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NhaA: A Unique Structural Fold of Secondary Active Transporters

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Michel,  Hartmut       
Department of Molecular Membrane Biology, Max Planck Institute of Biophysics, Max Planck Society;

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Citation

Padan, E., & Michel, H. (2015). NhaA: A Unique Structural Fold of Secondary Active Transporters. Israel Journal of Chemistry, 55(11/12), 1233-1239. doi:10.1002/ijch.201500044.


Cite as: https://hdl.handle.net/11858/00-001M-0000-002A-4114-B
Abstract
The Na+/H+ antiporters transport sodium (or several other monovalent cations) in exchange for H+ across lipid bilayers in all kingdoms of life. They are critical in the pH homeostasis of cytoplasm and/or organelles. In humans, these proteins are associated with the pathophysiology of various diseases. Yet, the most extensively studied Na+/H+ antiporter is Ec-NhaA, the main Na+/H+ antiporter of Escherichia coli. The crystal structure of an inactive, down-regulated Ec-NhaA, determined at acidic pH, has provided the first structural insights into the antiport mechanism and pH regulation of an Na+/H+ antiporter. It reveals a unique structural fold (called the NhaA fold) in which helical transmembrane segments (TMs) are organized in inverted-topology repeats, including two antiparallel non-helical extended chain regions that cross each other, forming a delicate electrostatic balance in the middle of the membrane. The NhaA fold contributes to the cation binding site and facilitates rapid conformational changes of Ec-NhaA. The NhaA fold has now been recognized to be shared by four Na+/H+ antiporters (bacterial and archaeal) and two Na+ symporters. Thus far, no crystal structure of any of the human Na+/H+ antiporters has been determined. Nevertheless, the Ec-NhaA crystal structure has enabled the structural modeling of NHE1, NHE9, and NHA2, which are involved in human pathophysiology. Future elucidation of structure-function relationships of eukaryotic and prokaryotic Na+/H+ antiporters are likely to provide insights into the human pathophysiology. Here, we will focus on the NhaA structural fold which underpins the antiporter functionality of Ec-NhaA and the secondary active transporters.