Thiol protease. Key protease responsible for the removal of the invariant chain from MHC class II molecules and MHC class II antigen presentation. The bond-specificity of this proteinase is in part similar to the specificities of cathepsin L. Similar to cathepsin L, but with much less activity on Z-Phe-Arg-|-NHMec, and more activity on the Z-Val-Val-Arg-|-Xaa compound. Monomer. Highest levels occur in the ileum followed by spleen, brain, thyroid, ovary and uterus. Low levels are found in the liver, kidney, jejunum and lung with lowest levels in the heart. By thyroid-stimulating hormone. Belongs to the peptidase C1 family.