Mouse FKBP23 mediates conformer-specific functions of BiP by catalyzing Pro117cis/trans isomerization

► FKBPs are cellular receptors for the immunosuppressant FK506 and anti-cancer agent rapamycin. ► FKBP23, a calcium-binding chaperone, acts as a trigger factor in regulating the ATPase activity of BiP. ► We found Pro117 in the ATPase domain of BiP was the target site for FKBP23. ► This regulatory mechanism is completely different from all of the well-known regulatory systems. ► We are the first to combine PPIase activity on peptides with that on the full-length substrate proteins.