Essential protease involved in epiderm development. Required for aleurone cell development in the endosperm probably by maintaining and restricting the aleurone and embryonic epidermal L1 cell-layer fates as well as meristems organization. Involved in the maintenance of adaxial/abaxial axis information in developing leaves, probably by regulating cell proliferation and expansion. Does not need calcium ions to be active. Expressed in most tissues at low levels ranging from 30 to 55 ppm. Present in all endosperm cells at transcript level, but confined to aleurones at protein level. In endosperm, accumulates during early developmental stages and declines near maturity. In embryos, levels peak at middevelopment with highest expression in the embryonic axis. Also present in young pericarp and immature ear tip and base. The transmembrane regions are not required for calpain activity but may play regulatory roles. Autocatalytic proteolytic cleavage leading to the production of mainly cytoplasmic localized subproducts of about 85 and 120 kDa. Loss of aleurone cells leading to white grains. Embryos arrest at the juvenile globoid stage and are devoid of shoot structures. Although homology to other calpains is high within the protease domain, the lack of calcium-binding sites suggests that this protein is a protease that may not be activated by calcium ions. Belongs to the peptidase C2 family.