Cathepsin E is an intracellular, non-lysosomal aspartic
protease expressed in a variety of cells and tissues. The
protease has proposed physiological roles in antigen presentation by the MHC class II system, in the biogenesis of the
vasoconstrictor peptide endothelin, and in neurodegeneration associated with
brain ischemia and aging.
Cathepsin E is the only A1 aspartic
protease that exists as a homodimer with a
disulfide bridge linking the two monomers. Like many other aspartic
proteases, it is synthesized as a
zymogen which is catalytically inactive towards its natural substrates at neutral pH and which auto-activates in an acidic environment. Here we report the crystal structure of an activation intermediate of human
cathepsin E at 2.35A resolution. The overall structure follows the general fold of aspartic
proteases of the A1 family, and the intermediate shares many features with the intermediate 2 on the proposed activation pathway of aspartic
proteases like
pepsin C and
cathepsin D. The pro-sequence is cleaved from the
protease and remains stably associated with the mature
enzyme by forming the outermost sixth strand of the interdomain beta-sheet. However, different from these other aspartic
proteases the pro-sequence of
cathepsin E remains intact after cleavage from the mature
enzyme. In addition, the active site of
cathepsin E in the crystal is occupied by N-terminal
amino acid residues of the mature
protease in the non-primed binding site and by an artificial N-terminal extension of the pro-sequence from a neighboring molecule in the primed site. The crystal structure of the
cathepsin E/pro-sequence complex, therefore, provides further insight into the activation mechanism of aspartic
proteases.