Heat shock protein 56 (hsp56) was previously identified as an immunophilin based on its ability to specifically bind to FK506-AffiGel 10. In this report, we have quantitated human Jurkat T cell hsp56 binding to3H-FK506, as well as to the immunosuppressant rapamycin. Binding was measured utilizing immunoadsorbed hsp56, and, in addition, we demonstrate that3H-FK506 binds to hsp56 in solution. Hsp56 bound to an antibody-Sepharose column binds3H-FK506 with an affinity of 19.4 ± 4.6 nM, as compared to 23.2 ± 6.8 nM for soluble hsp56. In competition experiments, the apparent affinity constant for rapamycin was 11.6 ± 2.8 nM, using immobilized hsp56, and 17.3 ± 7.7 nM, using the soluble hsp56 preparation. These results demonstrate that hsp56 binds FK506 and rapamycin with similar affinities, and suggest that hsp56 may play a role in mediating the cellular function of both of these drugs.
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