Protein phosphatase 2A promotes stomatal development by stabilizing SPEECHLESS in Arabidopsis

Stomatal guard cells control gas exchange that allows plant photosynthesis but limits water loss from plants to the environment. In Arabidopsis, stomatal development is mainly controlled by a signaling pathway comprising peptide ligands, membrane receptors, a mitogen-activated protein kinase (MAPK) cascade, and a set of transcription factors. The initiation of the stomatal lineage requires the activity of the bHLH transcription factor SPEECHLESS (SPCH) with its partners. Multiple kinases were found to regulate SPCH protein stability and function through phosphorylation, yet no antagonistic protein phosphatase activities have been identified. Here, we identify the conserved PP2A phosphatases as positive regulators of Arabidopsis stomatal development. We show that mutations in genes encoding PP2A subunits result in lowered stomatal production in Arabidopsis Genetic analyses place the PP2A function upstream of SPCH. Pharmacological treatments support a role for PP2A in promoting SPCH protein stability. We further find that SPCH directly binds to the PP2A-A subunits in vitro. In plants, nonphosphorylatable SPCH proteins are less affected by PP2A activity levels. Thus, our research suggests that PP2A may function to regulate the phosphorylation status of the master transcription factor SPCH in stomatal development.

Inactivation of PP2A by a recurrent mutation drives resistance to MEK inhibitors

The serine/threonine Protein Phosphatase 2A (PP2A) functions as a tumor suppressor by negatively regulating multiple oncogenic signaling pathways. The canonical PP2A holoenzyme comprises a scaffolding subunit (PP2A Aα/β), which serves as the platform for binding of both the catalytic C subunit and one regulatory B subunit. Somatic heterozygous missense mutations in PPP2R1A, the gene encoding the PP2A Aα scaffolding subunit, have been identified across multiple cancer types, but the effects of the most commonly mutated residue, Arg-183, on PP2A function have yet to be fully elucidated. In this study, we used a series of cellular and in vivo models and discovered that the most frequent Aα R183W mutation formed alternative holoenzymes by binding of different PP2A regulatory subunits compared with wild-type Aα, suggesting a rededication of PP2A functions. Unlike wild-type Aα, which suppressed tumorigenesis, the R183W mutant failed to suppress tumor growth in vivo through activation of the MAPK pathway in RAS-mutant transformed cells. Furthermore, cells expressing R183W were less sensitive to MEK inhibitors. Taken together, our results demonstrate that the R183W mutation in PP2A Aα scaffold abrogates the tumor suppressive actions of PP2A, thereby potentiating oncogenic signaling and reducing drug sensitivity of RAS-mutant cells.

Arabidopsis thaliana histone deacetylase 14 (HDA14) is an α-tubulin deacetylase that associates with PP2A and enriches in the microtubule fraction with the putative histone acetyltransferase ELP3

It is now emerging that many proteins are regulated by a variety of covalent modifications. Using microcystin-affinity chromatography we have purified multiple protein phosphatases and their associated proteins from Arabidopsis thaliana. One major protein purified was the histone deacetylase HDA14. We demonstrate that HDA14 can deacetylate α-tubulin, associates with α/β-tubulin and is retained on GTP/taxol-stabilized microtubules, at least in part, by direct association with the PP2A-A2 subunit. Like HDA14, the putative histone acetyltransferase ELP3 was purified on microcystin-Sepharose and is also enriched at microtubules, potentially functioning in opposition to HDA14 as the α-tubulin acetylating enzyme. Consistent with the likelihood of it having many substrates throughout the cell, we demonstrate that HDA14, ELP3 and the PP2A A-subunits A1, A2 and A3 all reside in both the nucleus and cytosol of the cell. The association of a histone deacetylase with PP2A suggests a direct link between protein phosphorylation and acetylation.

Protein phosphatase-2A is activated in pig brain following cardiac arrest and resuscitation

Protein phosphatase-2A (PP-2A) interacts with several regulators of cell death pathways and is therefore a potential component of signaling pathways linking global cerebral ischemia to cell death. Using a novel procedure to quantify PP-2A activity, we find that cardiac arrest with resuscitation and reperfusion leads to activation of PP-2A by 1.6-fold in pig brain extract and by 3.4-fold after partial purification of PP-2A. This is the first demonstration of PP-2A activation in a clinically relevant model of transient global cerebral ischemia. These results suggest that inhibition of PP-2A activity may be neuroprotective in global cerebral ischemia.