Human immune interferon-γ (HuIFN-γ) labeled with 32P was used to study the structure of IFN-γ receptor. When [32P]HuIFN-γ was bound and crosslinked to IFN-γ the receptor of human cells with a bifunctional crosslinker disuecinimidyl suberate (DSS), a single diffused 32P-labeled band corresponding to the IFN-γ · receptor complex was visualized by SDS-polyacrylamide gel electrophoresis and autoradiography. The size of the |32P]-HuIFN-γ · receptor complex was about 100-120 kD. Separation of crosslinked complex in reducing and nonreducing gels showed no size differences, suggesting the absence of interchain disulfide linkage. However, binding and formation of the crosslinked IFN-γ. receptor complex on cells was diminished in the presence of the disulfide reducing agent dithiothreitol (DTT). The reduction was DTT-dose-dependent, suggesting that intramolecular disulfides of the receptor are important for binding. Also, [32P]HuIFN-γ did not bind if cells were pretreated with and then washed free of DTT, suggesting an irreversible reduction of intrachain disulfide bonds, presumably of the receptor. [32P]HuIFN-γ also specifically binds to human placental membranes. Each placenta has about 170 ng of IFN-γ receptors. Covalent attachment of [32P]HuIFN-γ to placental plasma membranes via DSS produced 2 crosslinked complexes with the molecular sizes of 100-120 kD and 60-70 kD. The IFN-γ · receptor complex of placental membranes was solubilized with NP-40 after DSS treatment and partially purified with immobilized antibody to the carboxyl terminus of IFN-γ. Treatment of the receptor complex with trypsin and papain was used to demonstrate its differential proteolytic sensitivity.
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