Mechanism of TGFβ receptor inhibition by FKBP12 case of the activin-binding protein follistatin (Nakamura

case of the activin-binding protein follistatin (Nakamura Ye-Guang Chen, Fang Liu and et al., 1990; Hemmati-Brivanlou et al., 1994) or the BMPJoan Massagué1 binding proteins noggin (Holly et al., 1996; Zimmerman Cell Biology Program and Howard Hughes Medical Institute, et al., 1996) and chordin (Piccolo et al., 1996). However, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA little is known about regulation of the TGFβ signaling 1Corresponding author receptors themselves. e-mail: [email protected] The receptors for the TGFβ family are transmembrane protein serine/threonine kinases that form two structurally Transforming growth factor-β (TGFβ) signaling and functionally distinct groups known as the type I and requires phosphorylation of the type I receptor TβR-I the type II receptors, respectively (Massagué, 1996; ten by TβR-II. Although TGFβ promotes the association Dijke et al., 1996). Ligand binding induces the formation of TβR-I with TβR-II, these receptor components have of a complex in which the type II receptor phosphorylates affinity for each other which can lead to their ligandand activates the type I receptor which then propagates independent activation. The immunophilin FKBP12 the signal (Wrana et al., 1994). Phosphorylation of the binds to TβR-I and inhibits its signaling function. We type I receptor occurs at a cluster of serine and threonine investigated the mechanism and functional significance residues within a highly conserved region known as the of this effect. FKBP12 binding to TβR-I involves the GS domain and located immediately upstream of the rapamycin/Leu–Pro binding pocket of FKBP12 and kinase domain (Wrana et al., 1994; Wieser et al., 1995). a Leu–Pro sequence located next to the activating Roles for the type II receptor as a primary receptor and phosphorylation sites in TβR-I. Mutations in the bindthe type I receptor as a signal transducer are also suggested ing sites of FKBP12 or TβR-I abolish the interaction by the ability of constitutively active type I receptor between these proteins, leading to receptor activation mutants to signal in the absence of ligand or type II in the absence of added ligand. FKBP12 does not receptors (Wieser et al., 1995). inhibit TβR-I association with TβR-II, but inhibits Many of the resulting responses are mediated by the TβR-I phosphorylation by TβR-II. Rapamycin, which SMADs (Sekelsky et al., 1995; Baker and Harland, 1996; blocks FKBP12 binding to TβR-I, reverses the Graff et al., 1996; Hoodless et al., 1996; Lagna et al., inhibitory effect of FKBP12 on TβR-I phosphorylation. 1996; Liu et al., 1996; Savage et al., 1996; Zhang et al., By impeding the activation of TGFβ receptor complexes 1996), a family of cytoplasmic proteins that become formed in the absence of ligand, FKBP12 may provide phosphorylated in response to receptor activation (Eppert a safeguard against leaky signaling resulting from the et al., 1996; Hoodless et al., 1996; Lagna et al., 1996; innate tendency of TβR-I and TβR-II to interact with Zhang et al., 1996). SMAD phosphorylation is mediated each other. by a receptor-associated kinase activity (Macias-Silva