Image analysis of protein profiles from paired microvillous and basal syncytiotrophoblast plasma membranes from term human placenta and characterization of IgG binding to membrane vesicles.

Image analysis of SDS-PAGE profiles of highly purified, paired maternal-facing (microvillous; MVM) and fetal-facing (basal; BM) plasmalemma membrane vesicles from six term human placentae showed that, while individual MVM or BM profiles were extremely reproducible, the two membrane populations were substantially different--although all of the seven major bands of molecular mass, 98.4, 79.4, 71.1, 45.1, 40.9, 39.5 and 34.5 kDa found in MVM were present, albeit in differing amounts, in BM. BM were characterized by the presence of five low molecular weight bands which were not present in MVM. Despite this consistency of the membrane preparations, binding of 125I-IgG or its fragments showed marked variability in both MVM and BM. At pH 7.4, both MVM and BM bound similar amounts of 125I-IgG with Kd values of 5.2 +/- 1.9 x 10(-6) M (s.e., n = 8) and 2.9 +/- 0.4 x 10(-6) M respectively, (P > 0.05). There were 1.2-1.6 x 10(15) binding sites/mg protein. Affinity constants for Fc fragment binding to MVM and BM were similar to those for IgG, although the Bmax value for BM Fc binding was greatly reduced compared to that for IgG (P > 0.001). Fab binding to MVM and BM was also saturable but substantially lower than that of Fc, whereas binding of F(ab')2 was low and linear. Both MVM and BM bound marginally more IgG at pH 6.0 than at pH 7.4. These data provide further evidence for receptor-mediated transcytosis of maternal IgG across the placenta and confirm that the placental IgG transporter differs from classical Fc gamma receptors.