Na+/H+ antiporters

Abstract

Na(+)/H(+) antiporters are membrane proteins that play a major role in pH and Na(+) homeostasis of cells throughout the biological kingdom, from bacteria to humans and higher plants. The emerging genomic sequence projects already have started to reveal that the Na(+)/H(+) antiporters cluster in several families. Structure and function studies of a purified antiporter protein have as yet been conducted mainly with NhaA, the key Na(+)/H(+) antiporter of Escherichia coli. This antiporter has been overexpressed, purified and reconstituted in a functional form in proteoliposomes. It has recently been crystallized in both 3D as well as 2D crystals. The NhaA 2D crystals were analyzed by cryoelectron microscopy and a density map at 4 A resolution was obtained and a 3D map was reconstructed. NhaA is shown to exist in the 2D crystals as a dimer of monomers each composed of 12 transmembrane segments with an asymmetric helix packing. This is the first insight into the structure of a polytopic membrane protein. Many Na(+)/H(+) antiporters are characterized by very dramatic sensitivity to pH, a property that corroborates their role in pH homeostasis. The molecular mechanism underlying this pH sensitivity has been studied in NhaA. Amino acid residues involved in the pH response have been identified. Conformational changes transducing the pH change into a change in activity were found in loop VIII-IX and at the N-terminus by probing trypsin digestion or binding of a specific monoclonal antibody respectively. Regulation by pH of the eukaryotic Na(+)/H(+) antiporters involves an intricate signal transduction pathway (recently reviewed by Yun et al., Am. J. Physiol. 269 (1995) G1-G11). The transcription of NhaA has been shown to be regulated by a novel Na(+)-specific regulatory network. It is envisaged that interdisciplinary approaches combining structure, molecular and cell biology as well as genomics should be applied in the future to the study of this important group of transporters.

pH-dependent regulation of the multi-subunit cation/proton antiporter Pha1 system from Sinorhizobium meliloti

“…lower at neutral pH and significantly increased at alkaline pH. This type of pHdependency is frequently observed for bacterial cation/H + antiporters that have important roles in alkaline pH homeostasis (Padan et al, 2001;Radchenko et al, 2006). These observations support the previous suggestion that the Pha1 system is required to cope with an alkaline environment (Putnoky et al, 1998).…”

“…The involvement of Na + /H + antiporters in pH adaptation and Na + extrusion had been extensively studied in Escherichia coli (Padan et al, 2001) and several other species, particularly in alkaliphilic bacteria . While most of the antiporters studied to date are encoded by single genes, a wide variety of prokaryotic species possess characteristic multi-gene-encoded cation/H + antiporters called Mrp transporters (Swartz et al, 2005).…”

Alkalitolerance: A biological function for a multidrug transporter in pH homeostasis

“…The machinery responsible for internal pH stability comprises many cellular factors, and their intricate interplay enables E. coli to maintain an internal proton concentration, which allows growth over a broad range of external pH values. However, at external pH 9, the system begins to collapse and internal alkalization occurs, leading to the arrest of growth and cell death (26)(27)(28). Here, we present the surprising findings that the Mdr transporter, MdfA, confers resistance under relatively extreme alkaline pH conditions.…”

Characterizing folding, structure, molecular interactions and ligand gated activation of single sodium/proton antiporters

“…While NhaA is inactive at acidic conditions, it rapidly activates if the cytosolic pH increases to values between 7 and 8 (Taglicht et al 1991). This makes the protein an emergency and adaptation tool essential for cellular survival in alkaline and saline environment (Padan et al 2001). Upon adjusting the intracellular pH, NhaA removes sodium ions from the cytoplasm in exchange for protons and reduces the sodium toxicity.…”

“…Na + /H + antiporters regulate intracellular pH, cellular Na + concentration and cell volume of eukaryotic and prokaryotic organisms. In Escherichia coli two antiporters, NhaA and NhaB specifically exchange Na + and Li + ions for H + allowing the cell to adapt to high environmental salinity and to grow at an alkaline pH (Padan et al 2001). The activity of NhaA is highly dependent on intracellular pH and increases 2000-fold upon enhancing the pH from 7.0 to 8.0 (Taglicht et al 1991).…”