N-glycosylation controls functional activity of Oatp1, an organic anion transporter.
Rat Oatp1 (Slc21a1) is an organic anion-transporting polypeptide believed to be an anion exchanger. To characterize its mechanism of transport, Oatp1 was expressed in Saccharomyces cerevisiae under control of the GAL1 promoter. Protein was present at high levels in isolated S. cerevisiae secretory vesicles but had minimal posttranslational modifications and failed to exhibit taurocholate transport activity. Apparent molecular mass (M) of Oatp1 in yeast was similar to that of unmodified protein, approximately 62 kDa, whereas in liver plasma membranes Oatp1 has an M of approximately 85 kDa. To assess whether underglycosylation of Oatp1 in yeast suppressed functional activity, Oatp1 was expressed in Xenopus laevis oocytes with and without tunicamycin, a glycosylation inhibitor. With tunicamycin, M of Oatp1 decreased from approximately 72 to approximately 62 kDa and transport activity was nearly abolished. Mutations to four predicted N-glycosylation sites on Oatp1 (Asn to Asp at positions 62, 124, 135, and 492) revealed a cumulative effect on function of Oatp1, leading to total loss of taurocholate transport activity when all glycosylation sites were removed. M of the quadruple mutant was approximately 62 kDa, confirming that these asparagine residues are sites of glycosylation in Oatp1. Relatively little of the quadruple mutant was able to reach the plasma membrane, and most remained in unidentified intracellular compartments. In contrast, two of the triple mutants tested (N62/124/135D and N124/135/492D) were present in the plasma membrane fraction yet exhibited minimal transport activity. These results demonstrate that both membrane targeting and functional activity of Oatp1 are controlled by the extent of N-glycosylation.
Lee
Thomas K
TK
Dept. of Environmental Medicine, Univ. of Rochester School of Medicine, NY 14642, USA.
Koh
Albert S
AS
Cui
Zhifeng
Z
Pierce
Robert H
RH
Ballatori
Nazzareno
N
D000818:Animals; D001692:Biological Transport; D002462:Cell Membrane / Q000737:chemistry; D003470:Culture Media; D004970:Estrone / Q000031:analogs & derivatives* / Q000378:metabolism; D005260:Female; D005690:Galactose / Q000008:administration & dosage; D015870:Gene Expression; D006031:Glycosylation; D017997:Leukotriene C4 / Q000378:metabolism; D008099:Liver / Q000648:ultrastructure; D008970:Molecular Weight; D009154:Mutation; D009865:Oocytes / Q000378:metabolism; D029362:Organic Anion Transporters, Sodium-Independent / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D011401:Promoter Regions, Genetic; D051381:Rats; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000254:growth & development; D013329:Structure-Activity Relationship; D013656:Taurocholic Acid / Q000378:metabolism; D014162:Transfection; D014415:Tunicamycin / Q000494:pharmacology; D014982:Xenopus laevis
D003470:Culture Media; D029362:Organic Anion Transporters, Sodium-Independent; C421102:Slco1a1 protein, rat; D014415:Tunicamycin; D017997:Leukotriene C4; D004970:Estrone; D013656:Taurocholic Acid; C017296:estrone sulfate; D005690:Galactose