Distinct Clades of Protein Phosphatase 2A Regulatory B'/B56 Subunits Engage in Different Physiological Processes

Abstract

Protein phosphatase 2A (PP2A) is a strongly conserved and major protein phosphatase in all eukaryotes. The canonical PP2A complex consists of a catalytic (C), scaffolding (A), and regulatory (B) subunit. Plants have three groups of evolutionary distinct B subunits: B55, B' (B56), and B''. Here, the Arabidopsis B' group is reviewed and compared with other eukaryotes. Members of the B'α/B'β clade are especially important for chromatid cohesion, and dephosphorylation of transcription factors that mediate brassinosteroid (BR) signaling in the nucleus. Other B' subunits interact with proteins at the cell membrane to dampen BR signaling or harness immune responses. The transition from vegetative to reproductive phase is influenced differentially by distinct B' subunits; B'α and B'β being of little importance, whereas others (B'γ, B'ζ, B'η, B'θ, B'κ) promote transition to flowering. Interestingly, the latter B' subunits have three motifs in a conserved manner, i.e., two docking sites for protein phosphatase 1 (PP1), and a POLO consensus phosphorylation site between these motifs. This supports the view that a conserved PP1-PP2A dephosphorelay is important in a variety of signaling contexts throughout eukaryotes. A profound understanding of these regulators may help in designing future crops and understand environmental issues.

Keywords: Arabidopsis; B56; PP1; PP2A; brassinosteroid; cell cycle; flowering; shugoshin.

Figure 1

Simplified scheme of the best-established functions and subcellular localization of B’ subunits. Pathways to the left: PP2A complexes with B’ζ and η dephosphorylate the co-receptor BAK1 and act as negative regulators of signaling from flagellin recognized by FLS2. B’θ and B’ζ have been found associated with peroxisomes and mitochondria, respectively. Pathways in the middle: PP2A complexes with B’γ, ζ, δ, η, θ, κ dephosphorylate and inactivate the BR receptor BRI1 at the plasma lemma, whereas PP2A complexes with B’α, β dephosphorylate transcription factor BZR1 and keeps active BZR1 in the nucleus, thereby promoting BR signaling. Pathways to the right: Same B’ subunits as act negatively in BR signaling promote flowering (B’δ not tested), probably through inhibition of the FLC inhibitor, though interacting proteins have not been identified for this process.

Figure 2

PP2A engagement in the cell cycle. The cell cycle is divided into G1 (GAP1 phase), S (synthesis of DNA phase), G2 (GAP2 phase), and M (mitosis). Mitosis is divided into P (prophase), M (metaphase), A (anaphase), and T (telophase). The two major checkpoints are indicated with red lines. The RB(R) (RETINABLASTOMA (RELATED)) protein is conserved in eukaryotes, and phosphorylation of RB(R) by CDKs is crucial for G1 to S transition, whereas dephosphorylation keeps cells in G1. Activation of CDK2 in mammals (CDKB, A in Arabidopsis) and inactivation of PP2A-55 is crucial for bringing cells from G2 to M. To exit M, a dephosphorelay where PP1 activates PP2A-B55 and PP2A-B’ is found in fission yeast, and possibly conserved across eukaryotes.