Release of an N-terminal aspartate or glutamate from a peptide, with a preference for aspartate. Binds 2 Zn(2+) ions per subunit. Inhibited by zinc. Stimulated by calcium and bacitracin. Optimum pH is 7.5. Optimum temperature is 50 degrees Celsius. Tetrahedron-shaped homododecamer built from six homodimers. The N-terminus is blocked. Belongs to the peptidase M18 family.