Discovery and biochemical characterization of the D-aspartyl endopeptidase activity of the serine protease LACTB

Nonenzymatic D-isomerization of aspartic acid in proteins has been observed in lesions associated with age-related diseases, including cataracts and Alzheimer's disease. Given that D-isomerization of Asp disrupts the physiological conformation of proteins, it has been postulated that D-isomerization of Asp in proteins is a key factor in the pathogenesis of age-related diseases. D-Aspartyl endopeptidase (DAEP) activity, which cleaves proteins at the carboxy terminus of D-Asp and potentially induces degradation of abnormal proteins with D-isomerized Asp, has been observed in mitochondrial fractions of mammalian tissues. However, the specific proteins responsible for mammalian DAEP activity remain unknown. In this study, we identified mitochondrial serine β-lactamase-like protein (LACTB) as the first mammalian protein with DAEP activity by structural comparison with paenidase, a bacterial DAEP. LACTB exhibited DAEP activity similar to paenidase in an in vitro assay. In addition, LACTB cleaved a 10-residue peptide derived from amyloid β1-10 containing D-Asp at position 7, which was also observed with mammalian DAEP. LACTB has previously been characterized as a tumor suppressor and as a protein whose increased expression is associated with an increased risk of Alzheimer's disease. Therefore, our findings suggest that disruption of the proteostasis of D-Asp-containing proteins may underlie the pathogenesis of these diseases.