1. Inhibition of TGF Receptors FKBP12 Binding AsimFarooq ASAB - NUST

2. Introduction • TGFβ family plays a central role in controlling tissue development and homeostasis • Their activation, signaling and deactivation is carefully co-ordinated • Serine Threonine kinases, two subunits(I & II), having a phosphorylating GS domain, RII is a primary receptor and RI is a signal transducer • Mediator protein is SMAD, which accumulates in nucleus and binds to SNPs and transcriptional regulators

3. Continued • Type I receptors have ability to remain activated in absence of ligand or type II receptors • FKBP12 binds to the intracellular domain of receptor • FKBP12 proteins have prolylisomeraseactivity, bind to the Ca2+/calmodulin-dependent protein phosphatase calcineurin and inhibit its ability to mediate T-cell activation, bind to the ryanodine receptor and the inositol 1,4,5-triphosphate receptor, stabilizing the calcium channeling

4. Receptor Binding • FKBP12 interacts with the GS domain of TBR1 • The conserved sequence of binding is T185TSGSGSGLP • Missense mutations abolish the receptor interaction (L193G & P194K) • Mutations other than this does not have any effect on binding capability • FKBP12 recognizes a specific structure within the GS domain that includes the Leu–Pro motif.

5. FKBP12 Protects Against Leaky TGF Receptor Activation • Mutations in TBR1 result in elevated signaling • Overexpression of FKBP12 results in complete inhibition of signaling • Identification of Leu193–Pro as a FKBP12-binding site argue that the interaction involves the FKBP12 catalytic site

6. Mechanism of Inhibition • FBKP12 inhibits phosphorylation of TBR-I by TBR-II HOW??? • Overexpression of FKBP12 inhibits ligand-induced TBR-I phosphorylation, whereas the overexpression of mutants defective in TBR-I binding does not • Rapamycinaugments TbR-I phosphorylation and reverses the inhibitory effect of overexpressed FKBP12

7. Continued… • FKBP12 binding to this site is likely to hinder the entry of the phosphorylation sites into the catalytic center of the TBR-II kinase, thus protecting TBR-I from activation during ligand-independent interactions with TBR-II. • In normal physiological conditions, ligand binding removes FKBP12 and receptor is activated to carry out down stream signaling