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:293720, PubMed:1715244, PubMed:7543369). 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). The interaction between the 2 subunits is Ca(2+)-independent (PubMed:293720). Interacts with catalytic subunit PPP3CA/calcineurin A (PubMed:7543369, PubMed:1715244). Interacts with catalytic subunit PPP3CB/calcineurin A (By similarity). Interacts with CIB1 (via C-terminal region); the interaction increases upon cardiomyocyte hypertrophy. Interacts with RCAN1 (By similarity). Interacts with SPATA33 (via PQIIIT motif) (By similarity). Translocates from the cytosol to the sarcolemma in a CIB1-dependent manner during cardiomyocyte hypertrophy. This protein has four functional calcium-binding sites. Belongs to the calcineurin regulatory subunit family.