Presence of Gc (vitamin D-binding protein) and interactions with actin in human placental tissue

The distribution of Gc protein and actin and their interactions were studied in normal full-term human placentae. Both Gc and actin were detected by physicochemical analysis of isolated trophoblast membranes. Immunofluorescence of native placental sections showed fluorescence for both proteins on smooth muscle cells lining the fetal stem vessels, intervillous fibrin, villous fibrinoid, trophoblast membrane, and cytoplasm of villous stromal cells. Binding of Gc was demonstrated by prior incubation of sections with purified Gc which led to a striking increase in intensity of Gc fluorescence, but actin fluorescence was unaffected by this procedure and by preincubation with actin. Endogenous Gc and actin could also be removed by washing of tissue sections with chaotrope--3 M KCl or 3 M NH4SCN, denaturant--6 M urea, or glycine-HCl pH 3.8, as judged by fluorescence and SDS-PAGE of the wash supernatant. Phenotypic analysis of Gc eluted from trophoblast membranes and of corresponding matched maternal and fetal cord sera by isoelectric focusing indicated that trophoblast Gc was of predominantly maternal origin. Although the roles of Gc and actin in the placenta are unknown, these results indicate that Gc may be another maternal protein for which specific binding sites are expressed on the membrane of placental trophoblast.