The 12-kDa FK506-binding protein (FKBP12) is the target of the commonly used immunosuppressive drug FK506. The FKBP12-FK506 complex binds to calcineurin and inhibits its activity, leading to immunosuppression and preventing organ transplant rejection. Our recent characterization of crystal structures of FKBP12 proteins in pathogenic fungi revealed the involvement of the 80's loop residue (Pro90) in the active site pocket in self-substrate interaction providing novel evidence on FKBP12 dimerization in vivo. The 40's loop residues have also been shown to be involved in reversible dimerization of FKBP12 in the mammalian and yeast systems. To understand how FKBP12 dimerization affects FK506 binding and influences calcineurin function, we generated Aspergillus fumigatus FKBP12 mutations in the 40's and 50's loop (F37 MIL; W60V). Interestingly, the mutants exhibited variable FK506 susceptibility in vivo indicating differing dimer strengths. In comparison to the 80's loop P9OG and V91C mutants, the F37 M/L and W6OV mutants exhibited greater FK506 resistance, with the F37M mutation showing complete loss in calcineurin binding in vivo. Molecular dynamics and pulling simulations for each dimeric FKBP12 protein revealed a two-fold increase in dimer strength and significantly higher number of contacts for the F37M, F37L, and W6OV mutations, further confirming their varying degree of impact on FK506 binding and calcineurin inhibition in vivo. (C) 2020 Elsevier Inc. All rights reserved.