The levels of the A, B, and C subunits of protein phosphatase 2A in extracts from synchronized embryonic bovine tracheal cells were determined by immunoblotting with subunit-specific antibodies. A constant amount of each subunit was found in resting cells as well as in growing cells from all stages of the cell cycle. The phosphatase activity of protein phosphatase 2A was also constant. A quantitative comparison showed that the A and C subunits were present in similar amounts, whereas the B subunit was present at a significantly lower level. Together, the A, B, and C subunits represented approximately 0.2% of the total cellular protein.Control of the cell cycle in eukaryotes is achieved by the complex interplay of protein kinases and phosphatases (2,(17)(18)(19)25). Protein phosphatase 2A (PP2A), a serine/threonine-specific phosphatase, has been implicated in specific mitotic events in yeast cells (11) and amphibian oocytes (14). It is composed of a 37-kDa catalytic subunit (termed C) and two regulatory subunits of 65 and 55 kDa, termed A and B, respectively (for a review, see reference 3). The catalytic subunit is highly conserved between species and is a member of a gene family which includes the catalytic subunits of type 1 and 2B protein phosphatases (4). The A subunit, which is also highly conserved, has an unusual structure consisting of 15 nonidentical repeats that are 38 to 40 amino acids in length (10, 28). A key substrate of PP2A in oocytes is maturationpromoting factor (MPF) (14), a protein kinase with a catalytic subunit homologous to p34cd"2 of fission yeast (1,6,7,13,15) and a regulatory subunit called cyclin (5). PP2A removes phosphate from a specific site of the MPF catalytic subunit that correlates with loss of its enzymatic and maturation-promoting activity. PP2A also plays a role in neoplastic transformation by the small DNA tumor viruses polyomavirus and simian virus 40 (SV40). It forms specific complexes with the polyomavirus medium-sized tumor antigen (medium T) (8,9,12,20,21,29) and small tumor antigen (small T) (23) and with SV40 small T (21,22,27,31). Genetic experiments suggest that the complex formation is important for transformation (8). We have recently demonstrated that SV40 small T inhibits the dephosphorylation of various substrates, including SV40 large T and the growth suppressor protein p53, by binding to the A subunit of PP2A (24,30). This activity of small T might explain its growth-promoting effect, as in the case of the tumor-promoting effect of okadaic acid, which also inhibits PP2A (for a review, see reference 4).Since PP2A plays an important role in growth control, the questions of whether and how it is regulated itself arise. In the present communication, we report on the expression and activity of PP2A during the mitotic cell cycle. Embryonic bovine tracheal (EBTr) cells were synchronized by starvation in medium containing 0.2% fetal calf serum (FCS). After release from growth arrest with medium containing 10% FCS, the cells were harvested every 3 h over a total period...
