Integrity Assessment of Isolated Plant Mitochondria

The integrity of isolated mitochondria can be estimated functionally using enzymatic activities or the permeability of mitochondrial membranes to molecules of different sizes. Thus, the permeability of the outer membrane to the protein cytochrome c, the permeability of the inner membrane to protons, and the permeability of the inner membrane to NAD⁺, NADH and organic acids using soluble matrix dehydrogenases as markers have all been used. These assays all have limitations to how the data can be converted into a measure of integrity, are differently sensitive to artifacts and require widely variable amounts of material. Therefore, each method has a restricted utility for estimating integrity, depending on the type of mitochondria analysed. Here, we review the advantages and disadvantages of different integrity assays and present protocols for integrity assays that require relatively small amounts of mitochondria. They are based on the permeability of the outer membrane to cytochrome c, and the inner membrane to protons or NAD(H). The latter has the advantage of utilizing a membrane-bound activity (complex I) and the pore-forming peptide alamethicin to gain access to the matrix space. These methods together provide a toolbox for the determination of functionality and quality of isolated mitochondria.

Intracellular pH regulation: characterization and functional investigation of H+ transporters in Stylophora pistillata

BACKGROUND

Reef-building corals regularly experience changes in intra- and extracellular H⁺ concentrations ([H⁺]) due to physiological and environmental processes. Stringent control of [H⁺] is required to maintain the homeostatic acid-base balance in coral cells and is achieved through the regulation of intracellular pH (pH_i). This task is especially challenging for reef-building corals that share an endosymbiotic relationship with photosynthetic dinoflagellates (family Symbiodinaceae), which significantly affect the pH_i of coral cells. Despite their importance, the pH regulatory proteins involved in the homeostatic acid-base balance have been scarcely investigated in corals. Here, we report in the coral Stylophora pistillata a full characterization of the genomic structure, domain topology and phylogeny of three major H⁺ transporter families that are known to play a role in the intracellular pH regulation of animal cells; we investigated their tissue-specific expression patterns and assessed the effect of seawater acidification on their expression levels.

RESULTS

We identified members of the Na⁺/H⁺ exchanger (SLC9), vacuolar-type electrogenic H⁺-ATP hydrolase (V-ATPase) and voltage-gated proton channel (H_vCN) families in the genome and transcriptome of S. pistillata. In addition, we identified a novel member of the H_vCN gene family in the cnidarian subclass Hexacorallia that has not been previously described in any species. We also identified key residues that contribute to H⁺ transporter substrate specificity, protein function and regulation. Last, we demonstrated that some of these proteins have different tissue expression patterns, and most are unaffected by exposure to seawater acidification.

CONCLUSIONS

In this study, we provide the first characterization of H⁺ transporters that might contribute to the homeostatic acid-base balance in coral cells. This work will enrich the knowledge of the basic aspects of coral biology and has important implications for our understanding of how corals regulate their intracellular environment.

Expression of SLC4A11 protein in mouse and rat medulla: a candidate transporter involved in outer medullary ammonia recycling

SLC4A11 is a multifunctional membrane transporter involved with H+ transport, NH3 and alkaline pH stimulated H+ transport, and water transport. The role of SLC4A11 in the kidney is not well understood. A prior study has shown that in murine kidney, SLC4A11/LacZ staining is primarily in the long-looped descending thin limb (DTL) as determined by colocalization with aquaporin 1 (AQP1), a protein that is expressed in some, but not all, descending thin limb segments. Using a previously characterized polyclonal antibody, we demonstrate the selective expression of SLC4A11 in the upper DTLs (which are AQP1-positive) in the outer medulla and inner medulla with little or no expression in the lower DTLs (which are AQP-1-null). SLC4A11 also colocalized with AQP1 and the urea transporter UT-B in the mouse descending vasa recta, but was absent in mouse and rat ascending vasa recta. Mouse, but not rat, outer medullary collecting duct cells also labeled for SLC4A11. Our results are compatible with the hypothesis that in the inner stripe of the outer medulla, SLC4A11 plays a role in the countercurrent transport of ammonia absorbed from the outer medullary thick ascending limb and secreted into the long-looped DTLs. SLC4A11 can potentially modulate the rate of ammonia transport in the mouse outer medullary collecting duct. Our data suggest functionally unique SLC4A11 pathways in mouse and rat and complement previous studies of DTL Na+ , urea and water permeability indicating that the upper and lower DTLs of long-looped nephrons are functionally distinct.

