Evidence for major structural changes in the Manduca sexta midgut V1 ATPase due to redox modulation. A small angle X-ray scattering study

Plant Proton Pumps and Cytosolic pH-Homeostasis

Proton pumps create a proton motif force and thus, energize secondary active transport at the plasma nmembrane and endomembranes of the secretory pathway. In the plant cell, the dominant proton pumps are the plasma membrane ATPase, the vacuolar pyrophosphatase (V-PPase), and the vacuolar-type ATPase (V-ATPase). All these pumps act on the cytosolic pH by pumping protons into the lumen of compartments or into the apoplast. To maintain the typical pH and thus, the functionality of the cytosol, the activity of the pumps needs to be coordinated and adjusted to the actual needs. The cellular toolbox for a coordinated regulation comprises 14-3-3 proteins, phosphorylation events, ion concentrations, and redox-conditions. This review combines the knowledge on regulation of the different proton pumps and highlights possible coordination mechanisms.

The structural features of Acetobacterium woodii F-ATP synthase reveal the importance of the unique subunit γ-loop in Na+ translocation and ATP synthesis

The Na⁺ translocating F₁ F_O ATP synthase from Acetobacterium woodii shows a subunit stoichiometry of α₃ :β₃ :γ:δ:ε:a:b₂ :(c_2/3 )₉ :c₁ and reveals an evolutionary path between synthases and pumps involving adaptations in the rotor c-ring, which is composed of F- and vacuolar-type c subunits in a stoichiometry of 9 : 1. This hybrid turbine couples rotation with Na⁺ translocation in the F_O part and rotation of the central stalk subunits γ-ε to drive ATP synthesis in the catalytic α₃ :β₃ headpiece. Here, we isolated a highly pure recombinant A. woodii F-ATP synthase and present the first projected structure of this hybrid engine as determined by negative-stain electron microscopy and single-particle analysis. The uniqueness of the A. woodii F-ATP synthase is also reflected by an extra 17 amino acid residues loop (₁₉₅ TSGKVKITEETKEEKSK₂₁₁ ) in subunit γ. Deleting the loop-encoding DNA sequence (γ_Δ195-211 ) and purifying the recombinant F-ATP synthase γ_Δ195-211 mutant provided a platform to study its effect in enzyme stability and activity. The recombinant F-ATP synthase γ_Δ195-211 mutant revealed the same subunit composition as the wild-type enzyme and a minor reduction in ATP hydrolysis. When reconstituted into proteoliposomes ATP synthesis and Na⁺ transport were diminished, demonstrating the importance of the γ195-211 loop in both enzymatic processes. Based on a structural model, a coupling mechanism for this enzyme is proposed, highlighting the role of the γ-loop. Finally, the γ195-211 loop of A. woodii is discussed in comparison with the extra γ-loops of mycobacterial and chloroplasts F-ATP synthases described to be involved in species-specific regulatory mechanisms.

Ambiguity assessment of small-angle scattering curves from monodisperse systems

A novel approach is presented for an a priori assessment of the ambiguity associated with spherically averaged single-particle scattering. The approach is of broad interest to the structural biology community, allowing the rapid and model-independent assessment of the inherent non-uniqueness of three-dimensional shape reconstruction from scattering experiments on solutions of biological macromolecules. One-dimensional scattering curves recorded from monodisperse systems are nowadays routinely utilized to generate low-resolution particle shapes, but the potential ambiguity of such reconstructions remains a major issue. At present, the (non)uniqueness can only be assessed by a posteriori comparison and averaging of repetitive Monte Carlo-based shape-determination runs. The new a priori ambiguity measure is based on the number of distinct shape categories compatible with a given data set. For this purpose, a comprehensive library of over 14,000 shape topologies has been generated containing up to seven beads closely packed on a hexagonal grid. The computed scattering curves rescaled to keep only the shape topology rather than the overall size information provide a `scattering map' of this set of shapes. For a given scattering data set, one rapidly obtains the number of neighbours in the map and the associated shape topologies such that in addition to providing a quantitative ambiguity measure the algorithm may also serve as an alternative shape-analysis tool. The approach has been validated in model calculations on geometrical bodies and its usefulness is further demonstrated on a number of experimental X-ray scattering data sets from proteins in solution. A quantitative ambiguity score (a-score) is introduced to provide immediate and convenient guidance to the user on the uniqueness of the ab initio shape reconstruction from the given data set.

