Effects of Ala Substitution for Conserved Cys Residues in Mouse Ileal and Hepatic Na⁺-Dependent Bile Acid Transporters

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Although ileal and hepatic Na⁺-dependent bile acid transporters (SLC10A2 and SLC10A1 respectively) share structural similarities, the mutation of conserved amino acids often has distinct effects on them. We have identified two Cys residues in mouse Slc10a2 (Cys⁵¹ and Cys¹⁰⁶) the replacement of which by Ala remarkably reduces taurocholic acid (TCA) transport. Although Cys⁵¹ is conserved in Slc10a1 as Cys⁴⁴, Ala substitution gave no apparent difference in TCA uptake. Here, we further analyzed the kinetics of TCA uptake and cell surface localization of these mutants. The C51A and C106A mutants of Slc10a2 showed significantly reduced TCA uptake, while no apparent difference in TCA uptake was observed for the Slc10a1-C44A mutant. The K_m values for TCA uptake by these mutants were comparable, suggesting that these residues are not involved in the interaction with TCA.