Spermine modulates fungal morphogenesis and activates plasma membrane H+-ATPase during yeast to hyphae transition

Polyamines play a regulatory role in eukaryotic cell growth and morphogenesis. Despite many molecular advances, the underlying mechanism of action remains unclear. Here, we investigate a mechanism by which spermine affects the morphogenesis of a dimorphic fungal model of emerging relevance in plant interactions, Yarrowia lipolytica, through the recruitment of a phytohormone-like pathway involving activation of the plasma membrane P-type H⁺-ATPase. Morphological transition was followed microscopically, and the H⁺-ATPase activity was analyzed in isolated membrane vesicles. Proton flux and acidification were directly probed at living cell surfaces by a non-invasive selective ion electrode technique. Spermine and indol-3-acetic acid (IAA) induced the yeast-hypha transition, influencing the colony architecture. Spermine induced H⁺-ATPase activity and H⁺ efflux in living cells correlating with yeast-hypha dynamics. Pharmacological inhibition of spermine and IAA pathways prevented the physio-morphological responses, and indicated that spermine could act upstream of the IAA pathway. This study provides the first compelling evidence on the fungal morphogenesis and colony development as modulated by a spermine-induced acid growth mechanism analogous to that previously postulated for the multicellular growth regulation of plants.

Summary: This study presents a new mechanistic model for the integrative role of the polyamine spermine and hormone auxin in the signaling of yeast-to-hypha transition, filling an important gap in fungal morphogenesis.

Coupling of remote alternating-access transport mechanisms for protons and substrates in the multidrug efflux pump AcrB

Membrane transporters of the RND superfamily confer multidrug resistance to pathogenic bacteria, and are essential for cholesterol metabolism and embryonic development in humans. We use high-resolution X-ray crystallography and computational methods to delineate the mechanism of the homotrimeric RND-type proton/drug antiporter AcrB, the active component of the major efflux system AcrAB-TolC in Escherichia coli, and one most complex and intriguing membrane transporters known to date. Analysis of wildtype AcrB and four functionally-inactive variants reveals an unprecedented mechanism that involves two remote alternating-access conformational cycles within each protomer, namely one for protons in the transmembrane region and another for drugs in the periplasmic domain, 50 Å apart. Each of these cycles entails two distinct types of collective motions of two structural repeats, coupled by flanking α-helices that project from the membrane. Moreover, we rationalize how the cross-talk among protomers across the trimerization interface might lead to a more kinetically efficient efflux system.

DOI:http://dx.doi.org/10.7554/eLife.03145.001

The interior of living cells is separated from their external environment by an enveloping membrane that serves as a protective barrier. To regulate the chemical composition of their interior, cells are equipped with specialized proteins in their membranes that move substances in and out of cells. Membrane proteins that expel molecules from the inside to the outside of the cell are called efflux pumps.

In Escherichia coli bacteria, an efflux pump known as AcrB is part of a system that removes toxic substances from the bacterial cell—such as the antibiotics used to treat bacterial infections. AcrB and other closely related efflux pumps in pathogenic bacteria are often polyspecific transporters—they can transport a large number of different toxic molecules. These efflux pump systems are also more active in bacteria that have been targeted by antibiotics, and therefore they help bacteria to evolve resistance to multiple drugs. The emergence of bacterial multi-drug resistance is a global threat to human health; to combat this phenomenon, it is essential to understand its molecular basis.

Each AcrB protein has three main parts or domains. The periplasmic domain, which is located between the two membranes that surround E. coli, works via an ‘alternating-access cycle’; that is, the shape of the periplasmic domain changes between three different forms in such a way that antibiotic molecules are first captured and subsequently squeezed through the protein towards the outside of the cell. However, the mechanism of the transmembrane domain—which is embedded in the innermost membrane of the bacterium and is the source of energy for the transport process—was not understood.

Here, Eicher et al. use X-ray crystallography to examine the three-dimensional structures of the AcrB efflux pump—and several inactive variants—in high detail. Combining these results with computer simulations reveals the mechanism used by the transmembrane domain to take up protons from the exterior and transport them into the cell. Proton transport also proceeds according to an alternating-access mechanism—and, although the transmembrane and periplasmic domains are far apart, their movements are tightly linked. Thus, because proton uptake releases energy, the transmembrane domain effectively powers the periplasmic domain to expel drugs and other molecules. Eicher et al. note that a similar mechanism has not been seen before in other efflux pumps or transporter proteins.

