Ridaforolimus is a smallmolecule inhibitor of mTOR

by Selleckchem | Jan 30 2013

Upon activation of PI3-K, the serine?Cthreonine kinase phosphoinositide-dependent kinase 1 is translocated to the membrane by binding of its PH domain to the second messenger PIP3. PDK1 can activate a variety of kinases from the AGC family including PKB, p70 ribosomal S6 kinase and several Ridaforolimus isoforms of protein kinase C ; however, only PKB phosphorylation by PDK1 is PI3-K- and PIP3-dependent . Three closely related isoforms of PKB are produced in mammals, PKB??, PKB?? and PKB?? , all of which contain three domains: a PH domain at the N terminus with a module for lipid binding, a catalytic kinase domain related to other AGC family kinases and a hydrophobic motif at the C terminus, which forms a docking site for PDK1 . PKB is the most significant mediator of the PI3-K signalling cascade and is localised to the membrane by interactions between its PH domain and PIP3. PKB is brought into proximity with PDK1 at the membrane where its activation is elegantly regulated by two independent phosphorylation events . PDK1 phosphorylates PKB at threonine 308 located in the activation loop of the kinase domain .
The identity of the kinase responsible for phosphorylation of the SAR131675 HM at serine 473 was controversial until recently, with numerous candidate kinases identified that could replicate this event in vitro, yet no convincing in vivo data. Sarbassov et al. have since provided compelling evidence that the mammalian target of rapamycin complex 2 complex is the kinase responsible for Ser473 phosphorylation in vivo. Counteracting mTORC2 by dephosphorylating PKB at Ser473 are the PH domain and leucine-rich repeat protein phosphatases , PHLPP1 and PHLPP2, which have different specificities for each of the three mammalian isoforms of PKB . The multiprotein mTORC2 complex consists of mTOR, mammalian stress-activated protein kinase interacting protein 1 , mammalian Rucaparib counterpart of yeast LST8 , rapamycin-insensitive companion of mTOR and a protein associated with rictor . mTORC2 is often referred to as the "rapamycin-insensitive" complex of mTOR; however, it has since been found that in some cell lines, prolonged exposure to rapamycin leads to a decrease in PKB phosphorylation at Ser473, apparently due to rapamycin inhibiting the formation of the mTORC2 complex . Despite mTORC2's role in activating PKB, it is not essential for the successful phosphorylation of several PKB substrates in mice .
This may be due to the compensatory activity of other AGC kinases, or, alternatively, Ser473 phosphorylation might be unnecessary for full activation of PKB; however, the complex activity profile of mTORC2 in vivo remains uncertain at this time . mTOR together with mLST8, the regulatory associated protein of mTOR , and proline-rich Akt substrate 40 kDa forms another multiprotein complex SAR302503 known as mTORC1 which is specifically inhibited by rapamycin. PKB activates mTORC1 indirectly by phosphorylation of tuberous sclerosis complex 2 in the TSC1/TSC2 dimer. This phosphorylation event inhibits the guanosine triphosphatase activating protein Ruxolitinib activity of TSC2 and in turn leads to the activation of Rheb, which is active only in the guanosine triphosphate -bound form. Rheb-GTP does not directly activate mTORC1, but binds to another protein known as FKBP38 , a member of the FK506-binding protein family. The inhibited complex consists of FKBP38 bound to mTORC1; however, upon activation, Rheb-GTP binds to FKBP38, inducing its release from mTORC1 and thus activating the complex .