Large-scale purification of human BACE expressed in mammalian cells and removal of the prosegment with HIV-1 protease to improve crystal diffraction.

BACE, or β-secretase, is an attractive target in the treatment of Alzheimers Disease because of its involvement in the generation of amyloid β peptides. BACE is a type I transmembrane aspartyl protease composed of pre-, pro-, catalytic, transmembrane and cytoplasmic domains. For the present study, the coding sequence was truncated just before the transmembrane domain and the resulting construct was extended with the C-terminal addition of a (His)6 and expressed in several mammalian host cells. The enzyme expressed in CHO cells had the best crystallographic behavior and was purified in large quantities in a three step procedure. The purified BACE was comprised of two forms, namely the full length proBACE construct beginning with Thr1, and a derivative missing the first 24 amino acids beginning with E25. These BACE precursors co-crystallized in the presence of inhibitors yielding structures to 3.2 A resolution. HIV-1 protease treatment of this mixture resulted in complete cleavage of the F39-V40 bond, leaving the V40EM … ES432 (His)6 derivative that was purified yielding an enzyme that was no more active than untreated BACE but co-crystallized with inhibitors producing well shaped, bipyramidal co-crystals diffracting to 2.6 A resolution.