PP2A Phosphatases: The On-Off Regulatory Switches of Brassinosteroid Signaling

Inactivation of ligand-bound plasma membrane receptors is crucial for the regulation of their signaling outputs. The internalization of activated receptors and their subsequent targeting for recycling or degradation is controlled by posttranslational modifications, of which phosphorylation and dephosphorylation play an important role. Recent work suggests that a similar mechanism acts on the brassinosteroid (BR) receptor BR INSENSITIVE 1 (BRI1) in Arabidopsis thaliana to switch off BR signaling. The degradation of BRI1 requires a protein phosphatase 2A (PP2A)-mediated dephosphorylation that is specified by methylation of the phosphatase by a leucine carboxylmethyltransferase on membranes. PP2A is also reported to act positively on BR signaling by targeting the transcription factor BRASSINAZOLE-RESISTANT 1 (BZR1), a component downstream of BRI1. Thus, PP2A proteins play a dual role in the regulation of the BR pathway to switch between inhibition and activation of the BR signaling, depending on their substrate specificity and localization.