Protein phosphatase inhibitor that specifically inhibits protein phosphatase 2A (PP2A) (PubMed:28167675, PubMed:38123684). Inhibition of PP2A is enhanced when ARPP19 is phosphorylated (PubMed:28167675, PubMed:38123684). When phosphorylated at Ser-62 during mitosis, specifically interacts with PPP2R2D (PR55-delta) and inhibits its activity, leading to inactivation of PP2A, an essential condition to keep cyclin-B1-CDK1 activity high during M phase (PubMed:21164014). May indirectly enhance GAP-43 expression (By similarity). {ECO:0000250|UniProtKB:Q712U5, ECO:0000269|PubMed:21164014, ECO:0000269|PubMed:28167675, ECO:0000269|PubMed:38123684}. This is the function of ENSG00000128989 (ARPP19, cAMP regulated phosphoprotein 19).