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A more recent version of this article appeared on May 1, 2007
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Submitted on July 24, 2006
Revised on January 16, 2007
Accepted on February 16, 2007
*Department of Internal Medicine, National Taiwan University Hospital and National Taiwan University College of Medicine, Taipei, 100 Taiwan;
Department of Anatomy and Cell Biology, National Taiwan University College of Medicine, Taipei, 100 Taiwan;
Epithelial Pathobiology, Department of Surgery, University of Cincinnati College of Medicine, Cincinnati, OH 45267
Monitoring Editor: Howard Riezman
Podocalyxin/Gp135 was recently demonstrated to participate in the formation of a pre-apical complex to set up initial polarity in MDCK cells, a function presumably depending on the apical targeting of Gp135. We show that correct apical sorting of Gp135 depends on a bipartite signal composed of an extracellular O-glycosylation rich region and the intracellular PDZ domain binding motif. The function of this PDZ binding motif could be substituted with a fusion construct of Gp135 with Ezrin binding phosphoprotein 50 (EBP50). In accordance with this observation, EBP50 binds to newly synthesized Gp135 at Golgi apparatus, and facilitates oligomerization and sorting of Gp135 into clustering complex. Defective connection between Gp135 and EBP50 or EBP50 knock-down results in a delayed exit from the detergent resistant microdomain, failure of oligomerization, and basolateral missorting of Gp135. Furthermore, the basolaterally missorted EBP50 binding defective mutant of Gp135 was rapidly retrieved via a PKC-dependent mechanism. According to these findings, we propose a model by which a highly negative charged transmembrane protein could be packed into an apical sorting platform with the aid of its cytoplasmic partner EBP50.
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