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Published on September 1, 2006; 10.1104/pp.106.087064

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Received July 24, 2006
Accepted August 29, 2006

Di-acidic motif is required for efficient transport of the K+ channel KAT1 to the plasma membrane

Melanie Mikosch , Annette C. Hurst , Brigitte Hertel , and Ulrike Homann *

Institute of Botany, Darmstadt University of Technology, 64287 Darmstadt, Germany

* Corresponding author; email: homann-u{at}bio.tu-darmstadt.de.

For a number of mammalian ion channels trafficking to the plasma membrane was found to be controlled by intrinsic sequence motifs. Among these sequences are di-acidic motifs that function as endoplasmic reticulum (ER) export signals. So far it is unclear if similar motifs also exist in plant ion channels. In this study we analyzed the function of four di-acidic DXE/DXD motifs of the plant K+ channel KAT1. Mutation of the first di-acidic DXE motif resulted in a strong reduction of the KAT1 conductance in both guard cell protoplasts and HEK293 cells (human embryonic kidney cells). Confocal fluorescence microscopy of guard cells expressing the mutated KAT1 fused to green fluorescent protein revealed localization of the mutated channel only in intracellular structures around the nucleus. These structures could be identified as part of the ER via co-expression of KAT1 fused to yellow fluorescent protein with an ER retained protein (HDEL) fused to cyan fluorescent protein. Block of vesicle formation from the ER by overexpression of the small GTP-binding protein Sar1 fixed in its GDP bound form led to retention of wild type KAT1 in similar parts of the ER. Mutation of the three other di-acidic motifs had no effect.

Together the results demonstrate that one di-acidc motif of KAT1 is essential for ER export of the functional channel in both, guard cell protoplasts and HEK293 cells. This suggests that trafficking of plant plasma membrane ion channels is controlled via a conserved mechanism.




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