Proteins of human placental microvilli: II. Identification and topology of the plasma membrane proteins

We have investigated the location of proteins in the transverse plane of the plasma membrane of microvilli isolated from the syncytiotrophoblast layer of the human term placenta. Microvillous proteins were labelled with 125 I under reaction conditions where those proteins exposed on the maternal-facing surface of the microvilli were most heavily labelled. The proteins were then solubilized and subjected to one- and two-dimensional electrophoresis followed by protein isoelectric point or both, were identified, and these were classified into three groups: weakly, moderately heavily, and heavily labelled. The microvilli were in the form of intact vesicles that were correctly orientated (‘right-side out’). Thus the extent of labelling of each protein could be used as an indication of the extent of its exposure on the maternal-facing surface of the microvilli. Human serum albumin was present on the surface of the isolated, washed microvilli, but was probably a contaminant originating from maternal blood.