We have previously shown that type I IGF receptors are expressed primarily on the maternal-facing surface of the syncytiotrophoblast. We therefore began to examine the hypothesis that IGFBPs expressed at the placental-decidual interface interact with the syncytiotrophoblast membrane to modulate IGF function. Membrane vesicles were prepared specifically from the maternal-facing, microvillous membrane surface(MVM) and the fetal-facing, basal membrane surface (BM) of the syncytiotrophoblast. The association of IGFBPs with each membrane preparation was determined by ligand blot analysis using radiolabelled IGF-II. A doublet migrating at 38/42kD was detected in both MVM and BM preparations. Selective immunoprecipitation followed by ligand blot analysis identified this IGF binding species as IGFBP-3. Additionally, a prominent band migrating at approximately 29 kD was associated exclusively with the BM. This protein was identified as IGFBP-1 by both immunoprecipitation and immunoblotting techniques. Non-denaturing PAGE revealed five distinct bands corresponding to different degrees of phosphorylation. The phosphorylation pattern of the BM-associated IGFBP-1 was identical to that of native IGFBP-1 found in amniotic fluid. Next, we attempted to identify the IGFBP-1 binding site. Because IGFBP-1 contains an RGD sequence, it can potentially interact with specific integrin molecules. We demonstrated the presence of both αV and α5 integrins in BM by immunoblot analysis suggesting them as candidate receptors. Preliminary co-precipitation studies, however, failed to demonstrate their association with IGFBP-1. Finally, we studied the in situ location of IGFBP-1 in term placenta. Immunohistochemical analysis of frozen sections revealed IGFBP-1-specific staining of the syncytiotrophoblast and the fetal capillaries. In contrast to IGFBP-3, Northern analysis failed to detect expression of IGFBP-1 mRNA by the placenta. IGFBP-1, however, is known to be expressed by the fetal liver and maternal decidua. In summary IGFBP-1, a protein not produced by the human placenta, is localized to the basal surface of the syncytiotrophoblast. The localization of IGFBP-1 to a distinct compartment within the fetal placenta, not in proximity to the syncytiotrophoblast type I IGF receptor, suggests it may play a role in regulating/targeting IGF activity within the stromal compartment or by exerting IGF-independent effects on the basal surface of the syncytiotrophoblast. The nature of the IGFBP-1 binding site is not known.