FK506 BINDING-PROTEIN MUTATIONAL ANALYSIS - DEFINING THE SURFACE RESIDUE CONTRIBUTIONS TO STABILITY OF THE CALCINEURIN CO-COMPLEX

The 12 and 13-kDa FK506 binding proteins (FKBP12 and FKBP13) are cis-t rans peptidyl-prolyl isomerases that bind the macrolides FK506 (Tacrol imus) and rapamycin (Sirolimus). The FKBP12 . FK506 complex is immunos uppressive, acting as an inhibitor of the protein phosphatase calcineu rin, We have examined the role of the key surface residues of FKBP12 a nd FKBP13 in calcineurin interactions by generating substitutions at t hese residues by site-directed mutagenesis. All mutants are active cat alysts of the prolyl isomerase reaction, and bind FK506 or rapamycin w ith high affinity, Mutations at FKBP12 residues Asp-37, Arg-42, His-87 , and Ile-90 decrease calcineurin affinity of the mutant FKBP12 FK506 complex by as much as 2600-fold in the case of I90K, Replacement of th ree FKBP13 surface residues (Gln-50, Ala-95, and Lys-98) with the corr esponding homologous FKBP12 residues (Arg-42, His-87, and Ile-90) gene rates an FKBP13 variant that is equivalent to FKBP12 in its affinity f or FK506, rapamycin, and calcineurin, These results confirm the role o f two loop regions of FKBP12 (residues 40-44 and 84-91) as part of the effector face that interacts with calcineurin.