Regulatory subunit of calcineurin, a calcium-dependent, calmodulin stimulated protein phosphatase. Confers calcium sensitivity. Forms a complex composed of a calmodulin-dependent catalytic subunit (also known as calcineurin A) and a regulatory Ca(2+)-binding subunit (also known as calcineurin B) (PubMed:8524402, PubMed:12218175, PubMed:12357034, PubMed:17498738, PubMed:22343722, PubMed:23468591, PubMed:26794871, PubMed:27974827). There are three catalytic subunits, each encoded by a separate gene (PPP3CA, PPP3CB, and PPP3CC) and two regulatory subunits which are also encoded by separate genes (PPP3R1 and PPP3R2). Interacts with catalytic subunit PPP3CA/calcineurin A (PubMed:8524402, PubMed:12218175, PubMed:12357034, PubMed:17498738, PubMed:22343722, PubMed:23468591, PubMed:27974827). Interacts with catalytic subunit PPP3CB/calcineurin A (PubMed:26794871). Interacts with CIB1 (via C-terminal region); the interaction increases upon cardiomyocyte hypertrophy (By similarity). Interacts with RCAN1 (PubMed:12809556). Interacts with SPATA33 (via PQIIIT motif) (PubMed:34446558). Translocates from the cytosol to the sarcolemma in a CIB1-dependent manner during cardiomyocyte hypertrophy. This protein has four functional calcium-binding sites (PubMed:8524402, PubMed:12218175, PubMed:12357034, PubMed:17498738, PubMed:22343722, PubMed:23468591, PubMed:26794871, PubMed:27974827). Although African swine fever virus infects pigs and not humans, human PPP3R1 and PPP3CA have been used for the crystallization. PPP3CA and PPP3R1 interact with African swine fever virus Mal-047/A238L (via PKIIIT and FLCVK motifs); the interaction does not block catalytic activity per se but inhibits PPP3CA function by blocking the access to the two substrate recognition (PubMed:23468591). Belongs to the calcineurin regulatory subunit family.