Structural Biology of the Apical Bile Acid Transporter

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Bile acids have been recognized to function as second messengers and hormones, with clinical implications in glucose, lipid and energy metabolism. The human apical sodium- dependent bile acid transporter (hASBT) 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. Two recent bacterial crystal structures claim evolutionary relatedness to hASBT, yet their topology is in clear contrast to biochemical data in the literature. The current proposal directly confronts this controversy and our plan to advance understanding of bile acid transporter structure, function and regulation is organized around the following aims: 1) Do bacterial transporter structures provide representative structural models for eukaryotic SLC10 orthologs? wherein we hypothesize that analysis of genetic ancestry and substrate phenotype of orthologs can determine evolutionary and structural relatedness of (putative) prokaryotic and eukaryotic SLC10A family members; further, we aim to express, purify, crystallize and determine the x-ray structure of eukaryotic SLC10A orthologs; 2) To characterize and map the functional regulation and modulation of hASBT expression; where we will apply a proteomics approach to determining post- translational modifications temporally. Based on our preliminary data we will further investigate the organization of hASBT in higher-order dimers or multimers as a way of controlling function. 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 (SLC) proteins in general. Additionally, it may aid future development of specific therapeutic strategies against hypercholesterolemia and related cardiovascular and metabolic diseases.