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The major binding protein of the interferon antagonist sarcolectin in human placenta is a macrophage migration inhibitory factor.

The interferon antagonist and growth promotor sarcolectin has affinity for negatively charged carbohydrates. Isolation of cellular binding proteins will be a step to elucidate its physiological significance. Thus, resin-immobilized sarcolectin was employed as affinity ligand for chromatographic fractionation of extract from human placenta. Elution with 0.1 M NH4OH or with 0.1 M N-acetylneuraminic acid and 1 M NaCl resulted primarily in purification of a protein of molecular mass of about 12 kDa according to gel electrophoretic analysis under denaturing conditions in the presence or absence of reductive agent and 12,470 Da by laser desorption mass spectrometry. The native molecular mass, assessed by gel filtration, is approximately 28 kDa. No evidence for detectable post-translational modification by glycosylation was provided by treatment with N-glycosidase F or sialidase and subsequent electrophoretic analysis. The N-terminal sequence of the major sarcolectin-binding protein is identical to that deduced from the cDNA sequence of a human macrophage migration inhibitory factor (MIF), starting from its third amino acid, over the determined stretch of 22 amino acids. Comparison of the calculated molecular mass of 12,221 of this factor to the experimentally determined value of 12,470 excludes any extensive modification of the protein. The sarcolectin-binding protein reduces macrophage migration at a concentration of 100 ng/ml in MIF assays. Recombinant migration inhibitory factor and purified sarcolectin-binding protein reacted equally well with anti-MIF antibody in immunoblot analysis and in assays to block binding to sarcolectin. Binding of biotinylated sarcolectin, too, is nearly identical for the two protein preparations. It is optimal in the range pH 7-9 and is markedly impaired by increasing ionic strength. Chemical modification with group-specific reagents revealed that the integrity of carboxyl groups of the sarcolectin-binding protein and of lysine/arginine groups of sarcolectin are primarily important to maintain binding capacity. In addition to contribute to the understanding of the functional significance of sarcolectin this result provides a convenient procedure to purify a lymphokine.