Structural dynamics of Na+ and Ca 1 2+ interactions with full-size mammalian NCX

NCX proteins (Na+/Ca2+ exchangers) are allosterically regulated by cytosolic Ca2+ and Na+ to feedback ion signaling in diverse cell types. The recently discovered cryo-EM structure of the cardiac NCX1.1 provided breakthrough information on the mammalian NCX structure, although the structure-dynamic basis of allosteric regulation remains unresolved. Here, we analyzed the apo, Ca2+-, and Na+ -bound species of the brain NCX1.4 variant using hydrogen-deuterium exchange mass spectrometry (HDX-MS) in conjunction with molecular dynamics (MD) simulations. We show that Ca2+ binding to the regulatory CBD domains dramatically rigidifies the intracellular regulatory loop (5L6) and promotes its interaction with the membrane. Either Na+ or Ca2+ stabilizes the intracellular portions of TM3, TM4, TM9, TM10, and their connecting loops (3L4 and 9L10), exposing a putative cation binding site for allosteric regulation. Either Ca2+ or Na+ rigidifies TMH2, encompassing the palmitoylation domain, and the neighboring two-helix TM1/TM6 bundle(governing the alternating access transitions) to control the ion transport rates. Collectively, our results uncovered important details of allosteric regulation in mammalian NCX, while highlighting structure-dynamic features of functional regions not resolved in the cryo-EM structure. The present outcomes provide useful clues toward resolving the structure-dynamic determinants of regulatory diversity in NCX isoform/splice variants.