Peptidyl-prolyl cis-traps-isomerases (PPIases) are enzymes that can cis-traps-isomerize a Xaa-Pro peptide bond. Three families of PPIases are known: cyclophilins, FKBPs, and parvulins. The physiological functions of the PPIases are only poorly understood. In previous work, we reported that the mouse FK506-binding protein 23 (mFKBP23), which comprises an N-terminal PPIase domain and a C-terminal domain with Ca2+-binding sites, binds to mBiP in the endoplasmic reticulum (ER) and this binding is affected by the Ca2+ concentration. In this study, we demonstrate the ability of mFKBP23 to modulate the ATPase activity of BiP, and that the bound mFKBP23, but not the free mFKBP23, can suppress the ATPase activity of mBiP through its PPIase activity. (c) 2007 Elsevier Inc. All rights reserved.