Regulation of K-dependent Na/Ca-exchanger subtype 4, NCKX4, by palmitoylation.

Mammalian K-dependent Na/Ca exchangers (NCKX), encoded by the SLC24 gene family, are crucial for maintaining Ca homeostasis. NCKX4, widely expressed in the brain and sensory neurons, plays a key role in neuronal satiety and enamel formation. Despite its importance, the regulatory mechanisms of NCKX4 remain largely unexplored. This study investigates how palmitoylation, a post-translational modification affecting membrane proteins, regulates NCKX4 and influences its cellular localization and function. Using Acyl-RAC and palmitate-based click-chemistry, we found that approximately 14% of NCKX4 is palmitoylated at steady-state in both endogenous and transfected systems. The level of this modification is highly dynamic, being regulated by inhibitors of palmitoylation (2-bromopalmitate) and depalmitoylation (palmostatin B), resulting in greater than a two-fold decrease or increase, respectively. Site-directed mutagenesis of six cysteine residues revealed two key sites (Cys118 and Cys425) critical for NCKX4 palmitoylation. The subcellular distribution of palmitoylated NCKX4 was examined via proximity ligation and click-chemistry. NCKX4 was found across multiple membrane compartments, with a higher fraction localizing to the plasma membrane when palmitoylation was inhibited by 2-bromopalmitate. However, a Ca imaging assay in HEK293T cells showed no significant change in aggregate cellular NCKX4-mediated Ca transport upon modulation of palmitoylation status. These data suggest palmitoylation promotes internalization of the NCKX4 protein while also activating it, counter-acting effects that result in unchanged NCKX4-mediated cellular Ca transport activity. In summary, NCKX4 is subject to dynamic palmitoylation, which influences both distribution across cellular compartments and intrinsic Ca transport activity. These findings contribute to our understanding of the regulation and functional roles of NCKX4 in cellular physiology.