title: Structure of an integral membrane sterol reductase from Methylomicrobium alcaliphilum.
abstract: Sterols are essential biological molecules in the majority 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 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 MaSR1 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.
authors.affiliation: Laboratory of Cell Biology, Howard Hughes Medical Institute, The Rockefeller University, New York, New York 10065, USA.
authors.forename: Xiaochun
authors.initials: X
authors.lastname: Li
authors.affiliation: Department of Experimental Medicine, University of Perugia, Perugia 06132, Italy.
authors.forename: Rita
authors.initials: R
authors.lastname: Roberti
authors.affiliation: Laboratory of Cell Biology, Howard Hughes Medical Institute, The Rockefeller University, New York, New York 10065, USA.
authors.forename: Günter
authors.initials: G
authors.lastname: Blobel
mesh_terms: D001665:Binding Sites; D020134:Catalytic Domain; D002462:Cell Membrane / Q000378:metabolism*; D002784:Cholesterol / Q000096:biosynthesis; D018360:Crystallography, X-Ray; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism; D008772:Methylococcaceae / Q000201:enzymology*; D008958:Models, Molecular; D009249:NADP / Q000737:chemistry / Q000378:metabolism; D010088:Oxidoreductases / Q000737:chemistry* / Q000378:metabolism*; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry / Q000235:genetics; D013261:Sterols / Q000378:metabolism; D000097682:Lamin B Receptor
chemical_list: D002784:Cholesterol; D008565:Membrane Proteins; D009249:NADP; D010088:Oxidoreductases; D044925:Oxidoreductases Acting on CH-CH Group Donors; D018160:Receptors, Cytoplasmic and Nuclear; D013261:Sterols; D000097682:Lamin B Receptor; C019995:7-dehydrocholesterol reductase; C113387:TM7SF2 protein, human; C022672:delta(14)-sterol reductase