The A subunit of protein phosphatase 2A serves as a scaffolding molecule to coordinate the assembly of the catalytic subunit and a variable regulatory B subunit. Seems to act as a positive regulator of PP2A catalytic activity. Confers resistance to phosphatase inhibitors such as okadaic acid and cantharidin. Involved during developmental process such as seedling and floral developments, root gravitropism, and stomatal opening regulation. Involved in the regulation of auxin efflux, especially during basipetal (tips to base) auxin transport in roots, and appears to contribute to the perception of auxin efflux inhibitors such as 1-N-naphthylphthalamic acid (NPA) and to semicarbazone I (substituted phenylsemicarbazone of 2-acetylarylcarboxylic acids) (SCB-I). Modulates the magnitude of ethylene response in the hypocotyl and stem, and functions as a general positive transducer of early ABA signaling. The holoenzyme composed of PP2AA1, PP2A4 and B'ZETA or 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). PP2A consists of a common heterodimeric core enzyme, composed of a 36 kDa catalytic subunit (subunit C) and a 65 kDa constant regulatory subunit (subunit A), that associates with a variety of regulatory subunits such as subunits B (the R2/B/PR55/B55, R3/B''/PR72/PR130/PR59 and R5/B'/B56 families) and the regulatory subunits TON2. Interacts with CYP20-1/ROC7. Also interacts with phosphatidic acid (PA), a lipid signaling molecule. Interacts with CHIP. Interacts with SIC/RON3 (PubMed:26888284). Mostly expressed in cell-dividing tissues such as apical meristems. Ubiquitous, with higher levels in roots and flowers (at protein level). Each HEAT repeat appears to consist of two alpha helices joined by a hydrophilic region, the intrarepeat loop. The repeat units may be arranged laterally to form a rod-like structure (By similarity). Ubiquitinated. CHIP-mediated ubiquitination enhances phosphatase activity after an abiotic stress such as low temperature or darkness. Belongs to the phosphatase 2A regulatory subunit A family.