Stage-specific modification of G protein beta subunits in rat placenta

We previously analysed the plasma membrane proteins of rat placenta and prepared a database of 150 plasma membrane proteins, expressed in a stage-specific manner, utilizing two-dimensional gel electrophoresis (2D/E) [Mol. Cell. Endocrinol. 115(1995)149]. In this study, we focused on the proteins, tentatively named psL-I (MW 36.2 kDa, pI 5.3) and psL-II (35.9 kDa, 5.3), which were expressed mainly in late pregnancy. Close to psL-I and psL-II on 2D/E gels, we also recognized more abundant proteins [psC-I (36.2 kDa, 5.4) and psC-II (35.9 kDa, 5.4), respectively] arranged side by side with the same MW but different pI. Expression of psL-I and psL-II was detected only in junctional zone of placenta, whereas psC-I and psC-II were expressed in both labyrinth and junctional zones. In addition, psL-I and psL-II began to increase on day 16 of pregnancy and peaked at term, whereas expression of psC-I and psC-II was relatively constant. The analysis of these four proteins (psL-I, psL-II, psC-I and psC-II) by preparative 2D/E, peptide mapping, amino acid sequence and mass spectrometry (MALDI-TOF-MS) revealed that psC-I was a G protein β1 subunit, and psC-II was a β2 subunit, and showed that psL-I and psL-II were molecular modified forms of psC-I and psC-II, respectively. Expression of these G protein β subunits (psL-I, psL-II, psC-I and psC-II) was also observed in rat choriocarcinoma cells, Rcho-1 cells. Expression of psC-I and psC-II was much higher than those of psL-I and psL-II, and their level was relatively constant regardless of the stage of differentiation in vitro. Interestingly, expression of psL-I and psL-II gradually increased in association with the differentiation. Since the expression of β1 and β2 subunit proteins and their mRNAs was constant during the process of differentiation in Rcho-1 cells, the expression of these lower pI forms of G protein subunits (psL-I and psL-II) was thought to be post-translationally regulated. In conclusion, there are modified forms of G protein β1 and β2 subunits, in the placenta and Rcho-1 cells, which are expressed in a pregnancy-stage or differentiation stage specific manner.