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Protein and Vesicle Trafficking, Cytoskeleton

Shank2E binds NaP_i cotransporter at the apical membrane of proximal tubule cells

¹Department of Medicine, University of Colorado Health Sciences Center, Denver, Colorado; and ²Department of Biological Sciences, Korea Advanced Institute of Science and Technology, Daelon, Korea

Submitted 23 November 2004 ; accepted in final form 16 May 2005

Proteins expressing postsynaptic density (PSD)-95/Drosophila disk large (Dlg)/zonula occludens-1 (ZO-1) (PDZ) domains are commonly involved in moderating receptor, channel, and transporter activities at the plasma membrane in a variety of cell types. At the apical membrane of renal proximal tubules (PT), the type IIa NaP_i cotransporter (NaP_i-IIa) binds specific PDZ domain proteins. Shank2E is a spliceoform of a family of PDZ proteins that is concentrated at the apical domain of liver and pancreatic epithelial cell types and is expressed in kidney. In the present study, immunoblotting of enriched plasma membrane fractions and immunohistology found Shank2E concentrated at the brush border membrane of rat PT cells. Confocal localization of Flag-Shank2E and enhanced green fluorescent protein-NaP_i-IIa in cotransfected OK cells showed these proteins colocalized in the apical microvilli of this PT cell model. Shank2E coimmunoprecipitated with NaP_i-IIa from rat renal cortex tissue and HA-NaP_i-IIa coprecipitated with Flag-Shank2E in cotransfected human embryonic kidney HEK cells. Domain analysis showed that the PDZ domain of Shank2E specifically bound NaP_i-IIa and truncation of the COOH-terminal TRL motif from NaP_i-IIa abolished this binding, and Far Western blotting showed that the Shank2E- NaP_i-IIa interaction occurred directly between the two proteins. NaP_i-IIa activity is regulated by moderating its abundance in the apical membrane. High-P_i conditions induce NaP_i-IIa internalization and degradation. In both rat kidney PT cells and OK cells, shifting to high-P_i conditions induced an acute internal redistribution of Shank2E and, in OK cells, a significant degree of degradation. In sum, Shank2E is concentrated in the apical domain of renal PT cells, specifically binds NaP_i-IIa via PDZ interactions, and undergoes P_i-induced internalization.

PDZ domains; endocytosis; degradation; epithelia

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