Rabbit FKBP59-heat shock protein binding immunophillin (HBI) is a calmodulin binding protein

FKBP59-HBI, a heat shock protein hsp90-binding immunophilin that was originally detected in heterooligomer forms of steroid receptors, is retained on Calmodulin (CAM)-Sepharose 4B in the presence of 2 mM Ca2+ and is eluted by EGTA, demonstrating a specific p59-CAM interaction. The p59 amino acid sequence reveals the presence of two putative CAM binding sites in a helix regions of the protein, as well as PEST sequences which are generally present in CAM-binding proteins. In vitro proteolysis by calpain II (a Ca(2+)-activated neutral protease), another feature of CAM-binding proteins, generates shorter peptides revealed by the mAb EC1, but not by the pAb 173 which recognizes the C-terminal of the protein. The potential function of CAM binding by the hsp90-binding immunophilin is discussed.