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

We previously analysed the plasma membrane proteins of rat placenta and pre pared a database of 150 plasma membrane proteins, expressed in a stage-spec ific manner, utilizing two-dimensional gel electrophoresis (2D/E) [Mol. Cel l. Endocrinol. 115(1995)149]. In this study, we focused on the proteins, te ntatively named psL-I (MW 36.2 kDa, pI 5.3) and PsL-II (35.9 kDa, 5.3), whi ch 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) a nd psC-II (35.9 kDa, 5.3), respectively] arranged side by side with the sam e 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 bot h labyrinth and junctional zones. In addition, psL-I and psL-II began to in crease 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, ami no acid sequence and mass spectrometry (MALDI-TOF-MS) revealed that psC-I w as a G protein beta1 subunit, and psC-II was a beta2 subunit. and showed th at psL-I and PsL-II were molecular modified forms of psC-I and psC-II, resp ectively. Expression of these G protein beta 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 diff erentiation in vitro. Interestingly, expression of psL-I and PsL-II gradual ly increased in association with the differentiation. Since the expression of beta1 and beta2 subunit proteins and their mRNAs was constant during the process of differentiation in Rcho-1 cells; the expression of these lower pr forms of G protein subunits (psL-I and PsL-II) was thought to be post-tr anslationally regulated. In conclusion, there are modified forms of G prote in beta1 and beta2 subunits, in the placenta and Rcho-1 cells, which are ex pressed in a pregnancy-stage or differentiation stage specific manner. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.