Detection of the FK506-FKBP-calcineurin complex by a simple binding assay.

[3H]FK506-FK506-binding protein (FKBP) complex does not bind polyethyleneimine-treated glass fiber filters, on the other hand, it binds the filter when it makes a complex with calcineurin. This enabled us to develop a simple binding assay to detect the FK506-FKBP-calcineurin complex in the mixture and to estimate the amount of FKBP or calcineurin which can form the complex in the soluble extracts from rat tissues. The molar concentration of FKBP of every rat tissue is much higher than that of calcineurin, and FKBP/calcineurin molar ratio varies from 13 to 33 in various brain regions, and from 42 to 343 in peripheral tissues. These results suggest that most of the FKBP molecules have physiological roles which are not related to calcineurin, in spite of their potential ability to form the complex with FK506 and calcineurin.