Proteins of the apical and basal plasma membranes of the human placental syncytiotrophoblast: Immunochemical and electrophoretic studies

Proteins of the basal and microvillous plasma membranes of the human placental syncytiotrophoblast were compared to elucidate the basis for structural and functional differences in the two membranes. Among the proteins common to both membranes were actin, alkaline phosphatase, albumin, transferrin, immunoglobulin G, proteins of M r 35 000, 55 000 and 180 000 and five immunochemically detected proteins. Each membrane also contained unique proteins. Major microvillous cytoskeleton proteins of M r 68 000, 80 000 and 105 000 (α-actinin) were lacking or absent from basal membrane cytoskeletons which instead contained unique proteins of M r 14 000, 16 000 220 000 and 240 000. In addition, immunochemical analyses revealed four glycoprotein antigens unique to microvillous membrane and five unique to basal membrane. Fibronectin was also found to be exclusive to basal membrane. The differences in membrane-associated cytoskeletal proteins correlated with the different organization of actin microfilaments at the two membranes. The protein antigens unique to each of the two membranes provided further evidence for the polarization of membrane proteins and functions in the syncytium.