Received for publication March 27, 2006.
Revised August 1, 2006.
Accepted for publication August 2, 2006.

PDZ adaptor protein PDZK2 stimulates transport activity of organic cation/carnitine transporter OCTN2 by modulating cell-surface expression

Abstract

A part of organic cation transporter families (OCT3, OCTN1 and OCTN2) have recently been identified to physically interact with PDZ (PSD95, Dlg and ZO1) domain containing proteins, although the physiological relevance of such interaction has not yet been fully examined. Here we have examined stimulatory effect of PDZK2 (also named NaPi-Cap2 and intestinal and kidney-enriched PDZ protein, IKEPP) on those cation transporters. In HEK293 cells, coexpression with PDZK2 increased the uptake of carnitine by OCTN2 with minimal effect on its substrate recognition specificity, but not for transport activity of OCT3 or OCTN1. The stimulatory effect of PDZK2 on OCTN2 was compatible with approximately two times increase in transport capacity and can be accounted for by the increase in cell-surface expression of OCTN2. Coexpression of PDZK2 did not affect carnitine transport activity of OCTN2 with deletion of the last four amino acids that were found to be important for the interaction, suggesting involvement of physical interaction of the two proteins in increase of cell-surface expression of OCTN2. In mouse kidney, colocalization of PDZK2 and OCTN2 was predominantly located in the region which was close to, but not the same as, the surface of apical membranes where OCTN2 alone was observed, suggesting the existence of OCTN2 in subapical compartment that interacts with PDZK2. The present data have thus proposed "intracellular" pool for OCTN2 that may be relevant to the stabilization of cell-surface expression of OCTN2, thereby increasing transport activity for carnitine.

Key words: absorption, organic cation transport, protein analysis, protein-protein interactions, renal transport, transporters