ER Export of KAT1 is Correlated to the Number of Acidic Residues within a Triacidic Motif

For a number of ion channels, including the potassium (K⁺) inward rectifying channel from Arabidopsis thaliana (KAT1), diacidic endoplasmic reticulum (ER) export motifs have been identified. These motifs consist of two acidic amino acids (aspartate (D) and/or glutamate (E)) separated by any amino acid. To specify the role of single acidic amino acids for efficiency of ER export, we analysed a sequence of KAT1 that included the originally identified diacidic ER export motif (DxE) plus an additional D just upstream of the diacidic motif. Analysis of single, double and triple mutations of the acidic amino acids of the DxDxE motif revealed a gradual reduction of ER export depending on the number of mutated acidic residues. The amount of reduction in ER export was not related to the position, but only to the number of mutated acidic amino acids. These results show that a triacidic motif is essential for efficient ER export of KAT1. Function of the triacidic motif probably involves cooperative binding to Sec24.