The apical sodium-dependent bile acid transporter (ASBT) plays a key role in the enterohepatic recycling of bile salts, cholesterol homeostasis, and serves as a molecular target for hypercholesterolemic agents and pharmaceutical prodrug strategies. Despite its clinical significance, ASBT is poorly characterized at the molecular level. The proposed research will focus on the structural biology of ASBT. Using a novel approach that combines molecular and computational biology our long-term goal is to delineate the three-dimensional structure, ligand-binding domains, and cellular transport mechanism of ASBT. The following specific aims will be addressed: 1) To determine amino acids in critical protein domains that participate in substrate and sodium binding and translocation;we will use a combination of site-directed thiol modification, second-site suppressor mutagenesis and kinetic analysis of key mutants to address the hypothesis that amino acids lining the hydrophilic cleft of ASBT participate in substrate translocation. 2) To determine the structural organization and helical packing of ASBT transmembrane domains;here, we will use bifunctional chemical cross-linking reagents to study helical proximity and orientation in double cysteine mutant constructs;furthermore, we aim to determine the organization of ASBT in functional multimeric states. 3) To employ molecular dynamics simulations to refine the homology model of ASBT and probe conformations and conformational changes associated with the transport process;these studies will use the distance constraints obtained in aim 2 to refine our homology model of ASBT protein, which will be simulated in a fully solvated lipid bilayer system. Information gained by these studies will significantly increase our understanding of the structural interactions that drive bile acid transport and further our structural knowledge of solute carrier proteins in general. Additionally, it may aid future development of specific therapeutic strategies against hypercholesterolemia and related cardiovascular diseases.
Bile acids play an invaluable role in the intestinal absorption of food-derived lipids and lipid-soluble vitamins and drugs. The human bile acid pool is efficiently conserved through recirculation by bile acid transporters expressed in the distal ileum and the liver. Fecal loss of bile acids is compensated by de novo synthesis in the liver from its precursor, cholesterol;thus, bile acid transporters play an intricate role in cholesterol catabolism and they may be used as a target for anti-hypercholesterolemic drugs. Furthermore, the intestinal bile acid transporter, ASBT, may be exploited as a drug delivery target for poorly permeable therapeutics. The present proposal builds upon our previous work that helped us identify key residues for ASBT that play a role in drug- protein interactions;this, in turn allowed the development of a functional three- dimensional model for human ASBT which can be used in the rational design of novel therapeutics aimed at lowering plasma cholesterol levels or prodrugs designed for enhanced intestinal permeability.
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