From Proteopediaproteopedia link
ATOMIC STRUCTURE OF FKBP-FK506, AN IMMUNOPHILIN-IMMUNOSUPPRESSANT COMPLEX
[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
The structure of the human FK506 binding protein (FKBP), complexed with the immunosuppressant FK506, has been determined to 1.7 angstroms resolution by x-ray crystallography. The conformation of the protein changes little upon complexation, but the conformation of FK506 is markedly different in the bound and unbound forms. The drug's association with the protein involves five hydrogen bonds, a hydrophobic binding pocket lined with conserved aromatic residues, and an unusual carbonyl binding pocket. The nature of this complex has implications for the mechanism of rotamase catalysis and for the biological actions of FK506 and rapamycin.
Atomic structure of FKBP-FK506, an immunophilin-immunosuppressant complex.,Van Duyne GD, Standaert RF, Karplus PA, Schreiber SL, Clardy J Science. 1991 May 10;252(5007):839-42. PMID:1709302
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.