Controlled hydroxylations of diterpenoids allow for plant chemical defense without autotoxicity.

Many plant specialized metabolites function in herbivore defense, and abrogating particular steps in their biosynthetic pathways frequently causes autotoxicity. However, the molecular mechanisms underlying their defense and autotoxicity remain unclear. Here, we show that silencing two cytochrome P450s involved in diterpene biosynthesis in the wild tobacco Nicotiana attenuata causes severe autotoxicity symptoms that result from the inhibition of sphingolipid biosynthesis by noncontrolled hydroxylated diterpene derivatives. Moreover, the diterpenes' defensive function is achieved by inhibiting herbivore sphingolipid biosynthesis through postingestive backbone hydroxylation products. Thus, by regulating metabolic modifications, tobacco plants avoid autotoxicity and gain herbivore defense. The postdigestive duet that occurs between plants and their insect herbivores can reflect the plant's solutions to the "toxic waste dump" problem of using potent chemical defenses.

Hemolymph protease-5 links the melanization and Toll immune pathways in the tobacco hornworm, Manduca sexta.

Proteolytic activation of phenoloxidase (PO) and the cytokine Spätzle during immune responses of insects is mediated by a network of hemolymph serine proteases (HPs) and noncatalytic serine protease homologs (SPHs) and inhibited by serpins. However, integration and conservation of the system and its control mechanisms are not fully understood. Here we present biochemical evidence that PO-catalyzed melanin formation, Spätzle-triggered Toll activation, and induced synthesis of antimicrobial peptides are stimulated via hemolymph (serine) protease 5 (HP5) in Manduca sexta Previous studies have demonstrated a protease cascade pathway in which HP14 activates proHP21; HP21 activates proPAP2 and proPAP3, which then activate proPO in the presence of a complex of SPH1 and SPH2. We found that both HP21 and PAP3 activate proHP5 by cleavage at ESDR176*IIGG. HP5 then cleaves proHP6 at a unique site of LDLH112*ILGG. HP6, an ortholog of Drosophila Persephone, activates both proHP8 and proPAP1. HP8 activates proSpätzle-1, whereas PAP1 cleaves and activates proPO. HP5 is inhibited by Manduca sexta serpin-4, serpin-1A, and serpin-1J to regulate its activity. In summary, we have elucidated the physiological roles of HP5, a CLIPB with unique cleavage specificity (cutting after His) that coordinates immune responses in the caterpillar.

Identification of a Conserved Prophenoloxidase Activation Pathway in Cotton Bollworm Helicoverpa armigera.

Melanization is a prominent insect humoral response for encapsulation of and killing invading pathogens. It is mediated by a protease cascade composed of a modular serine protease (SP), and clip domain SPs (cSPs), which converts prophenoloxidase (PPO) into active phenoloxidase (PO). To date, melanization pathway in cotton bollworm Helicoverpa armigera, an important agricultural pest, remains largely unclear. To biochemically reconstitute the pathway in vitro, the putative proteases along with modified proteases containing the factor Xa cleavage site were expressed by Drosophila S2 cell expression system. Purified recombinant proteins were used to examine their role in activating PPO. It is revealed that cascade is initiated by a modular SP-SP41, followed by cSP1 and cSP6. The three-step SP41/cSP1/cSP6 cascade could further activate PPO, and the PO activity was significantly enhanced in the presence of two cSP homologs (cSPHs), cSPH11 and cSPH50, suggesting the latter are cofactors for PPO activation. Moreover, baculovirus infection was efficiently blocked by the reconstituted PPO activation cascade, and the effect was boosted by cSPH11 and cSPH50. Taken together, we unraveled a conserved PPO activation cascade in H. armigera, which is similar to that exists in lepidopteran biochemical model Manduca sexta and highlighted its role in antagonizing viral infection.

Allosteric activation of the nitric oxide receptor soluble guanylate cyclase mapped by cryo-electron microscopy.

Soluble guanylate cyclase (sGC) is the primary receptor for nitric oxide (NO) in mammalian nitric oxide signaling. We determined structures of full-length Manduca sexta sGC in both inactive and active states using cryo-electron microscopy. NO and the sGC-specific stimulator YC-1 induce a 71° rotation of the heme-binding ß H-NOX and PAS domains. Repositioning of the ß H-NOX domain leads to a straightening of the coiled-coil domains, which, in turn, use the motion to move the catalytic domains into an active conformation. YC-1 binds directly between the ß H-NOX domain and the two CC domains. The structural elongation of the particle observed in cryo-EM was corroborated in solution using small angle X-ray scattering (SAXS). These structures delineate the endpoints of the allosteric transition responsible for the major cyclic GMP-dependent physiological effects of NO.

Instability in a coiled-coil signaling helix is conserved for signal transduction in soluble guanylyl cyclase.

