Calcium-binding protein maintaining matrix calcium levels at low concentration. Regulates mitochondrial calcium dynamics in planta by restricting influx. Located at the inner mitochondrial membrane and/or in the intermenbrane space. Expressed in both green and non-green tissues, including roots, shoots, floral buds and pollen. The EF-hand domains have high affinity for calcium. No visible phenotype. Belongs to the MICU1 family. MICU1 subfamily.