The pregnancy-specific beta1-glycoprotein (PSG) form a family of very closely related blood proteins which are essential to pregnancy. Despite being present at high levels in the maternal blood, the roles of the PSG are unknown. However, reduced serum levels are associated with spontaneous abortion, a condition that affects 1 in every 300 conceptions. The applicant has demonstrated that one of the PSG interacts with cells of the immune system, suggesting that it may have a role in modifying the maternal response to the antigenically foreign fetus. The long term goal of this research is to identify and characterize the biological roles of the PSG and explore their clinical use, during pregnancy and for contraception. The applicant has shown previously that PSG11s, one member of the PSG, interacts with the surface of promonocyte cells via an arginine-glycine- aspartate (RGD) peptide motif present in the N-terminal domain of the protein. In this study, the receptor for pSG11s will be identified by cross-linking a radiolabeled PSG11s RGD-containing peptide to the surface of promonocyte cells. The receptor will be isolated by affinity chromatography and parts of its amino acid sequence determined. This information will be used to isolate and characterize the receptor cDNA. Expression of the PSG11s receptor transcript will be examined, in a variety of cells, using PCR, and those cells expressing the transcript will be tested for the PSG11s binding activity. Using a baculovirus expression system, PSG11s and the related PSG1a, will be expressed and then purified. The binding activity of these proteins will then be compared with the binding activity of the PSG11s RGD- containing peptide initially used to characterize the receptor activity. Antisera raised against the RGD motif, and synthetic competitor peptides, will also be used to characterize the binding interaction more fully.