The B regulatory subunit may modulate substrate selectivity and catalytic activity, and also may direct the localization of the catalytic enzyme to a particular subcellular compartment (By similarity). The holoenzyme composed of PP2AA1, PP2A4 and B'ETA acts as negative regulator of plant innate immunity by controlling BAK1 phosphorylation state and activation in surface-localized immune receptor complexes (PubMed:25085430). Required for the formation of the PP2A holoenzyme that negatively regulates brassinosteroid signaling by dephosphorylating and inactivating BRI1 in the cytoplasm (PubMed:26517938). PP2A consists of a common heteromeric enzyme, composed of a catalytic subunit (subunits C), a constant regulatory subunit (subunit A), and a variety of regulatory subunits such as subunits B (the R2/B/PR55/B55, R3/B''/PR72/PR130/PR59 and R5/B'/B56 families) (By similarity). Interacts with BZR1 (PubMed:21258370). Interacts with BRI1 (PubMed:26517938). A number of isoforms are produced. According to EST sequences. Induced by epibrassinolide. Delayed flowering. Belongs to the phosphatase 2A regulatory subunit B56 family.