The BTC cytoplasmic domain contains a monoleucine-based basolateral sorting motif (EExxxL). In the top panel, the wild-type BTC cytoplasmic domain sequence is depicted with a putative basolateral-sorting motif highlighted in red. In subsequent panels, amino acid substitutions within the cytoplasmic domain are displayed. Cell-surface biotinylation was performed as described in Fig. 2. TM, triple mutant in which the monoleucine and glutamic acid residues were converted to alanine; C3, all three juxtamembrane cysteine residues were converted to alanine. Dashes on the left of western blots indicate a molecular mass of 55 kDa. (B) Quantification of western blots. % Basolateral=([BL]/[Ap]+[BL])×100. Combined results from at least three independent experiments are represented as mean±s.e.m.; *statistically significant difference (P<0.05, two-tailed unpaired t-test) compared with wild type (WT). C3/TM, combined C3 and TM mutant.