High Yield Expression of Human BACE Constructs in Eschericia coli for Refolding, Purification, and High Resolution Diffracting Crystal Forms
Abstract:BACE (β-site APP cleaving enzyme) or β-secretase, the enzyme responsible for processing APP to give the Nterminal portion of the Aβ peptide, is a membrane bound aspartyl protease consisting of an ectodomain catalytic unit, a Cterminal transmembrane segment and a cytoplasmic domain. Three BACE constructs, pET11a-BACE, pQE80L-BACE, and pQE70-BACE were designed to terminate at a position just before the transmembrane domain (Ser432) and are described schematically below.
(2) pQE80L-Met-R-G-S-(His)6-G-S-I-E-T-D-(T1QH...QTDES432), and
(3) pQE70-Met-BACE (R36GSFVEMG....PQTDES432(His)6)
Each construct was over-expressed in Escherichia coli as inclusion bodies. The inclusion body proteins were solubilized in urea and refolded by dilution in water to yield active enzyme. Maximal activity for pET11a-BACE and pQE80L-BACE was usually reached at day 3 to 4, while construct pQE70-BACE required about 21 days. Active BACE was purified to homogeneity by anion-exchange chromatography and affinity chromatography over a column of immobilized peptide inhibitor. The process, easily scalable to 60 liters of cell culture, yielded in excess of 400 mg of active enzyme for crystallographic analysis. Highly purified pET11a-BACE and pQE70-BACE formed complexes with various inhibitors, the latter protein giving crystals diffracting up to 1.45 Å resolution. In addition, a crystal form that does not require the presence of an inhibitor has been obtained for pQE70-BACE. This ligand-free crystal form has proven useful for the preparation of BACE-inhibitor complexes in soaking experiments.
Document Type: Research Article
Publication date: 2008-02-01
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