Importance of uncharged polar residues and proline in the proximal two-thirds (Pro107–Ser128) of the highly conserved region of mouse ileal Na+-dependent bile acid transporter, Slc10a2, in transport activity and cellular expression

To elucidate the functional importance of this region, we previously performed mutational analysis of the uncharged polar residues and proline in the distal one-third (Thr130–Pro142) of the highly conserved region in mouse Slc10a2. In this study, proline and uncharged polar residues in the remaining two-thirds of this region in mouse Slc10a2 were subjected to mutational analysis, and taurocholic acid uptake and cell surface localization were examined. Cell surface localization of Slc10a2 is necessary for bile acid absorption. Mutants in which Asp or Leu were substituted for Pro107 (P107N or P107L) were abundantly expressed, but their cell surface localization was impaired. The S126A mutant was completely impaired in cellular expression. The T110A and S128A mutants exhibited remarkably enhanced membrane expression. The S112A mutant was properly expressed at the cell surface but transport activity was completely lost. Replacement of Tyr117 with various amino acids resulted in reduced transport activity. The degree of reduction roughly depended on the van der Waals volume of the side chains.The functional importance of proline and uncharged polar residues in the highly conserved region of mouse Slc10a2 was determined. This information will contribute to the design of bile acid-conjugated prodrugs for efficient drug delivery or SLC10A2 inhibitors for hypercholesterolemia treatment.Bile acids are synthesized from cholesterol in the liver and secreted into the small intestine as components of bile for the digestion and absorption of lipids and lipid-soluble vitamins. In addition to the detergent action of bile acids, which aids in the digestion and absorption of lipid and lipid-soluble nutrients by forming micelles with biliary phospholipids and cholesterol, bile acids are now appreciated as signaling molecules that control lipid metabolism and energy production [1-6]. At the distal end of the ileum, 95%–98% of bile acids are effectively reabsorbed by an ileal sodium-depe