From Proteopediaproteopedia link
SOLUTION STRUCTURE OF FKBP, A ROTAMASE ENZYME AND RECEPTOR FOR FK506 AND RAPAMYCIN
[FKB1A_HUMAN] Keeps in an inactive conformation TGFBR1, the TGF-beta type I serine/threonine kinase receptor, preventing TGF-beta receptor activation in absence of ligand. Recruites SMAD7 to ACVR1B which prevents the association of SMAD2 and SMAD3 with the activin receptor complex, thereby blocking the activin signal. May modulate the RYR1 calcium channel activity. PPIases accelerate the folding of proteins. It catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides. 
Publication Abstract from PubMed
Immunophilins, when complexed to immunosuppressive ligands, appear to inhibit signal transduction pathways that result in exocytosis and transcription. The solution structure of one of these, the human FK506 and rapamycin binding protein (FKBP), has been determined by nuclear magnetic resonance (NMR). FKBP has a previously unobserved antiparallel beta-sheet folding topology that results in a novel loop crossing and produces a large cavity lined by a conserved array of aromatic residues; this cavity serves as the rotamase active site and drug-binding pocket. There are other significant structural features (such as a protruding positively charged loop and an apparently flexible loop) that may be involved in the biological activity of FKBP.
Solution structure of FKBP, a rotamase enzyme and receptor for FK506 and rapamycin.,Michnick SW, Rosen MK, Wandless TJ, Karplus M, Schreiber SL Science. 1991 May 10;252(5007):836-9. PMID:1709301
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.