Functional Studies on Extra-lysosomal Legumain

Normally, the enzyme legumain is found within the endo-lysosomal system of our cells. In there, legumain is an important player of our immune system. It is cleaving foreign antigens like e.g. toxis in smaller peptides, so they can be recognized by our immune system. Mainly under pathophysiologic conditions legumain was found mislocalized to the cytosol, the nucleus or extracellularly. Although mislocalization of legumain is associated with severe disorders like cancer and Alzheimers disease, its function at these non-classical locations is still not understood. However, because of its relevance in a number of very different disorders, legumain became an interesting target for drug development. To develop save and efficient drugs, we need a detailed understanding of our target. Therefore, within this project we aim to study the molecular function of mislocalized, cytosolic legumain. In a previous study we found that legumain exists in three distinct maturations states, that correspond to a newborn, a child and an adult form. These different forms of legumain have different functions and different properties. While the adult form will only survive in the acidic endo-lysosome, newborn and child legumain can also tolerate neutral pH, like it can be found in the cytosol. Furthermore, child and adult legumain harbor, in addition to their protease activity, also a ligase activity. They can not only cut other proteins, but they can also link them. Importantly, legumain has the ligase function only at neutral pH. Based on these findings we hypothesize that (i) different forms of legumain have environment-specific functions, (ii) if the adult form of legumain is mislocalized it must be stabilized by an interaction partner and (iii) that mislocalized legumain is performing its pathophysiologic function not only by cutting but also by ligating other proteins. To test these hypotheses, we aim to develop a method that allows us to monitor the ligase function of legumain in real time. Furthermore, we aim to identify which form of legumain is mislocalizing and who its interaction partners are. This knowledge will allow us to unravel the pathophysiologic interaction network of mislocalized legumain and to develop new therapeutic and diagnostic tools.