title: Extracellular disulfide bonds support scavenger receptor class B type I-mediated cholesterol transport.
abstract: Scavenger receptor class B type I (SR-BI) binds high-density lipoprotein (HDL) and mediates the selective uptake of cholesteryl esters (CE). Although the extracellular domain of SR-BI is critical for function, the structural characteristics of this region remain elusive. Using sulfhydryl labeling strategies, we report the novel finding that all six cysteine (Cys) residues in the extracellular domain of SR-BI are involved in disulfide bond formation that is intramolecular by nature. We hypothesized that an SR-BI conformation stabilized by extracellular disulfide bonds is a prerequisite for SR-BI-mediated cholesterol transport. Thus, single-Cys mutant SR-BI receptors (C251S-, C280S-, C321S-, C323S-, C334S-, and C384S-SR-BI), as well as Cys-less SR-BI, a mutant SR-BI receptor void of all Cys residues, were created, and plasma membrane localization was confirmed. Functional assays revealed that C280S-, C321S-, C323S-, and C334S-SR-BI and Cys-less SR-BI mutant receptors displayed weakened HDL binding and subsequent selective uptake of HDL-CE. However, only C323S-SR-BI and Cys-less SR-BI were unable to mediate wild-type levels of efflux of free cholesterol (FC) to HDL. None of the Cys mutations disrupted SR-BI's ability to redistribute plasma membrane FC. Taken together, the intramolecular disulfide bonds in the extracellular domain of SR-BI appear to maintain the receptor in a conformation integral to its cholesterol transport functions.
authors.affiliation: Department of Medicine, Division of Endocrinology, Metabolism and Clinical Nutrition, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, Wisconsin 53226, United States.
authors.forename: Gabriella A
authors.initials: GA
authors.lastname: Papale
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Hanson
authors.forename: Daisy
authors.initials: D
authors.lastname: Sahoo
mesh_terms: D000818:Animals; D001692:Biological Transport; D018955:CD36 Antigens / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D019556:COS Cells; D002522:Chlorocebus aethiops; D002784:Cholesterol / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D008076:Cholesterol, HDL / Q000737:chemistry / Q000378:metabolism*; D004220:Disulfides / Q000737:chemistry* / Q000378:metabolism; D004705:Endocytosis; D045604:Extracellular Fluid / Q000737:chemistry* / Q000378:metabolism; D006868:Hydrolysis; D051379:Mice; D011485:Protein Binding; D011487:Protein Conformation
chemical_list: D018955:CD36 Antigens; D008076:Cholesterol, HDL; D004220:Disulfides; C023511:cholesteryl oleyl ether; D002784:Cholesterol