Hierarchy of mechanisms involved in generating Na/K-ATPase polarity in MDCK epithelial cells.

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Abstract
We have studied mechanisms involved in generating a polarized distribution of Na/K-ATPase in the basal-lateral membrane of two clones of MDCK II cells. Both clones exhibit polarized distributions of marker proteins of the apical and basal-lateral membranes, including Na/K-ATPase, at steady state. Newly synthesized Na/K-ATPase, however, is delivered from the Golgi complex to both apical and basal-lateral membranes of one clone (II/J), and to the basal-lateral membrane of the other clone (II/G); Na/K-ATPase is selectively retained in the basal-lateral membrane resulting in the generation of complete cell surface polarity in both clones. Another basal-lateral membrane protein, E-cadherin, is sorted to the basal-lateral membrane in both MDCK clones, demonstrating that there is not a general sorting defect for basal-lateral membrane proteins in clone II/J cells. A glycosyl-phosphatidylinositol (GPI)-anchored protein (GP-2) and a glycosphingolipid (glucosylceramide, GlcCer) are preferentially transported to the apical membrane in clone II/G cells, but, in clone II/J cells, GP-2 and GlcCer are delivered equally to both apical and basal-lateral membranes, similar to Na/K-ATPase. To examine this apparent inter-relationship between sorting of GlcCer, GP-2 and Na/K-ATPase, sphingolipid synthesis was inhibited in clone II/G cells with the fungal metabolite, Fumonisin B1 (FB1). In the presence of FB1, GP-2 and Na/K-ATPase are delivered to both apical and basal-lateral membranes, similar to clone II/J cells; FB1 had no effect on sorting of E-cadherin to the basal-lateral membrane of II/G cells. Addition of exogenous ceramide, to circumvent the FB1 block, restored GP-2 and Na/K-ATPase sorting to the apical and basal-lateral membranes, respectively. These results show that the generation of complete cell surface polarity of Na/K-ATPase involves a hierarchy of sorting mechanisms in the Golgi complex and plasma membrane, and that Na/K-ATPase sorting in the Golgi complex of MDCK cells may be regulated by exclusion from an apical pathway(s). These results also provide new insights into sorting pathways for other apical and basal-lateral membrane proteins.

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