Regulatory subunit of the calcium-regulated non-lysosomal thiol-protease which catalyzes limited proteolysis of substrates involved in cytoskeletal remodeling and signal transduction. Essential for embryonic development (PubMed:10825211). Homodimer or heterodimer of a large (catalytic) and a small (regulatory) subunit. In presence of calcium, the heterodimer dissociates (By similarity). Translocates to the plasma membrane upon calcium binding. The contact of the 5th EF-hand domain from each monomer allows the formation of the homodimer and also appears to mediate the contact between the large catalytic subunit and small regulatory subunit for the formation of the heterodimer. EF-hand domains are paired. EF-hand 1 is paired with EF-hand 2 and EF-hand 3 is paired with EF-hand 4. The fifth EF-hand domain, left unpaired, does not bind the calcium but is responsible of the dimerization by EF-embrace. The first four EF-hand domains bind calcium, however it is not sure if the binding of EF-hand 4 to calcium is physiologically relevant. Embryonic lethality by 11.5 dpc, with indications of defects in both vasculogenesis and erythropoiesis.