTORC1), a master regulator of cell growth, protein synthesis, and autophagy [2, 4, 13, 14]. Dysregulation of the mTOR pathway, often involving mutations that affect the FRB domain's gatekeeper function, is a hallmark of various cancers and metabolic disorders [3, 5, 10]. Consequently, the FRB domain is a critical therapeutic target for first-generation mTOR inhibitors (rapalogs) used in oncology and transplant medicine [6, 10]. While rapalogs are effective against mTORC1, they generally do not inhibit mTOR Complex 2 (mTORC2) because its structural components block access to the FRB domain [13]. This domain also interacts with endogenous regulators like phosphatidic acid and FKBP38, which modulate mTOR activity in response to nutrient availability [9, 12]. Understanding the structural biology of the FRB domain has been essential for developing next-generation inhibitors that overcome resistance mechanisms associated with FRB mutations [5, 8, 10].

Other names

FRB domainFKBP12-rapamycin binding domainFKBP-rapamycin binding domainFRAP FRB domainRAFT1 FRB domainmTOR FRB

Allosteric inhibition of mTORC1 through the formation of a ternary complex with FKBP12 and rapamycin (or rapalogs), which sterically hinders substrate access to the kinase active site.