A protein phosphatase 2A (PP2A) inhibition assay using a recombinant enzyme for rapid detection of microcystins.

Worldwide blooms of toxic cyanobacteria (blue-green algae) commonly occur in freshwater, often in drinking water sources, necessitating routine monitoring of water quality. Microcystin-LR and related cyanobacterial toxins strongly inhibit protein phosphatase 2A (PP2A) and are therefore assayable by measuring the extent of PP2A inhibition. In this study, we evaluated the suitability of the catalytic subunit of recombinant PP2A (rPP2Ac) expressed with a baculovirus system for use in a microplate microcystin assay. Five microcystin analogs, microcystin-LR, -RR, -YR, -LF, and -LW, and nodularin strongly inhibited rPP2Ac activity with IC(50) values of 0.048, 0.072, 0.147, 0.096, 0.114, and 0.54 nM, respectively. Microcystin-LR in a water sample could be assayed from 0.005 to 5 ng/ml. The assay could detect the toxin at a far lower level than required by the World Health Organization for regulation of microcystin-LR or its equivalent (1 microg/L). Pretreatment or concentration of water samples with low toxin concentrations was not necessary. The microplate assay using rPP2Ac was more sensitive than an enzyme-linked immunosorbent assay (ELISA) method and a cytotoxicity assay. The genetically engineered rPP2Ac was more stable than a commercially available dimeric enzyme, producing accurate and reproducible results. Our results confirm that the rPP2Ac we prepared is an excellent tool for detecting and quantifying microcystins in water.

Methylation of the C-terminal leucine residue of the PP2A catalytic subunit is unnecessary for the catalytic activity and the binding of regulatory subunit (PR55/B).

Protein phosphatase 2A (PP2A) is composed of structural (A), catalytic (C), and regulatory (B) subunits. The catalytic subunit (PP2A(C)) undergoes reversible carboxyl-methylation and -demethylation at its C-terminal leucine residue (Leu309), catalyzed by PP2A-methyltransferase (PMT) and PP2A methylesterase (PME-1), respectively. In this study, we observed that the activity of PP2A was largely unaffected by the addition of PME-1, and that the regulatory subunit (PR55/B) could bind demethylated PP2A(D). Furthermore, to study the precise effect of Leu309 demethylation on PP2A activity, we generated two His(8)-tagged mutant versions of PP2A(C) containing an alanine residue in place of Leu309, and a deletion of Leu309. Both recombinant mutants exhibited phosphatase activity. In addition, we demonstrated that both mutants could constitute a holoenzyme with the regulatory A and B subunits. Our collective results indicate that methylation of Leu309 of PP2A(C) is unnecessary for the PP2A activity and the binding of PR55/B.

Baculovirus expression, purification, and characterization of human protein phosphatase 2A catalytic subunits alpha and beta.

Protein phosphatase 2A (PP2A) contains a 36-kDa catalytic subunit (PP2Ac), a 65-kDa structural subunit (PR65/A), and a regulatory B subunit. The core enzyme consists of the structural and catalytic subunits. The catalytic subunit exists as two closely related isoforms, alpha and beta. Several natural toxins, including okadaic acid (OA) and microcystins, specifically inhibit PP2A. To obtain biologically active recombinant PP2A and to compare the properties of the PP2A catalytic subunit alpha and beta isoforms, we expressed human PP2Acalpha and cbeta in High Five insect cells. The recombinant PP2Acalpha and cbeta possess similar phosphatase activities using p-NPP and phosphopeptide as substrates and are strongly inhibited by OA and microcystin-LR to similar degrees. In addition, PP2Acalpha or cbeta was co-expressed with PR65/A and co-purified as a core dimer, PP2AD (Aalpha/calpha and Aalpha/cbeta) with PR65alpha/Aalpha. The recombinant PP2AD bound to the B subunit in vitro. These results show that the recombinant PP2Acalpha and cbeta are identical in their ability to associate with the A and B subunits, in their phosphatase activities, and in carboxyl-methylation. Furthermore, our results show that High Five insect cells can produce biologically active recombinant PP2A, which should be a valuable tool for detecting natural toxins and investigating the mechanism of PP2A catalysis and other protein interactions.