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Correspondence to: Kai Simons, European Molecular Biology Laboratory (EMBL), Cell Biology and Biophysics Programme, Postfach 102209, Meyerhofstrasse 1, D-69012 Heidelberg, Germany., Simons{at}EMBL-Heidelberg.de (E-mail), 49-6221-387-334 (phone), 49-6221-387-512 (fax)
Glycosyl-phosphatidylinositol (GPI)- anchored proteins are preferentially transported to the apical cell surface of polarized Madin-Darby canine kidney (MDCK) cells. It has been assumed that the GPI anchor itself acts as an apical determinant by its interaction with sphingolipid-cholesterol rafts. We modified the rat growth hormone (rGH), an unglycosylated, unpolarized secreted protein, into a GPI-anchored protein and analyzed its surface delivery in polarized MDCK cells. The addition of a GPI anchor to rGH did not lead to an increase in apical delivery of the protein. However, addition of N-glycans to GPI-anchored rGH resulted in predominant apical delivery, suggesting that N-glycans act as apical sorting signals on GPI-anchored proteins as they do on transmembrane and secretory proteins. In contrast to the GPI-anchored rGH, a transmembrane form of rGH which was not raft-associated accumulated intracellularly. Addition of N-glycans to this chimeric protein prevented intracellular accumulation and led to apical delivery.
Key Words: lipid rafts, N-glycans, GPI-anchored proteins, Madin-Darby canine kidney cells, sorting
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