Band 3 variants occur rather frequently in different populations. Based on sodium dodecyl sulfate (SDS)-polyacrylamide electrophoretic properties, a widespread polymorphism (band 3 Memphis) has been previously described. It corresponds to a protein that has been hypothesized to be elongated in its N-terminal cytoplasmic domain. Band 3 from a heterozygote subject for this polymorphism and that displays a normal reactivity towards stilbene disulfonates has been isolated and its primary structure determined by protein chemistry. Reverse-phase high performance liquid chromatography tryptic peptide mapping showed, as the only difference with controls, that the enzymatic cleavage between the two N-terminal peptides did not occur, yielding a 69 residue-long fragment. Further cleavages of this peptide (cyanogen bromide, V8 protease), amino acid composition, and sequence analyses demonstrated that the lysine at position 56 was replaced by a glutamic acid. Thus, surprisingly, a single amino acid change is responsible for the large difference in the electrophoretic behavior. This result suggests that single amino acid substitutions may similarly be involved in the structural modification of several other protein variants, described as elongated or shortened based only on SDS-polyacrylamide electrophoresis studies. When deletions/insertions were confirmed by sequence analysis, their extent was often different from that expected from electrophoresis.
ABCB4 (ATP-binding cassette subfamily B member 4) is a hepatocanalicular floppase involved in biliary phosphatidylcholine (PC) secretion. Variations in the ABCB4 gene give rise to several biliary diseases, including progressive familial intrahepatic cholestasis type 3 (PFIC3), an autosomal recessive disease that can be lethal in the absence of liver transplantation. In this study, we investigated the effect and potential rescue of ten ABCB4 missense variations in NBD1:NBD2 homologous positions (Y403H/Y1043H, K435M/K1075M, E558K/E1200A, D564G/D1206G and H589Y/H1231Y) all localized at the conserved and functionally critical motifs of ABC transporters, six of which are mutated in patients. By combining structure analysis and in vitro studies, we found that all ten mutants were normally processed and localized at the canalicular membrane of HepG2 cells, but showed dramatically impaired PC transport activity that was significantly rescued by treatment with the clinically approved CFTR potentiator ivacaftor. Our results provide evidence that functional ABCB4 mutations are rescued by ivacaftor, paving the way for the repositioning of this potentiator for the treatment of selected patients with PFIC3 caused by mutations in the ATP-binding sites of ABCB4.
Glycosylation was considered the major signal candidate for apical targeting of transmembrane proteins in polarized epithelial cells. However, direct demonstration of the role of glycosylation has proved difficult because non-glycosylated apical transmembrane proteins usually do not reach the cell surface. Here we were able to follow the targeting of the apical transmembrane glycoprotein NPP3 both when glycosylated and non-glycosylated. Transfected in polarized MDCK and Caco-2 cells, NPP3 was exclusively expressed at the apical membrane. The transport kinetics of the protein to the cell surface were studied after metabolic (35)S-labeling and surface immunoprecipitation. The newly synthesized protein was mainly targeted directly to the apical surface in MDCK cells, whereas 50% transited through the basolateral surface in Caco-2 cells. In both cell types, the basolaterally targeted pool was effectively transcytosed to the apical surface. In the presence of tunicamycin, NPP3 was not N-glycosylated. The non-glycosylated protein was partially retained intracellularly but the fraction that reached the cell surface was nevertheless predominantly targeted apically. However, transcytosis of the non-glycosylated protein was partially impaired in MDCK cells. These results provide direct evidence that glycosylation cannot be considered an apical targeting signal for NPP3, although glycosylation is necessary for correct trafficking of the protein to the cell surface.
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