NhaA antiporter functions using 10 helices, and an additional 2 contribute to assembly/stability

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biophysics

Background:

• The Escherichia coli Na(+)/H(+) antiporter (Ec-NhaA) is crucial for maintaining cellular ion homeostasis.
• Ec-NhaA possesses a unique 12-helix structure, including a VI-VII helical hairpin absent in related transporters.
• This hairpin is situated within the homodimer interface, alongside a beta-sheet.

Purpose of the Study:

• To investigate the functional and structural significance of the unique VI-VII helical hairpin in Ec-NhaA.
• To determine the role of this hairpin in Ec-NhaA stability, dimerization, transport activity, and pH regulation.

Main Methods:

• Computational analyses including evolutionary conservation (ConSurf) and normal mode analysis.
• Experimental characterization of truncated Ec-NhaA mutants lacking the alpha-hairpin and/or beta-sheet.
• Biophysical studies in membrane and detergent micelle environments at physiological pH.

Main Results:

• Evolutionary analysis revealed lower conservation of the VI-VII helical hairpin compared to other transmembrane regions.
• Normal mode analysis suggested similar dynamics between intact and alpha-hairpin-deleted Ec-NhaA.
• Truncated Ec-NhaA mutants retained significant transport activity and pH regulatory properties.
• Mutants exhibited defects in Ec-NhaA dimer assembly and stability.

Conclusions:

• The VI-VII helical hairpin is not essential for Ec-NhaA transport or pH regulation.
• While dispensable for core function, the hairpin contributes to the stability and proper assembly of the Ec-NhaA dimer.
• These findings provide insights into the structural plasticity and functional adaptability of Na(+)/H(+) exchangers.