Novel type Arabidopsis thaliana H(+)-PPase is localized to the Golgi apparatus

Functional complementation of yeast cytosolic pyrophosphatase by bacterial and plant H+-translocating pyrophosphatases

Two types of proteins that hydrolyze inorganic pyrophosphate (PPi), very different in both amino acid sequence and structure, have been characterized to date: soluble and membrane-bound proton-pumping pyrophosphatases (sPPases and H(+)-PPases, respectively). sPPases are ubiquitous proteins that hydrolyze PPi releasing heat, whereas H+-PPases, so far unidentified in animal and fungal cells, couple the energy of PPi hydrolysis to proton movement across biological membranes. The budding yeast Saccharomyces cerevisiae has two sPPases that are located in the cytosol and in the mitochondria. Previous attempts to knock out the gene coding for a cytosolic sPPase (IPP1) have been unsuccessful, thus suggesting that this protein is essential for growth. Here, we describe the generation of a conditional S. cerevisiae mutant (named YPC-1) whose functional IPP1 gene is under the control of a galactose-dependent promoter. Thus, YPC-1 cells become growth arrested in glucose but they regain the ability to grow on this carbon source when transformed with autonomous plasmids bearing diverse foreign H+-PPase genes under the control of a yeast constitutive promoter. The heterologously expressed H+-PPases are distributed among different yeast membranes, including the plasma membrane, functional complementation by these integral membrane proteins being consistently sensitive to external pH. These results demonstrate that hydrolysis of cytosolic PPi is essential for yeast growth and that this function is not substantially affected by the intrinsic characteristics of the PPase protein that accomplishes it. Moreover, this is, to our knowledge, the first direct evidence that H+-PPases can mediate net hydrolysis of PPi in vivo. YPC-1 mutant strain constitutes a convenient expression system to perform studies aimed at the elucidation of the structure-function relationships of this type of proton pumps.

A vacuolar sorting receptor PV72 on the membrane of vesicles that accumulate precursors of seed storage proteins (PAC vesicles)

A novel vesicle, referred to as a precursor-accumulating (PAC) vesicle, mediates the transport of storage protein precursors to protein storage vacuoles in maturing pumpkin seeds. PV72, a type I integral membrane protein with three repeats of epidermal growth factor, was found on the membrane of the PAC vesicles. PV72 had an ability to bind to pro2S albumin, a storage protein precursor, in a Ca(2+)-dependent manner, via the C-terminal region of pro2S albumin, which was found to function as a vacuolar targeting signal. This implies that PV72 is a vacuolar sorting receptor of the storage protein. PV72 was specifically and transiently accumulated at the middle stage of seed maturation in association with the synthesis of storage proteins. Subcellular fractionation showed that PV72 was also accumulated in the microsomal fraction. A fusion protein consisting of GFP and the transmembrane domain and the cytosolic tail of PV72 was localized in Golgi complex. PV72 in the isolated PAC vesicles had a complex type of oligosaccharide, indicating that PV72 passed though the Golgi complex. These results suggest that PV72 is recycled between PAC vesicles and Golgi complex/post-Golgi compartments. PV72 appears to be responsible for recruiting pro2S albumin molecules from the Golgi complex to the PAC vesicles.

H+-pyrophosphatase of Rhodospirillum rubrum. High yield expression in Escherichia coli and identification of the Cys residues responsible for inactivation my mersalyl

H(+)-translocating pyrophosphatase (H(+)-PPase) of the photosynthetic bacterium Rhodospirillum rubrum was expressed in Escherichia coli C43(DE3) cells. Recombinant H(+)-PPase was observed in inner membrane vesicles, where it catalyzed both PP(i) hydrolysis coupled with H(+) transport into the vesicles and PP(i) synthesis. The hydrolytic activity of H(+)-PPase in E. coli vesicles was eight times greater than that in R. rubrum chromatophores but exhibited similar sensitivity to the H(+)-PPase inhibitor, aminomethylenediphosphonate, and insensitivity to the soluble PPase inhibitor, fluoride. Using this expression system, we showed that substitution of Cys(185), Cys(222), or Cys(573) with aliphatic residues had no effect on the activity of H(+)-PPase but decreased its sensitivity to the sulfhydryl modifying reagent, mersalyl. H(+)-PPase lacking all three Cys residues was completely resistant to the effects of mersalyl. Mg(2+) and MgPP(i) protected Cys(185) and Cys(573) from modification by this agent but not Cys(222). Phylogenetic analyses of 23 nonredundant H(+)-PPase sequences led to classification into two subfamilies. One subfamily invariably contains Cys(222) and includes all known K(+)-independent H(+)-PPases, whereas the other incorporates a conserved Cys(573) but lacks Cys(222) and includes all known K(+)-dependent H(+)-PPases. These data suggest a specific link between the incidence of Cys at positions 222 and 573 and the K(+) dependence of H(+)-PPase.

