{'fraction_non_alphanumeric': 0.039769082745349585, 'fraction_numerical': 0.019884541372674792, 'mean_word_length': 5.09375, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Ca2+ regulation in the Na+/Ca2+ exchanger features a dual electrostatic switch mechanism

Regulation of ion-transport in the Na+/Ca2+ exchanger (NCX) occurs via its cytoplasmic Ca2+-binding domains, CBD1 and CBD2. Here, we present a mechanism for NCX activation and inactivation based on data obtained using NMR, isothermal titration calorimetry (ITC) and small-angle X-ray scattering (SAXS). We initially determined the structure of the Ca2+-free form of CBD2-AD and the structure of CBD2-BD that represent the two major splice variant classes in NCX1. Although the apo-form of CBD2-AD displays partially disordered Ca2+-binding sites, those of CBD2-BD are entirely unstructured even in an excess of Ca2+. Striking differences in the electrostatic potential between the Ca2+-bound and -free forms strongly suggest that Ca2+-binding sites in CBD1 and CBD2 form electrostatic switches analogous to C2-domains. SAXS analysis of a construct containing CBD1 and CBD2 reveals a conformational change mediated by Ca2+-binding to CBD1. We propose that the electrostatic switch in CBD1 and the associated conformational change are necessary for exchanger activation. The response of the CBD1 switch to intracellular Ca2+ is influenced by the closely located cassette exons. We further propose that Ca2+-binding to CBD2 induces a second electrostatic switch, required to alleviate Na+-dependent inactivation of Na+/Ca2+ exchange. In contrast to CBD1, the electrostatic switch in CBD2 is isoform- and splice variant-specific and allows for tailored exchange activities.