Regulation of the V-type ATPase by redox modulation

ATP-hydrolysis and proton pumping by the V-ATPase (vacuolar proton-translocating ATPase) are subject to redox regulation in mammals, yeast and plants. Oxidative inhibition of the V-ATPase is ascribed to disulfide-bond formation between conserved cysteine residues at the catalytic site of subunit A. Subunits containing amino acid substitutions of one of three conserved cysteine residues of VHA-A were expressed in a vha-A null mutant background in Arabidopsis. In vitro activity measurements revealed a complete absence of oxidative inhibition in the transgenic line expressing VHA-A C256S, confirming that Cys256 is necessary for redox regulation. In contrast, oxidative inhibition was unaffected in plants expressing VHA-A C279S and VHA-A C535S, indicating that disulfide bridges involving these cysteine residues are not essential for oxidative inhibition. In vivo data suggest that oxidative inhibition might not represent a general regulatory mechanism in plants.

Purification, crystallization and preliminary crystallographic analysis of the vacuole-type ATPase subunit E from Pyrococcus horikoshii OT3

The vacuole-type ATPases in eukaryotic cells translocate protons across various biological membranes including the vacuolar membrane by consuming ATP molecules. The E subunit of the multisubunit complex V-ATPase from Pyrococcus horikoshii OT3, which has a molecular weight of 22.88 kDa, has been cloned, overexpressed in Escherichia coli, purified and crystallized by the microbatch method using PEG 4000 as a precipitant at 296 K. A data set to 1.85 A resolution with 98.8% completeness and an Rmerge of 6.5% was collected from a single flash-cooled crystal using synchrotron radiation. The crystal belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 52.196, b = 55.317, c = 77.481 A, and is most likely to contain one molecule per asymmetric unit.

The insect plasma membrane H+ V-ATPase: intra-, inter-, and supramolecular aspects

The plasma membrane H+ V-ATPase from the midgut of larval Manduca sexta, commonly called the tobacco hornworm, is the sole energizer of epithelial ion transport in this tissue, being responsible for the alkalinization of the gut lumen up to a pH of more than 11 and for any active ion movement across the epithelium. This minireview deals with those topics of our recent research on this enzyme that may contribute novel aspects to the biochemistry and physiology of V-ATPases. Our research approaches include intramolecular aspects such as subunit topology and the inhibition by macrolide antibiotics, intermolecular aspects such as the hormonal regulation of V-ATPase biosynthesis and the interaction of the V-ATPase with the actin cytoskeleton, and supramolecular aspects such as the interactions of V-ATPase, K+/H+ antiporter, and ion channels, which all function as an ensemble in the transepithelial movement of potassium ions.

New insight into the structure and regulation of the plant vacuolar H+-ATPase

Plant cells are characterized by a highly active secretory system that includes the large central vacuole found in most differentiated tissues. The plant vacuolar H+-ATPase plays an essential role in maintaining the ionic and metabolic gradients across endomembranes, in activating transport processes and vesicle dynamics, and, hence, is indispensable for plant growth, development, and adaptation to changing environmental conditions. The review summarizes recent advances in elucidating the structure, subunit composition, localization, and regulation of plant V-ATPase. Emerging knowledge on subunit isogenes from Arabidopsis and rice genomic sequences as well as from Mesembryanthemum illustrates another level of complexity, the regulation of isogene expression and function of subunit isoforms. To this end, the review attempts to define directions of future research on plant V-ATPase.

Subunit composition, structure, and distribution of bacterial V-type ATPases

The overall structure of V-ATPase complexes resembles that of F-type ATPases, but the stalk region is different and more complex. Database searches followed by sequence analysis of the five water-soluble stalk region subunits C-G revealed that (i) to date V-ATPases are found in 16 bacterial species, (ii) bacterial V-ATPases are closer to archaeal A-ATPases than to eukaryotic V-ATPases, and (iii) different groups of bacterial V-ATPases exist. Inconsistencies in the nomenclature of types and subunits are addressed. Attempts to assign subunit positions in V-ATPases based on biochemical experiments, chemical cross-linking, and electron microscopy are discussed. A structural model for prokaryotic and eukaryotic V-ATPases is proposed. The prokaryotic V-ATPase is considered to have a central stalk between headpiece and membrane flanked by two peripheral stalks. The eukaryotic V-ATPases have one additional peripheral stalk.

