Modeling the interaction between FK506 and FKBP12: a mechanism for formation of the calcineurin inhibitory complex

FK506 is a naturally occurring immunosuppressant whose mode of action involves formation of an initial complex with the cytosolic protein FKBP12. The composite surface of this complex then binds to and inhibits the protein phosphatase calcineurin (PP2B). To investigate why FK506 does not inhibit calcineurin directly we have conducted molecular modeling and conformational studies on published structures of FK506 both alone and in complex with FKBP12. From studies of the structure of FK506 in CDCl3 and Z-Arg32-ascomycin in water (a water soluble analogue of FK506) we suggest that the FK506 molecule can be viewed as consisting of three separate regions. The pipecolate region which extends from C24 to C10 including the pipecolate ring shows strongly conserved conformation in both solvents. The loop region which extends from C25 to C16 shows general conservation of the loop structure and the pyranose region made up of the pyranose ring and C15-C17 which shows highly variable conformation depending on solvent. Comparison of the structure of Z-Arg32-ascomycin in water with structures of FK506 bound to FKBP12 indicate that the conformation of the pipecolate region is conserved during the binding process. The conformation of the loop region was generally conserved but a significant reduction (approximately 1.7 A) in the diameter of the loop in the bound structure was observed. The conformation of the pyranose ring and C15-C17 region was found to be significantly altered in the bound structure resulting in displacements of the C13 and C15 methoxyl groups of 2.8 and 3.5 A, respectively. From computer models and molecular dynamics simulations of interactions between FK506 and FKBP12 we suggest that the conformational changes observed in bound FK506 are induced by the interaction between the 80's loop of FKBP12 and the pyranose ring of FKBP12. These interactions result in the formation of a complex with the both correct shape and surface polarity for interaction with calcineurin.