A motif in PSG11s mediates binding to a receptor on the surface of the promonocyte cell line THP-1.

The pregnancy-specific glycoproteins (PSG) form a large family of essential pregnancy proteins, but their biological function is unknown. We have investigated whether one function of the PSG is to interact with cells of the maternal immune system. Normal human peripheral blood mononuclear cells, activated with phorbol ester, are shown to bind to purified placental PSG. This binding activity can be mimicked using a chemically synthesized peptide ligand containing the Arg-Gly-Asp (RGD) motif present in the N-terminal domain of PSG11s. The PSG11s receptors are present on cells of the myeloid cell lineage but not of the T cell or B cell lineages. The binding is mediated in part by the RGD motif and can be competed against by appropriate RGD-containing, but not Arg-Ala-Asp (RAD)-containing, ligands. Ligand binding requires a functional cytoskeleton. By examining the U937 and THP-1 promonocyte cell lines, the presence of receptors with two different binding characteristics are demonstrated. The THP-1 receptor is identified by chemical cross-linking as a protein of 46 kilodaltons (kDa), and affinity chromatography demonstrates the presence of three protein species of 32 kDa, 16.8 kDa, and 15.9 kDa, suggesting the receptor has multiple subunits.