How nitric oxide (NO) activates its primary receptor, α1/ß1 soluble guanylyl cyclase (sGC or GC-1), remains unknown. Likewise, how stimulatory compounds enhance sGC activity is poorly understood, hampering development of new treatments for cardiovascular disease. NO binding to ferrous heme near the N-terminus in sGC activates cyclase activity near the C-terminus, yielding cGMP production and physiological response. CO binding can also stimulate sGC, but only weakly in the absence of stimulatory small-molecule compounds, which together lead to full activation. How ligand binding enhances catalysis, however, has yet to be discovered. Here, using a truncated version of sGC from Manduca sexta, we demonstrate that the central coiled-coil domain, the most highly conserved region of the ~150,000 Da protein, not only provides stability to the heterodimer but is also conformationally active in signal transduction. Sequence conservation in the coiled coil includes the expected heptad-repeating pattern for coiled-coil motifs, but also invariant positions that disfavor coiled-coil stability. Full-length coiled coil dampens CO affinity for heme, while shortening of the coiled coil leads to enhanced CO binding. Introducing double mutation αE447L/ßE377L, predicted to replace two destabilizing glutamates with leucines, lowers CO binding affinity while increasing overall protein stability. Likewise, introduction of a disulfide bond into the coiled coil results in reduced CO affinity. Taken together, we demonstrate that the heme domain is greatly influenced by coiled-coil conformation, suggesting communication between heme and catalytic domains is through the coiled coil. Highly conserved structural imperfections in the coiled coil provide needed flexibility for signal transduction.

Comparative analysis of seven types of superoxide dismutases for their ability to respond to oxidative stress in Bombyx mori.

Insects are well adapted to changing environmental conditions. They have unique systems for eliminating reactive oxygen species (ROS). Superoxide dismutase (SOD) is a key enzyme that plays a primary role in removing ROS. Bombyx mori is a lepidopteran insect, whose body size is larger than the model insect Drosophila melanogaster, which enabled us to more easily examine gene expression at the tissue level. We searched B. mori SOD (BmSOD) genes using genome database, and we analyzed their function under different type of oxidative stress. Consequently, we identified four new types of BmSODs in addition to the three types already known. Two of the seven types had a unique domain architecture that has not been discovered previously in the SOD family, and they were expressed in different tissues and developmental stages. Furthermore, these BmSODs responded differently to several kinds of stressors. Our results showed that the seven types of BmSODs are likely to play different roles in B. mori; therefore, B. mori could be used to distinguish the functions of each SOD for resistance to oxidative stress that changes with the environmental conditions.

Manduca sexta hemolymph protease-2 (HP2) activated by HP14 generates prophenoloxidase-activating protease-2 (PAP2) in wandering larvae and pupae.

Melanization is a universal defense mechanism of insects against microbial infection. During this response, phenoloxidase (PO) is activated from its precursor by prophenoloxidase activating protease (PAP), the terminal enzyme of a serine protease (SP) cascade. In the tobacco hornworm Manduca sexta, hemolymph protease-14 (HP14) is autoactivated from proHP14 to initiate the protease cascade after host proteins recognize invading pathogens. HP14, HP21, proHP1*, HP6, HP8, PAP1-3, and non-catalytic serine protease homologs (SPH1 and SPH2) constitute a portion of the extracellular SP-SPH system to mediate melanization and other immune responses. Here we report the expression, purification, and functional characterization of M. sexta HP2. The HP2 precursor is synthesized in hemocytes, fat body, integument, nerve and trachea. Its mRNA level is low in fat body of 5th instar larvae before wandering stage; abundance of the protein in hemolymph displays a similar pattern. HP2 exists as an active enzyme in plasma of the wandering larvae and pupae in the absence of an infection. HP14 cleaves proHP2 to yield active HP2. After incubating active HP2 with larval hemolymph, we detected higher levels of PO activity, i.e. an enhancement of proPO activation. HP2 cleaved proPAP2 (but not proPAP3 or proPAP1) to yield active PAP2, responsible for a major increase in IEARpNA hydrolysis. PAP2 activates proPOs in the presence of a cofactor of SPH1 and SPH2. In summary, we have identified a new member of the proPO activation system and reconstituted a pathway of HP14-HP2-PAP2-PO. Since high levels of HP2 mRNA were present in integument and active HP2 in plasma of wandering larvae, HP2 likely plays a role in cuticle melanization during pupation and protects host from microbial infection in a soil environment.

Manduca sexta hemolymph protease-1, activated by an unconventional non-proteolytic mechanism, mediates immune responses.