Understanding how AcrB works opens up new avenues that could be exploited to develop new drugs against multidrug resistant bacteria. Furthermore, Eicher et al. suggest that efflux pumps in humans closely related to AcrB might function in a similar way—including those required for regulation of cellular cholesterol, and for the correct development of embryos.

DOI:http://dx.doi.org/10.7554/eLife.03145.002

Point mutations in the extracytosolic loop between transmembrane segments M5 and M6 of the yeast Pma1 H+-ATPase: alanine-scanning mutagenesis

Membrane-spanning segments M4, M5, M6, and M8 of the H(+)-, Ca(2+)-, and K(+), Na(+)-ATPases, which belong to the P2-type pumps are the core through which cations are transported. M5 and M6 loop is a short extracytoplasmic stretch of the seven amino acid residues (714-DNSLDID) connecting two of these segments, M5 and M6, where residues involved in the formation of the proton-binding site(s) are located. In the present study, we have used alanine-scanning mutagenesis to explore the structural and functional relationships within this loop of the yeast plasma membrane Pma1 H(+)-ATPase. Of the 7 Ala mutants made, substitution for the most conserved residue (Leu-717) has led to a severe misfolding and complete block in biogenesis of the mutant enzyme. The replacement of Asp-714 has also caused misfolding leading to significant decrease in the expression of the mutant and loss of activity. The remaining mutants were expressed in secretory vesicles at 21-119% of the wild-type level and were active enough to be analyzed in detail. One of these mutants (I719A) showed five- to threefold decrease in both expression and ATP hydrolyzing and H(+) pumping activities and also threefold reduction in the coupling ratio between ATP hydrolysis and H(+) transport. Thus, Ala substitutions at three positions of the seven seriously affected biogenesis, folding, stability and/or functioning of the enzyme. Taken together, these results lead to suggestion that M5 and M6 loop play an important role in the protein stability and function and is responsible for proper arrangement of transmembrane segments M5 and M6 and probably other domains of the enzyme. Results for additional conserved substitutions (Asn and Glu) at Asp-714 and Asp-720 confirmed this suggestion.

Multiple Transport Pathways for Mediating Intracellular pH Homeostasis: The Contribution of H(+)/ion Exchangers

Intracellular pH homeostasis is an essential process in all plant cells. The transport of H(+) into intracellular compartments is critical for providing pH regulation. The maintenance of correct luminal pH in the vacuole and in compartments of the secretory/endocytic pathway is important for a variety of cellular functions including protein modification, sorting, and trafficking. It is becoming increasingly evident that coordination between primary H(+) pumps, most notably the V-ATPase, and secondary ion/H(+) exchangers allows this endomembrane pH maintenance to occur. This article describes some of the recent insights from the studies of plant cation/H(+) exchangers and anion/H(+) exchangers that demonstrate the fundamental roles of these transporters in pH homeostasis within intracellular compartments.

Role of prostaglandin in the regulation of gastric H(+)-Transporting system

Prostaglandins and (PG) have been reported to be an important gastric acid suppressive factor. However, the mechanism underlying is yet to be clearly established. In vitro study with gastric microsomes in presence of both PGE(2) and PGI(2) shows a stimulation of gastric H(+) K(+)-ATPase activity below 1X10(-6)M and 2.5X10(-7)M concentrations respectively. However, with further increase in concentrations of both PGE(2) and PGI(2), H(+), K(+)-ATPase activity shows an inhibition but PGI(2) completely obliterates the K(+) stimulated part of H(+), K(+)-ATPase activity at higher concentration. The H(+)-ion transport study using chambered frog gastric mucosa shows that both PGE(2) and PGI(2) inhibit H(+)-ion transport at 5X10(-6) M and 10X10(-6)M concentrations respectively but the effect of PGI(2) is reversible. These differential effects of PGE(2) and PGI(2) on microsomal H(+), K(+)-ATPase and on H(+) transport my be caused by the differential effects of these phospholipid mediators with the gastric mucosal cell membrane. This in vitro investigation shows the role of prostaglandin (s) as a physiological switch/regulator of gastric H(+) ion transport leading to the cessation of gastric acid secretion.