Subunit arrangement in V-ATPase from Thermus thermophilus

The V0V1-ATPase of Thermus thermophilus catalyzes ATP synthesis coupled with proton translocation. It consists of an ATPase-active V1 part (ABDF) and a proton channel V0 part (CLEGI), but the arrangement of each subunit is still largely unknown. Here we found that acid treatment of V0V1-ATPase induced its dissociation into two subcomplexes, one with subunit composition ABDFCL and the other with EGI. Exposure of the isolated V0 to acid or 8 m urea also produced two subcomplexes, EGI and CL. Thus, the C subunit (homologue of d subunit, yeast Vma6p) associates with the L subunit ring tightly, and I (homologue of 100-kDa subunit, yeast Vph1p), E, and G subunits constitute a stable complex. Based on these observations and our recent demonstration that D, F, and L subunits rotate relative to A3B3 (Imamura, H., Nakano, M., Noji, H., Muneyuki, E., Ohkuma, S., Yoshida, M., and Yokoyama, K. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 2312-2315; Yokoyama, K., Nakano, M., Imamura, H., Yoshida, M., and Tamakoshi, M. (2003) J. Biol. Chem. 278, 24255-24258), we propose that C, D, F, and L subunits constitute the central rotor shaft and A, B, E, G, and I subunits comprise the surrounding stator apparatus in the V0V1-ATPase.

Defined subcomplexes of the A1 ATPase from the archaeon Methanosarcina mazei Gö1: biochemical properties and redox regulation

The potential A(1) ATPase genes ahaA, ahaB, ahaC, ahaD, ahaE, ahaF, and ahaG from the anaerobic archaeon Methanosarcina mazei Gö1 were overexpressed in Escherichia coli DK8 (pTL2). An A(1) complex was purified to apparent homogeneity and shown by Western blot and N-terminal sequence analyses to contain subunits A, B, C, D, and F but to be devoid of subunits E and G. Further removal of subunit C was without effect on ATPase activity. The enzyme was most active at pH 5.2 and required bisulfite and acetate for maximal activity. Kinetic studies confirmed three new inhibitors for A(1) ATPases (diethylstilbestrol and its derivatives hexestrol and dienestrol) and identified redox modulation as a new type of regulation of archaeal A(1) ATPases.

Resolution of the V1 ATPase from Manduca sexta into subcomplexes and visualization of an ATPase-active A3B3EG complex by electron microscopy

The effect of the ATPase activity of Manduca sexta V(1) ATPase by the amphipathic detergent lauryldimethylamine oxide (LDAO) and the relationship of these activities to the subunit composition of V(1) were studied. The V(1) was highly activated in the presence of 0.04-0.06% LDAO combined with release of the subunits H, C, and F from the enzyme. Increase of LDAO concentration to 0.1-0.2% caused the characterized subcomplexes A(3)B(3)HEGF and A(3)B(3)EG with a remaining ATPase activity of 52 and 65%, respectively. The hydrolytic-active A(3)B(3)EG subcomplex has been visualized by electron microscopy showing six major masses of density in a pseudo-hexagonal arrangement surrounding a seventh mass. The compositions of the various subcomplexes and fragments of V(1) provide an organization of the subunits in the enzyme in the framework of the known three-dimensional reconstruction of the V(1) ATPase from M. sexta (Radermacher, M., Ruiz, T., Wieczorek, H., and Grüber, G. (2001) J. Struct. Biol. 135, 26-37).

Structure–function relationships of A-, F- and V-ATPases

Ion-translocating ATPases, such as the F1Fo-, V1Vo- and archaeal A1Ao enzymes, are essential cellular energy converters which transduce the chemical energy of ATP hydrolysis into transmembrane ionic electrochemical potential differences. Based on subunit composition and primary structures of the subunits, these types of ATPases are related through evolution; however, they differ with respect to function. Recent work has focused on the three-dimensional structural relationships of the major, nucleotide-binding subunits A and B of the A1/V1-ATPases and the corresponding β and α subunits of the F1-ATPase, and the location of the coupling subunits within the stalk that provide the physical linkage between the regions of ATP hydrolysis and ion transduction. This review focuses on the structural homologies and diversities of A1-, F1- and V1-ATPases, in particular on significant differences between the stalk regions of these families of enzymes.