The overall goal of this project is to understand the molecular basis for the function of the cholesterol homeostasis signaling protein Scap. Integrated into the endoplasmic reticulum (ER) membrane of mammalian cells, Scap binds cholesterol and undergoes ligand-gated conformational changes that modulate the maturation of SREBP transcription factors. Scap is essential for cell survival, mediates the LDL-lowering activity of the statin drugs, and may be valuable as a drug target for cardiovascular and metabolic diseases such as hepatic steatosis. Despite the biological significance of Scap, remarkably little is known about the biophysical mechanism of its cholesterol binding and conformational changes. What is Scap's three- dimensional structure, and how is it changed by cholesterol binding to modulate Scap's interaction with downstream proteins(e.g. COPII) in the SREBP pathway? To understand how cholesterol binds to Scap, we will engineer a soluble form of Scap's cholesterol binding domain (CBD) that is amenable to biophysical characterization and structure determination. Comparison of cholesterol-free and bound CBD structures will provide our first molecular insights into sterol gating of Scap function, and will guide structure-based mutagenesis and functional assays. To discover how Scap links cholesterol binding to larger-scale structural changes that propagate across the membrane and ultimately modulate SREBP trafficking, we will identify an ortholog that is biochemically stable and tractable, and solve structures of the entire transmembrane region by single-particle cryoEM and X-ray crystallography. The combination of innovative protein engineering strategies, cryoEM, and lipid-mediated crystallography methods will overcome obstacles associated with membrane proteins and provide a detailed molecular picture of this physiologically essential sterol sensor. In the future we will use the knowledge and reagents developed in this project to help discover small-molecule Scap modulators.

Public Health Relevance

This goal of this project is to understand the structure and molecular mechanism of the cholesterol homeostasis protein Scap. We will use the biophysical techniques of X-ray crystallography and single-particle electron microscopy to reveal how sterols bind to Scap and influence its structure to control downstream signaling. Understanding this mechanism will benefit public health because Scap is an essential and conserved mammalian signaling protein with significant roles in cardiovascular and metabolic physiology, as well as potential as a therapeutic target.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Biochemistry and Biophysics of Membranes Study Section (BBM)
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Nie, Zhongzhen
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University of Texas Sw Medical Center Dallas
Schools of Medicine
United States
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Clark, Lindsay D; Dikiy, Igor; Chapman, Karen et al. (2017) Ligand modulation of sidechain dynamics in a wild-type human GPCR. Elife 6:
Zhang, Yinxin; Lee, Kwang Min; Kinch, Lisa N et al. (2016) Direct Demonstration That Loop1 of Scap Binds to Loop7: A CRUCIAL EVENT IN CHOLESTEROL HOMEOSTASIS. J Biol Chem 291:12888-96
Shao, Zhenhua; Yin, Jie; Chapman, Karen et al. (2016) High-resolution crystal structure of the human CB1 cannabinoid receptor. Nature :
Lee, Jyh-Yeuan; Kinch, Lisa N; Borek, Dominika M et al. (2016) Crystal structure of the human sterol transporter ABCG5/ABCG8. Nature 533:561-4
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Clark, Lindsay; Zahm, Jacob A; Ali, Rustam et al. (2015) Methyl labeling and TROSY NMR spectroscopy of proteins expressed in the eukaryote Pichia pastoris. J Biomol NMR 62:239-45