人β-分泌酶 (BACE1) 在毕赤酵母中分泌表达及纯化

Expression and purification of human β-secretase (BACE1) in Pichia pastoris

摘要:

为了获得活性良好的重组人β-分泌酶 (β-secreatase, BACE1)，用于研究其与抑制剂的作用模式，构建了携带β-分泌酶proBACE1和BACE1编码序列的重组表达质粒pPIC9K-MetBACE22和pPIC9K-MetBACE46，通过电击法转入毕赤酵母GS115中，分别得到重组子9k-B22和9k-B46。重组菌株在诱导表达培养基中诱导外源基因表达，结果显示9k-B22的上清活性明显高于9k-B46的上清活性。9k-B22表达上清浓缩后经HisTrap亲和柱纯化得到的蛋白具有良好的BACE1活性, SDS-PAGE/高碘酸-希夫试剂染色发现其为糖蛋白，并且其糖基侧链可以被Endo Hf完全切除，得到50 kDa左右的两条蛋白带。肽质量指纹图谱鉴定发现，这两个蛋白分别与proBACE1和BACE1匹配。活性检测发现糖基化BACE1和去糖基化BACE1的活性均低于HEK-293细胞表达的商品BACE1，这说明糖基化及其类型对BACE1的活性非常重要。然而已知的BACE1抑制剂对三者的抑制率无显著差异，这说明糖基化并不影响与抑制剂的相互作用。经过一系列培养条件优化BACE1纯化产量提高到1 mg/L，这为发现并优化BACE1新型抑制剂的相关研究奠定了物质基础。

Abstract:

To generate active recombinant human β-secreatase (BACE1) for studying its interaction with its inhibitors, we constructed two recombinant plasmids, pPIC9K-MetBACE22 (bearing pro-bace1 gene) and pPIC9K-MetBACE46 (bearing bace1 gene). These two plasmids were then transformed into Pichia pastoris GS115 by electroporation to obtain the recombinant strains 9k-B22 and 9k-B46. After induction in buffered methanol complex medium, we found the supernatant activity of 9k-B22 significantly higher than that of 9k-B46. The culture filtrate of 9k-B22 was concentrated, and then purified by HisTrap affinity column. The purified proteins, showing good BACE1 protease activity, were found to be a mixture of glycoproteins because they can be stained by periodic acid-Schiff reagent. After this mixture was treated with Endo Hf (a recombinant protein of endoglycosidase H), we found two new adjacent bands around 50 kDa on SDS-PAGE. These two bands were cut and subjected to peptide mass fingerprint analysis, and identified as proBACE1 and BACE1 proteins. Enzyme assays revealed that the activities of both BACE1 proteins in glycosylated and deglycosylated form were lower than that of commercial BACE1 (expressed in HEK-293), inferring glycosylation and the type of glycosylation are crucial to the activity. However, we found no apparent difference in the inhibition of those all above three enzyme forms by one known BACE1 inhibitor. This observation demonstrated that the glycosylation of BACE1 by Pichia pastoris does not affect its interaction with this inhibitor. After optimization of culture conditions, the production of BACE1 in Pichia pastoris was enhanced to about 1 mg/L. This work enables us to further investigate the interaction of BACE1 and its inhibitors, and assists in discovering and optimizing BACE1 inhibitors as anti-Alzheimer’s disease agents.