Calcium-regulated non-lysosomal thiol-protease which catalyzes limited proteolysis of substrates involved in cytoskeletal remodeling and signal transduction. This small subunit may act as a tissue-specific chaperone of the large subunit, possibly by helping it fold into its correct conformation for activity. Heterodimer of a large (catalytic) and a small (regulatory) subunit. Translocates to the plasma membrane upon calcium binding.