Structure of an integral membrane sterol reductase from Methylomicrobium alcaliphilum

摘要

固醇见于动物、植物、真菌和一些原核生物,在其中它们发挥一系列不同的生物功能。动物中最丰富的固醇是胆固醇,它可帮助维持质膜的强度和可滲透性。胆固醇的合成需要几十种"膜内在酶",而其中只有少数几种的结构被确定。在这篇论文中,李晓春(音译)等人以2.7A的分辨率报告了来自嗜甲烷细菌Methylomicrobium alcaliphilum的一种△~(14)-固醇还原酶的X-射线晶体结构。这种酶是人类C14SR和DHCR7(还原胆固醇生物合成路径中特定碳一碳双键的酶)的一种同源物。它的结构显示了两个相互连接的"袋"：一个面朝向胞质、含有NADPH结合袋；另一个含有面朝向"脂质双层"的一个空腔,该空腔可能含有'固醇结合袋。对这种酶结构进行分析,可以为了解DHCR7和LBR发生的特定突变何以会导致人类疾病提供一些线索。%Sterols are essential biological molecules in the majority of life forms. Sterol reductases including △~(14)-sterol reductase (C14SR, also known as TM7SF2), 7-dehydrocholesterol reductase (DHCR7) and 24-dehydrocholesterol reductase (DHCR24) reduce specific carbon-carbon double bonds of the sterol moiety using a reducing cofactor during sterol biosynthesis. Lamin B receptor (LBR), an integral inner nuclear membrane protein, also contains a functional C14SR domain. Here we report the crystal structure of a △~(14)-sterol reductase (MaSR1) from the methanotrophic bacterium Methylomicrobium alcaiiphilum 20Z (a homologue of human C14SR, LBR and DHCR7) with the cofactor NADPH. The enzyme contains ten transmembrane segments (TM1-10). Its catalytic domain comprises the carboxy-terminal half (containing TM6-10) and envelops two interconnected pockets, one of which faces the cytoplasm and houses NADPH, while the other one is accessible from the lipid bilayer. Comparison with a soluble steroid 5β-reductase structure suggests that the reducing end of NADPH meets the sterol substrate at the juncture of the two pockets. A sterol reductase activity assay proves that MaSRl can reduce the double bond of a cholesterol biosynthetic intermediate, demonstrating functional conservation to human C14SR. Therefore, our structure as a prototype of integral membrane sterol reductases provides molecular insight into mutations in DHCR7 and LBR for inborn human diseases.