Catalytic subunit of calcineurin, a calcium-dependent, calmodulin stimulated protein phosphatase which plays an essential role in the transduction of intracellular Ca(2+)-mediated signals. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Binds 1 Fe(3+) ion per subunit. Binds 1 zinc ion per subunit. Activated by Ca(2+)-bound calmodulin following an increase in intracellular Ca(2+) (PubMed:11352578, PubMed:10608845, PubMed:9765812, PubMed:8681950). At low Ca(2+) concentrations, the catalytic subunit (also known as calcineurin A) is inactive and is bound to the regulatory subunit (also known as calcineurin B) in which only two high-affinity binding sites are occupied by Ca(2+) (PubMed:11352578). In response to elevated calcium levels, the occupancy of the low-affinity sites on calcineurin B by Ca(2+) causes a conformational change of the C-terminal regulatory domain of calcineurin A, resulting in the exposure of the calmodulin-binding domain and in the partial activation of calcineurin A (PubMed:11352578). The subsequent binding of Ca(2+)-bound calmodulin leads to the displacement of the autoinhibitory domain from the active site and possibly of the autoinhibitory segment from the substrate binding site which fully activates calcineurin A (PubMed:11352578, PubMed:10608845, PubMed:9765812, PubMed:8681950). Activated by arachidonic acid, linoleic acid and oleic acid to the same extent as calmodulin (PubMed:10608845). Forms a complex composed of a catalytic and calmodulin-dependent subunit, calcineurin A, and a regulatory Ca(2+)-binding subunit, calcineurin B (PubMed:11352578). In response to an increase in Ca(2+) intracellular levels, forms a complex composed of canA/calcineurin A, cnbA/calcineurin B and calA/calmodulin (PubMed:11352578). Interacts (via calcineurin B binding domain) with regulatory subunit cnbA/calcineurin B (PubMed:11352578). Interacts (via calmodulin-binding domain) with calmodulin; the interaction depends on calmodulin binding to Ca(2+) (PubMed:10608845, PubMed:11352578). Highest levels are found after 10 hours of development. Levels decrease during multicellular development (at protein level). The autoinhibitory domain prevents access to the catalytic site. The autoinhibitory segment prevents access to the substrate binding site. Possible isomerization of Pro-358 within the SAPNY motif triggers a conformation switch which affects the organization and thus accessibility of the active site and the substrate binding region (PxIxIF motif). The trans- to cis-transition may favor calcineurin A activation and substrate binding. The reverse cis- to trans-transition may be enhanced by peptidyl-prolyl isomerases such as PPIA. The N-terminus is blocked. Belongs to the PPP phosphatase family. PP-2B subfamily.