Developmental regulation of FKBP65 - An ER-localized extracellular matrix binding-protein

FKBP65 (65-kDa FK506-binding protein) is a member of the highly conserved f amily of intracellular receptors called immunophilins. All have the propert y of peptidyl-prolyl cis-trans isomerization, and most have been implicated in folding and trafficking events. In an earlier study, we identified that FKBP65 associates with the extracellular matrix protein tropoelastin durin g its transport through the cell. In the present study, we have carried out a detailed investigation of the subcellular localization of FKBP65 and its relationship to tropoelastin. Using subcellular fractionation, Triton X-11 4 phase separation, protease protection assays, and immunofluorescence micr oscopy (IF), we have identified that FKBP65 is contained within the lumen o f the endoplasmic reticulum (ER). Subsequent IF studies colocalized FKBP65 with tropoelastin and showed that the two proteins dissociate before reachi ng the Golgi apparatus. Immunohistochemical localization of FKBP65 in devel oping lung showed strong staining of vascular and airway smooth muscle cell s. Similar areas stained positive for the presence of elastic fibers in the extracellular matrix. The expression of FKBP65 was investigated during dev elopment as tropoelastin is not expressed in adult tissues. Tissue-specific expression of FKBP65 was observed in 12-d old mouse tissues; however, the pattern of expression of FKBP65 was not restricted to those tissues express ing tropoelastin. This suggests that additional ligands for FKBP65 likely e xist within the ER. Remarkably, in the adult tissues examined, FKBP65 expre ssion was absent or barely detectable. Taken together, these results suppor t an ER-localized FKBP65-tropoelastin interaction that occurs specifically during growth and development of tissues.