期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2020
卷号：117
期号：19
页码：10313-10321
DOI：10.1073/pnas.1918604117
出版社：The National Academy of Sciences of the United States of America
摘要：The H /Ca 2 (calcium ion) antiporter (CAX) plays an important role in maintaining cellular Ca 2 homeostasis in bacteria, yeast, and plants by promoting Ca 2 efflux across the cell membranes. However, how CAX facilitates Ca 2 balance in response to dynamic cytosolic Ca 2 perturbations is unknown. Here, we identified a type of Ca 2 “mini-sensor” in YfkE, a bacterial CAX homolog from Bacillus subtilis. The mini-sensor is formed by six tandem carboxylate residues within the transmembrane (TM)5-6 loop on the intracellular membrane surface. Ca 2 binding to the mini-sensor triggers the transition of the transport mode of YfkE from a high-affinity to a low-affinity state. Molecular dynamics simulation and fluorescence resonance energy transfer analysis suggest that Ca 2 binding to the mini-sensor causes an adjacent segment, namely, the exchanger inhibitory peptide (XIP), to move toward the Ca 2 translocation pathway to interact with TM2a in an inward-open cavity. The specific interaction was demonstrated with a synthetic peptide of the XIP, which inhibits YfkE transport and interrupts conformational changes mediated by the mini-sensor. By comparing the apo and Ca 2 -bound CAX structures, we propose the following Ca 2 transport regulatory mechanism of YfkE: Ca 2 binding to the mini-sensor induces allosteric conformational changes in the Ca 2 translocation pathway via the XIP, resulting in a rearrangement of the Ca 2 -binding transport site in the midmembrane. Since the Ca 2 mini-sensor and XIP sequences are also identified in other CAX homologs and/or Ca 2 transporters, including the mammalian Na /Ca 2 exchanger (NCX), our study provides a regulatory mechanism for the Ca 2 /cation transporter superfamily.
