Intermolecular cross-linking of monomers in Helicobacter pylori Na+/H+ antiporter NhaA at the dimer interface inhibits antiporter activity

Functional characterization of the NhaA Na+/H+ antiporter from the green picoalga Ostreococcus tauri

Transmembrane ion transport is a critical process in the cellular response to salt stress. Among the known functional membrane transporters that are involved in the salt stress response, Na⁺/H⁺ antiporters have been extensively studied. These ubiquitous membrane proteins are crucial for salt tolerance and are associated with the regulation of internal pH, cell volume, morphogenesis, and vesicular trafficking. Molecular and functional analyses of Na⁺/H⁺ antiporters have been characterized among taxa but little is known about algal Na⁺/H⁺ antiporters. Here, we analyzed putative Na⁺/H⁺ antiporters from the complete genome sequence of the marine picoalga Ostreococcus tauri. At least 10 putative Na⁺/H⁺ antiporters belonging to the SOS1, NHX, and KEA/Kef families were found. Surprisingly, a bacterial type NhaA sequence (OtNhaA) was also found. Topological modeling of OtNhaA predicted 12 possible transmembrane segments with a long N-terminus. The full-length (FL_OtNhaA) and N-terminal truncated (ΔN112_OtNhaA) versions of OtNhaA were constructed, expressed in the salt-sensitive mutant Escherichia coli TO114, and functionally characterized. Complementation analysis revealed that FL_OtNhaA- and ΔN112_OtNhaA-expressing cells exhibited increased tolerance to high NaCl concentrations up to 700 mM. Antiporter activity assays showed that both FL_OtNhaA and ΔN112_OtNhaA proteins predominantly exhibited Na⁺/H⁺ and Ca²⁺/H⁺ antiporter activities at alkaline pH conditions. Intriguingly, the ΔN112_OtNhaA exhibited higher Na⁺/H⁺ and Ca²⁺/H⁺ antiporter activities compared to FL_OtNhaA. Kinetic analysis revealed that FL_OtNhaA has a high affinity for Na⁺ and Ca²⁺ ions with a K_m of 1.1 ± 0.23 mM for Na⁺ (at pH 8.5) and a K_m of 0.3 ± 0.07 mM for Ca²⁺ (at pH 8.5). Since NhaA has shown striking diversity among taxa, our results provide insight into the functional properties of the algal NhaA Na⁺/H⁺ antiporter. These results will contribute to the understanding of Na⁺/H⁺ antiporters that have various implications in all kingdoms of life.

Human and viral Golgi anti-apoptotic proteins (GAAPs) oligomerize via different mechanisms and monomeric GAAP inhibits apoptosis and modulates calcium

Golgi anti-apoptotic proteins (GAAPs) are hydrophobic proteins resident in membranes of the Golgi complex. They protect cells from a range of apoptotic stimuli, reduce the Ca²⁺ content of intracellular stores, and regulate Ca²⁺ fluxes. GAAP was discovered in camelpox virus, but it is highly conserved throughout evolution and encoded by all eukaryote genomes examined. GAAPs are part of the transmembrane Bax inhibitor-containing motif (TMBIM) family that also includes other anti-apoptotic and Ca²⁺-modulating membrane proteins. Most TMBIM members show multiple bands when analyzed by SDS-PAGE, suggesting that they may be oligomeric. However, the molecular mechanisms of oligomerization, the native state of GAAPs in living cells and the functional significance of oligomerization have not been addressed. TMBIM members are thought to have evolved from an ancestral GAAP. Two different GAAPs, human (h) and viral (v)GAAP were therefore selected as models to examine oligomerization of TMBIM family members. We show that both hGAAP and vGAAP in their native states form oligomers and that oligomerization is pH-dependent. Surprisingly, hGAAP and vGAAP do not share the same oligomerization mechanism. Oligomerization of hGAAP is independent of cysteines, but oligomerization of vGAAP depends on cysteines 9 and 60. A mutant vGAAP that is unable to oligomerize revealed that monomeric vGAAP retains both its anti-apoptotic function and its effect on intracellular Ca²⁺ stores. In conclusion, GAAP can oligomerize in a pH-regulated manner, and monomeric GAAP is functional.