Protein phosphatase 2A holoenzyme and its subunits from Medicago sativa

We detected an about 200 kDa holoenzyme of protein phosphatase 2A (PP2A) in the crude extract of Medicago sativa microcallus cells by gel permeation c hromatography. By polymerase chain reaction (PCR) we isolated two M. sativa cDNA fragments corresponding to the catalytic (C) subunit, and one each co ding for the A and the B regulatory subunits of PP2A. The C subunit sequenc es were different from that published previously, indicating the existence of at least three different isoforms in M. sativa. Using the PCR fragments as probes, we obtained two distinct full-length clones for both the A and B subunits from an alfalfa cDNA library. Our results demonstrate that the co mponents of the PP2A holoenzyme, namely the catalytic and regulatory subuni ts, are present in alfalfa in several isoforms and that their sequences are highly similar to their plant, yeast and animal counterparts. The distinct regulatory subunit genes are constitutively expressed during the cell cycl e. Interestingly, two A-B subunit pairs had parallel mRNA steady-state leve ls in different plant tissues suggesting that not all of the possible isofo rm combinations are present in all tissues. The expression of the MsPP2A B beta subunit form was induced by abscisic acid indicating a specific functi on for this protein in the stress response.