Abstract: A new homogeneous cytosolic binding (HCB) protein, having a specific binding activity of about 26 .mu.g FK-506 per mg protein and a molecular weight of about 10-12 kilodaltons, reversibly binds the immunosuppressant FK-506 but not cyclosporine A (CSA). The protein is stable to heating at 56 degrees C. for 30 minutes retaining its FK-506 binding affinity, and has the (partial) amino terminal amino acid sequence: H.sub.2 N G y V l G n V l G u T r I e S r P o G y A p G y A g T r P e P o L s A g-Gly-Gln-Thr-X-Val-Val-Val-His-Tyr-Thr-Gly-Met-Leu-Glu-Asp-Fly-Lys-Phe-AS p (wherein X is undefined). The HCBV protein is isolated from the cytosol of mammalian tissues, preferably human neoplastic T-cell lines, e.g. Jurkat, and can be used in diagnostic and purification procedures involving FK-506 macrolide type immunosuppressants. The HCB protein also catalyses the cis-trans isomerization of proline-containing peptide bonds.

Abstract: A new homogeneous cytosolic binding (HCB) protein, having a specific binding activity of about 26 .mu.g FK-506 per mg protein and a molecular weight of about 10-12 kilodaltons, reversibly binds the immunosuppressant FK-506 but not cyclosporine A (CSA). The protein is stable to heating at 56 degrees C. for 30 minutes retaining its FK-506 binding affinity, and has the (partial) amino terminal amino acid sequence: H.sub.2 N-Gly-Val-Gln-Val-Glu-Thr-Ile-Ser-Pro- Gly-Asp-Gly-Arg-Thr-Phe-Pro-Lys- Ar g-Gly-Gln-Thr-X-Val-Val-His-Tyr-Thr-Gly-Met-Leu-Glu-Asp-Gly-Lys-Lys-Phe-As p (wherein X is undefined). The HCB protein is isolated from the cytosol of mammalian tissues, preferably human neoplastic T-cell lines, e.g., Jurkat, and can be used in diagnostic and purification procedures involving FK-506 macrolide type immunosuppressants. The HCB protein also catalyzes the cis-trans isomerization of proline-containing peptide bonds.