CHARACTERIZATION OF SURFACE BINDING-SITES FOR CYCLOPHILIN-B ON A HUMAN TUMOR T-CELL LINE

Cyclophilin B (CyPB) is a cyclosporin-binding protein, known to be loc ated mainly within the endoplasmic reticulum vesicles. Its previous ch aracterization in human milk implies that the protein may be released from the secretory pathway and recovered in biological fluids. In an a ttempt to understand the role of the extracellular CyPB, we have inves tigated the binding capacity of the protein to cells derived from huma n T- and B-lymphocytes. We present here evidence that CyPB binds to T- lymphocytes and that the binding to the Jurkat T-cell surface is speci fic, saturable, and reversible. The dissociation constant K-d was 12 n M, and the number of binding sites was estimated to 35,000/cell. We re port that the surface-bound CyPB was internalized at 37 degrees C and subsequently degraded in the cell. We also show that the immunosuppres sive drug cyclosporin A does not inhibit the surface binding of CyPB, and does not interfere with internalization of the protein. These resu lts support the hypothesis that the selective action of the immunosupp ressive drug results in part from its interaction with the extracellul ar form of CyPB.