Basic and editing mechanisms underlying ion transport and regulation in NCX variants

Structure-dynamic analysis of archaeal NCX (NCX_Mj) provided new insights into the underlying mechanisms of ion selectivity, ion-coupled alternating access, ion occlusion, and transport catalysis. This knowledge is relevant, not only for prokaryotic and eukaryotic NCXs, but also for other families belonging to the superfamily of Ca²⁺/CA antiporters. In parallel with the ion transport mechanisms, the structure-dynamic determinants of regulatory CBD1 and CBD2 domains have been resolved according to which the Ca²⁺-induced allosteric signal is decoded at the two-domain interface and "secondarily" modified by a splicing segment at CBD2. The exon-dependent combinations within the splicing segment control the number of Ca²⁺ binding sites (from zero to three) at CBD2, as well as the Ca²⁺ binding affinity and Ca²⁺ off-rates at both CBDs. The exon-dependent combinations specifically rigidify the local segments at CBDs, yielding the Ca²⁺-dependent activation (through Ca²⁺ binding to CBD1) and Ca²⁺-dependent alleviation of Na⁺-induced inactivation (through Ca²⁺ binding with CBD2). The exon-dependent synergistic interactions between CBDs characteristically differ in NCX1 and NCX3, thereby underscoring the physiological relevance of structure-controlled shaping of ion-dependent regulation in tissue-specific NCX variants. How the ion-dependent regulatory modules operate in conjunction with other regulators (PIP₂, palmitoylation, XIP, among the others) of NCX is an open question that remains to be determined.