Human syncytiotrophoblast membrane proteins defined using a heterologous antiserum

Detergent extracts of human trophoblast plasma membranes (TrPM) and of other fetal tissues from the same conceptus were prepared and the proteins separated by use of isoelectric focusing. A major band of isoelectric point (pI) 4.1 was present in TrPM together with minor bands of (pI) 4.9, 5.9, 7.3, 7.8 and 7.9. These bands were absent from non-trophoblastic fetal tissues. The pI 4.1 protein reacted with an 125I-labelled IgG fraction of a heterologous antiserum raised against TrPM and was found to have an apparent molecular weight of 100,000. This protein did not appear to be placental alkaline phosphatase, SP1, transferrin or human chorionic gonadotrophin. Many neoplastic cell lines tested also revealed a major protein band of pI 4.1 but it has not been established that this was the same protein as found in the TrPM preparations.