LOCALIZATION OF THE FK506-BINDING PROTEIN, FKBP-13, TO THE LUMEN OF THE ENDOPLASMIC-RETICULUM

The function of the immunophilins, FKBP 12 and FKBP 13, which are bind ing proteins for the immunosuppressant drug FK506 and rapamycin, remai ns poorly defined, although it has been suggested that immunophilins a nd immunophilin-like proteins may play a role in protein sorting/foldi ng and intracellular calcium ion regulation. As a first step towards u nderstanding the function of FKBP 13, we studied its subcellular local ization by immunoblotting of well-defined subcellular fractions from a canine pancreatic homogenate and immunocytochemical analysis of an ov erexpressed cloned cDNA for FKBP 13. Whereas FKBP 12 fractionated enti rely into the cytosol, virtually all FKBP 13 was found in the rough mi crosomal fraction which consisted of highly purified rough endoplasmic reticulum (ER), along with several well-characterized ER markers [the immunoglobulin heavy-chain binding protein (BiP), grp 94 and ribophor in I]. Moreover, FKBP 13 co-banded with the ER markers on isopycnic su crose gradients. By immunofluorescence, the overexpressed cDNA for FKB P 13 in Hela cells gave an ER-staining pattern highly similar to that of known ER proteins. Addition of the ligand FK506 did not appear to a lter the distribution of FKBP 13. Separation of the ER luminal content s and membrane revealed FKBP 13 to be a luminal ER protein. Since the lumen of the ER is where the folding of membrane and secreted proteins occurs, as well as a major site of intracellular calcium storage, it seems possible that FKBP 13 may be involved in one of these functions.