ASYMMETRICAL DISTRIBUTION OF G-PROTEINS IN SYNCYTIOTROPHOBLASTIC BRUSH-BORDER AND BASAL-PLASMA MEMBRANES OF HUMAN TERM PLACENTA

In human placental syncytiotrophoblast brush-border (BBM, facing the m other) and basal-plasma membranes (BPM, facing to fetus) we have recen tly demonstrated the presence of calcaemic hormone-specific receptors for parathyroid hormone and calcitonin, which could be implicated in c alcium transport from the mother to the fetus. It is well recognized t hat signal transducing G proteins (guanosine nucleotide-binding protei ns) can associate with various transmembrane receptors and effector pr oteins, and regulate a variety of second-messenger systems and ion cha nnels. In this present paper, we investigated the presence of a variet y of alpha and beta subunits of G proteins in both syncytiotrophoblast , BBM and BPM by Western blot technique. For the first time, we were a ble to demonstrate the presence of G proteins in the bipolar syncytiot rophoblast membranes, which were evaluated by immunoblotting using aff inity purified antiserum raised against the alpha subunits of G(i1), G (?(i1/i2)), G(i3), G(0), G(q), G(s), G(2) and against the beta subunit s. In BBM, we identified the alpha subunits of G(i1), G(i3), G(0), G(q ), G(s) (42, 46 kDa), G(2) and beta subunits. The same alpha subunits of G proteins were found in BPM, although alpha subunits of G(i3), G(q ), G(s) (46 kDa) were located predominantly in the BBM, and the alpha subunit of G(0) was found preferentially in BPM. Moreover, in BBM and BPM, a purified antisera raised against the alpha subunits of G(i1), a nd G(s), detected a 105 kDa protein and a 67 kDa protein, respectively . Interestingly, the 67 kDa protein was preferentially located in BBM, and none of these proteins were detectable in membranes prepared from brain (control). The asymmetrical distribution of the alpha subunits of G proteins among the two different placental bipolar membranes migh t reflect the very specialized function of these syncytiotrophoblast m embranes in ions and nutrients transport from the mother to the fetus. (C) 1996 W. B. Saunders Company Ltd