Human aspartic protease memapsin 2 cleaves the beta-secretase site of beta-amyloid precursor protein

The cDNAs of two new human membrane-associated aspartic proteases, memapsin 1 and memapsin 2, have been cloned and sequenced. The deduced amino acid s equences show that each contains the typical pre, pro, and aspartic proteas e regions, but each also has a C-terminal extension of over 80 residues, wh ich includes a single transmembrane domain and a C-terminal cytosolic domai n. Memapsin 2 mRNA is abundant in human brain. The protease domain of memap sin 2 cDNA was expressed in Escherichia coil and was purified. Recombinant memapsin 2 specifically hydrolyzed peptides derived from the beta-secretase site of both the wild-type and Swedish mutant beta-amyloid precursor prote in (APP) with over 60-fold increase of catalytic efficiency for the latter. Expression of APP and memapsin 2 in Hela cells showed that memapsin 2 clea ved the beta-secretase site of APP intracellularly. These and other results suggest that memapsin 2 fits all of the criteria of beta-secretase, which catalyzes the rate-limiting step of the in vivo production of the beta-amyl oid (A beta) peptide leading to the progression of Alzheimer's disease. Rec ombinant memapsin 2 also cleaved a peptide derived from the processing site of presenilin 1, albeit with poor kinetic efficiency. Alignment of cleavag e site sequences of peptides indicates that the specificity of memapsin 2 r esides mainly at the S-1' subsite, which prefers small side chains such as Ala, Ser, and Asp.