Membrane Topology of Human ASBT (SLC10A2) Determined by Dual Label Epitope Insertion Scanning Mutagenesis. New Evidence for Seven Transmembrane Domains

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Abstract
The membrane topology of the human apical sodium-dependent bile acid transporter (hASBT) remains unresolved. Whereas N-glycosylation analysis favors a 7 transmembrane (TM) model, membrane insertion scanning supports a 9TM topology. In order to resolve this controversy, we used dual label epitope insertion to systematically examine the topological framework of hASBT. Two distinct epitopes, hemagglutinin (HA) and FLAG, were individually inserted by inverted PCR mutagenesis at strategic positions along the hASBT sequence. Cell surface biotinylation and immunoblotting with epitope-specific and anti-hASBT antibodies confirmed expression and trafficking of the mutants to the plasma membrane. Confocal microscopy confirmed membrane localization of epitope-tagged hASBT in saponin-treated (permeabilized) and nonpermeabilized transfected COS-1 and MDCK cells. Tags at positions 116, 120, 186, 270, and 284 were accessible to the epitope antibodies in nonpermeabilized cells, indicative of the extracellular localization of loops 1 (99−130), 2 (180−191), and 3 (253−287). The corresponding positions in the 9TM model were predicted to be intracellular or membrane bound. Epitope mutants at residues 56, 92, 156, and 221 were only detected after treatment with saponin, indicating the intracellular localizations of loops 1 (50−73), 2 (150−160), 3 (215−227) as predicted by a 7TM model. Our results also confirm the exofacial and cytosolic localization of N- and C-terminal tails, respectively. With the exception of constructs inserted at position 120, epitope mutants displayed active, sodium-dependent taurocholate uptake. Consequently, our study strongly supports a 7TM topology for hASBT and refutes the previously proposed 9TM model.

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