Cellular localization of the K + ‐dependent Na + –Ca 2+ exchanger NCKX 5 and the role of the cytoplasmic loop in its distribution in pigmented cells

NCKX 5 is a bidirectional K + ‐dependent Na + –Ca 2+ exchanger, which belongs to the SLC 24A gene family. In particular, the A111T mutation of NCKX 5 has been associated with reduced pigmentation in European populations. In contrast to other NCKX isoforms, which function in the plasma membrane ( PM ), NCKX 5 has been shown to localize either in the trans‐Golgi network ( TGN ) or in melanosomes. Moreover, sequences responsible for retaining its intracellular localization are unknown. This study addresses two major questions: (i) clarification of intracellular location of NCKX5 and (ii) identification of sequences that retain NCKX5 inside the cell. We designed a set of cDNA constructs representing NCKX 5 loop deletion mutants and NCKX 2– NCKX 5 chimeras to address these two questions after expression in pigmented MNT 1 cells. Our results show that NCKX 5 is not a PM resident and is exclusively located in the TGN . Moreover, the large cytoplasmic loop is the determinant for retaining NCKX 5 in the TGN .