A proton pumping pyrophosphatase in the Golgi apparatus and plasma membrane vesicles of Trypanosoma cruzi

The proton pumping pyrophosphatase (H(+)-PPase) is an enzyme that has been identified in membranes of plant vacuoles, in the Golgi complex of plants and Chlamydomonas reinhardtii, and more recently in acidocalcisomes of different trypanosomatids and apicomplexan parasites. Immunofluorescence and immunoelectron microscopy studies using antibodies against the plant enzyme also suggested a plasma membrane localization in different stages of Trypanosoma cruzi. In this report we provide immunogold electron microscopy evidence of the presence of the H(+)-PPase in the Golgi complex and plasma membrane of epimastigotes of T. cruzi. Pyrophosphate promoted acidification of plasma membrane vesicles as determined using acridine orange. This activity was stimulated by K(+) ions, inhibited by the pyrophosphate analogs imidodiphosphate (IDP) and aminomethylenediphosphonate (AMDP) by KF, NaF and DCCD, and it had different responses to ions and inhibitors as compared with the activity present in acidocalcisomes. Surface localization of the H(+)-PPase was confirmed by experiments using biotinylation of cell surface proteins and immunoprecipitation with antibodies against H(+)-PPase. Taken together, these results are consistent with the presence of a functional H(+)-PPase in the plasma membrane of these parasites.

Pyrophosphate import and synthesis by plant mitochondria

The matrix level of pyrophosphate (PPi) in mitochondria isolated from etiolated pea (Pisum sativum L. cv. Alaska) stems was evaluated, on the basis of an enzymatic assay, to be approx. 0.2 mM. Pyrophosphate could enter from the cytoplasm to the mitochondria via adenine nucleotide translocase (ANT), because F- and Ca2+ (two penetrating PPiase inhibitors) and atractylate (ANT inhibitor) inhibited PPiase activity in isolated mitochondria supplied with PPi. This result was also confirmed by measuring oxygen consumption and membrane potential (DeltaPsi) in succinate-energized mitochondria. In a medium free of phosphate (Pi), the addition of PPi before the substrate rendered possible an ADP-stimulated oxygen consumption that was inhibited by F- or Ca2+. In a similar experiment, ADP induced the dissipation of DeltaPsi when it was added after the succinate-generated DeltaPsi had reached a steady state and, again, F- inhibited this dissipation. These results imply that PPi enters the mitochondria where it is hydrolyzed to 2 Pi which become available for the H+-ATPase (EC 3.6.1.34). In addition, PPi may be synthesized by the H+-PPiase (EC 3.6.1.1), acting as a synthase. This evidence arises from the observation that Pi stimulated an oxygen consumption (respiratory control ratio of 1.7) that was inhibited by F- or Ca2+. The physiological role of the mitochondrial H+-PPiase is discussed in the light of the consideration that this enzyme can catalyse a readily reversible reaction.

Vacuolar type H+ pumping pyrophosphatases of parasitic protozoa

Trans-membrane proton pumping is responsible for a myriad of physiological processes including the generation of proton motive force that drives bioenergetics. Among the various proton pumping enzymes, vacuolar pyrophosphatases (V-PPases) form a distinct class of proton pumps, which are characterised by their ability to translocate protons across a membrane by using the potential energy released by hydrolysis of the phosphoanhydride bond of inorganic pyrophosphate. Until recently, V-PPases were known to be the purview of only plant vacuoles and plasma membranes of phototrophic bacteria. Recent discoveries of V-PPases in kinetoplastid and apicomplexan parasites, however, have expanded our view of the evolutionary reach of these enzymes. The lack of V-PPases in the vertebrate hosts of these parasites makes them potentially excellent targets for developing broad-spectrum antiparasitic agents. This review surveys the current understanding of V-PPases in parasitic protozoa with an emphasis on malaria parasites. Topological predictions suggest remarkable similarity of the parasite enzymes to their plant homologues with 15-16 membrane spanning domains and conserved sequences shown to constitute critical catalytic residues. Remarkably, malaria parasites have been shown to possess two V-PPase genes, one is an apparent orthologue of the canonical plant enzyme, whereas the other is a more distantly related paralogue with homology to a recently identified new class of K+-insensitive plant V-PPases. V-PPases appear to localise both to the plasma membrane and cytoplasmic organelles believed to be acidocalcisomes or polyphosphate bodies. Gene transfer experiments suggest that one of the malarial V-PPases is predominantly localised to the surface of intraerythrocytic parasites. We suggest a model in which V-PPase localised to the malaria parasite plasma membrane may serve as an electrogenic pump utilising pyrophosphate as an energy source, thus sparing the more precious ATP. Searching for V-PPase inhibitors could prove fruitful as a novel means of antiparasitic chemotherapy.