Arabidopsis PHOSPHOTYROSYL PHOSPHATASE ACTIVATOR Is Essential for PROTEIN PHOSPHATASE 2A Holoenzyme Assembly and Plays Important Roles in Hormone Signaling, Salt Stress Response, and Plant Development    

Abstract

PROTEIN PHOSPHATASE 2A (PP2A) is a major group of serine/threonine protein phosphatases in eukaryotes. It is composed of three subunits: scaffolding subunit A, regulatory subunit B, and catalytic subunit C. Assembly of the PP2A holoenzyme in Arabidopsis (Arabidopsis thaliana) depends on Arabidopsis PHOSPHOTYROSYL PHOSPHATASE ACTIVATOR (AtPTPA). Reduced expression of AtPTPA leads to severe defects in plant development, altered responses to abscisic acid, ethylene, and sodium chloride, and decreased PP2A activity. In particular, AtPTPA deficiency leads to decreased methylation in PP2A-C subunits (PP2Ac). Complete loss of PP2Ac methylation in the suppressor of brassinosteroid insensitive1 mutant leads to 30% reduction of PP2A activity, suggesting that PP2A with a methylated C subunit is more active than PP2A with an unmethylated C subunit. Like AtPTPA, PP2A-A subunits are also required for PP2Ac methylation. The interaction between AtPTPA and PP2Ac is A subunit dependent. In addition, AtPTPA deficiency leads to reduced interactions of B subunits with C subunits, resulting in reduced functional PP2A holoenzyme formation. Thus, AtPTPA is a critical factor for committing the subunit A/subunit C dimer toward PP2A heterotrimer formation.

Methylation of protein phosphatase 2A—Influence of regulators and environmental stress factors

“…Growth of roots was clearly impaired in the lcmt1 mutant, with both shorter main roots and poor growth of lateral roots (Figure 5g). Enhanced effects of ABA and NaCl on root growth in a lcmt1 knockout has been reported previously, substantiating impaired root growth in lcmt1 under various conditions (Chen et al, 2014).…”

“…A tendency of lower PP2A activity extracted from lcmt1 relative to WT would be expected according to previous experiments from Chen et al (). Variable assay methods between research groups have probably contributed to a smaller difference between WT and lcmt1 in our experiments.…”

“…Conversely, overexpression of PP2A-C5 led to salt resistance (Hu et al, 2016;Pernas et al, 2007). Knockdown of the PP2A activator PTPA (phosphotyrosyl phosphatase activator/PP2A activator) resulted in hypersensitivity to NaCl and impaired root growth (Chen, Hu, Zhu, Shen, & Zhang, 2014). Furthermore, one of the B′ subunit (B′ζ) was induced fourfold, and B″α was induced twofold under salt stress (Lillo et al, 2014).…”

PP2A-3 interacts with ACR4 and regulates formative cell division in the Arabidopsis root

“…Phosphorylation of the tail of the PP2A catalytic subunit plays an important role in regulating the assembly-and thus activity-of PP2A holoenzymes (39). We therefore explored PP2A activity in cellular extracts prepared from acr4 seedlings using a PP2A phosphatase assay system (35,43,44). In our hands, this revealed a decrease in PP2A activity of about 19% in acr4 compared with wild type, which was similar to that associated with the PP2A regulatory subunit mutant roots curl in npa 1 (rcn1) (22%) and the catalytic subunit mutant pp2a-3 (16%) (Fig.…”

Overexpression of AtPTPA in Arabidopsis increases protein phosphatase 2A activity by promoting holoenzyme formation and ABA negatively affects holoenzyme formation

“…1 PP2A is made of 3 subunits: the scaffolding subunit A, the regulatory subunit B, and the catalytic subunit C. 2 We recently reported the functional characterization of AtPTPA, a critical regulator for PP2A activity in Arabidopsis. 3 AtPTPA exerts its action on PP2A through binding to PP2A's AC dimer, and the action of AtPTPA in changing the conformation of the C subunits (i.e. PP2Ac) is critical for PP2A holoenzyme formation and maintenance of PP2A activity.…”