Membrane proteins, like the K+ channel, are transported to the plasma membrane through the secretory pathway. In this study it was shown for the first time that ER export motifs, which play an important role in the transport of membrane proteins, are functional in plant cells. A diacidic ER export motif was identified in the K+ channel KAT1 from Arabidopsis thaliana. This motif was shown to be required for efficient transport of the protein to the plasma membrane. Mutation of the diacidic motif resulted in ER retention of KAT1 in Vicia faba guard cells and in HEK293 cells. Further analysis of the diacidic motif and the surrounding amino acids revealed that the diacidic motif was not only diacidic but contained at least three acid residues at position D392, D394 and E396. Patch clamp studies demonstrated that the ER export was almost completely inhibited only when all three acidic amino acids were mutated to alanine. Mutation of one or two of the three residues led to a reduction of conductance corresponding to the number of mutated residues. Confocal analysis of these mutants revealed an almost complete ER retention for K+ channels with three and two mutated acidic amino acids but a clear plasma membrane staining for mutation of only one acidic amino acid. Plasma membrane localisation of the ER export mutant of KAT1 could be rescued upon heterotetramerisation with the wildtype KAT1 channel. Therefore, ER export motifs can affect also subunit composition of K+ channel tetramers in the plasma membrane. Functional analysis of the identified ER export motif by FRET measurements demonstrated that KAT1 binds to Sec24, a subunit of COPII. The interaction of the K+ channel KAT1 with Sec24 was dependent on the acidic ER export motif. This implies that binding of the diacidic motif of KAT1 to Sec24 is essential for recruitment of the channel into COPII vesicles and efficient export out of the ER. Altogether, the experiments show that ER export signals are crucial for the regulation of ion channel density and composition. Analysis of ER export of KAT1 in plant and animal cells implies that the mechanism of ER export is highly conserved among eukaryotes.