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 beta1 subunit, and psC-II was a beta2 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 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 differentiation in vitro. Interestingly, expression of psL-I and psL-II gradually 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 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 beta1 and beta2 subunits, in the placenta and Rcho-1 cells, which are expressed in a pregnancy-stage or differentiation stage specific manner.
Ishimura
R
R
Laboratory of Cellular Biochemistry, Department of Veterinary Medical Sciences/Animal Resource Sciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, 113-8657, Tokyo, Japan.
Yoshida
K
K
Kimura
H
H
Dohmae
N
N
Takio
K
K
Ogawa
T
T
Tanaka
S
S
Shiota
K
K
D000595:Amino Acid Sequence; D000818:Animals; D005260:Female; D020962:Heterotrimeric GTP-Binding Proteins / Q000096:biosynthesis* / Q000737:chemistry / Q000302:isolation & purification; D008297:Male; D013058:Mass Spectrometry; D008565:Membrane Proteins / Q000737:chemistry; D008969:Molecular Sequence Data; D010449:Peptide Mapping; D010766:Phosphorylation; D010920:Placenta / Q000737:chemistry* / Q000378:metabolism; D011247:Pregnancy; D011499:Protein Processing, Post-Translational; D051381:Rats; D017208:Rats, Wistar; D017421:Sequence Analysis; D014018:Tissue Distribution
D008565:Membrane Proteins; D020962:Heterotrimeric GTP-Binding Proteins