Sterols are essential biologicalmolecules in themajority of life forms. Sterol reductases including δ 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 δ 14-sterol reductase (MaSR1) from the methanotrophic bacterium Methylomicrobium alcaliphilum 20Z (a homologue of human C14SR, LBR and DHCR7) with the cofactorNADPH. The enzymecontains ten transmembrane segments (TM1-10). Its catalytic domain comprises the carboxy-terminal half (containingTM6-10) 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 structure3 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 MaSR1 can reduce the double bond of a cholesterol biosynthetic intermediate, demonstrating functional conservation to humanC14SR. Therefore, our structure as a prototype of integral membrane sterol reductases providesmolecular insight intomutations inDHCR7 and LBR for inborn human diseases.
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