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). 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) (By similarity). Interacts with SPATA33 (via PQIIIT motif) (By similarity). Localizes in the mitochondria in a SPATA33-dependent manner. This protein has four functional calcium-binding sites. Belongs to the calcineurin regulatory subunit family.