Tissue damage or pathogen invasion triggers the auto-proteolysis of an initiating serine protease (SP), rapidly leading to sequential cleavage activation of other cascade members to set off innate immune responses in insects. Recently, we presented evidence that Manduca sexta hemolymph protease-1 zymogen (proHP1) is a member of the SP system in this species, and may activate proHP6. HP6 stimulates melanization and induces antimicrobial peptide synthesis. Here we report that proHP1 adopts an active conformation (*) to carry out its function, without a requirement for proteolytic activation. Affinity chromatography using HP1 antibodies isolated from induced hemolymph the 48 kDa proHP1 and also a 90 kDa band (detected by SDS-PAGE under reducing conditions) containing proHP1 and several serpins, as revealed by mass spectrometric analysis. Identification of tryptic peptides from these 90 kDa complexes included peptides from the amino-terminal regulatory part of proHP1, indicating that proHP1* was not cleaved, and that it had formed a complex with the serpins. As suicide inhibitors, serpins form SDS-stable, acyl-complexes when they are attacked by active proteases, indicating that proHP1* was catalytically active. Detection of M. sexta serpin-1, 4, 9, 13 and smaller amounts of serpin-3, 5, 6 in the complexes suggests that it is regulated by multiple serpins in hemolymph. We produced site-directed mutants of proHP1b for cleavage by bovine blood coagulation factor Xa at the designed proteolytic activation site, to generate a form of proHP1b that could be activated by Factor Xa. However, proHP1b cut by Factor Xa failed to activate proHP6 and, via HP6, proHP8 or proPAP1. This negative result is consistent with the suggestion that proHP1* is a physiological mediator of immune responses. Further research is needed to investigate the conformational change that results in conversion of proHP1 to active proHP1*.

Identification of Aminopeptidase-N2 as a Cry2Ab binding protein in Manduca sexta.

Bacillus thuringiensis Cry2Ab toxin has been used in combination with Cry1Ac for resistance management on the Bt-cotton that is widely planted worldwide. However, little is known regarding Cry2Ab mode of action. Particularly, there is a gap of knowledge on the identification of insect midgut proteins that bind Cry2Ab and mediate toxicity. In the case of Cry1Ab toxin, a transmembrane cadherin protein and glycosyl-phosphatidylinositol (GPI) anchored proteins like aminopeptidase-N1 (APN1) or alkaline-phosphatase (ALP) from Manduca sexta, have been shown to be important for oligomer formation and insertion into the membrane. Binding competition experiments showed that Cry2Ab toxin does not share binding sites with Cry1Ab toxin in M. sexta brush border membrane vesicles (BBMV). Also, that Cry2Ab shows reduced binding to the Cry1Ab binding molecules cadherin, APN1 or ALP. Finally, ligand blot experiments and protein sequence by LC-MS/MS identified APN2 isoform as a Cry2Ab binding protein. Cloning and expression of APN2 confirmed that APN2 is a Cry2Ab binding protein.

EPR Studies of V-ATPase with Spin-Labeled Inhibitors DCC and Archazolid: Interaction Dynamics with Proton Translocating Subunit†c.

Vacuolar-type H(+) -ATPases (V-ATPases) have gained recent attention as highly promising anticancer drug targets, and therefore detailed structural analyses and studies of inhibitor interactions are very important research objectives. Spin labeling of the V-ATPase holoenzyme from the tobacco hornworm Manduca sexta and V-ATPase in isolated yeast (Saccharomyces cerevisiae) vacuoles was accomplished by two novel methods involving the covalent binding of a (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) derivative of N,N'-dicyclohexylcarbodiimide (DCC) to the essential glutamate residue in the active site and the noncovalent interaction of a radical analogue of the highly potent inhibitor archazolid, a natural product from myxobacteria. Both complexes were evaluated in detail by electron paramagnetic resonance (EPR) spectroscopic studies and double electron-electron resonance (DEER) measurements, revealing insight into the inhibitor binding mode, dynamics, and stoichiometry as well as into the structure of the central functional subunit c of these medicinally important hetero-multimeric proton-translocating proteins. This study also demonstrates the usefulness of natural product derived spin labels as tools in medicinal chemistry.

Structure of the vacuolar H+-ATPase rotary motor reveals new mechanistic insights.

Vacuolar H(+)-ATPases are multisubunit complexes that operate with rotary mechanics and are essential for membrane proton transport throughout eukaryotes. Here we report a ∼ 1 nm resolution reconstruction of a V-ATPase in a different conformational state from that previously reported for a lower-resolution yeast model. The stator network of the V-ATPase (and by implication that of other rotary ATPases) does not change conformation in different catalytic states, and hence must be relatively rigid. We also demonstrate that a conserved bearing in the catalytic domain is electrostatic, contributing to the extraordinarily high efficiency of rotary ATPases. Analysis of the rotor axle/membrane pump interface suggests how rotary ATPases accommodate different c ring stoichiometries while maintaining high efficiency. The model provides evidence for a half channel in the proton pump, supporting theoretical models of ion translocation. Our refined model therefore provides new insights into the structure and mechanics of the V-ATPases.

Multicopper oxidase-1 orthologs from diverse insect species have ascorbate oxidase activity.

Members of the multicopper oxidase (MCO) family of enzymes can be classified by their substrate specificity; for example, ferroxidases oxidize ferrous iron, ascorbate oxidases oxidize ascorbate, and laccases oxidize aromatic substrates such as diphenols. Our previous work on an insect multicopper oxidase, MCO1, suggested that it may function as a ferroxidase. This hypothesis was based on three lines of evidence: RNAi-mediated knock down of Drosophila melanogaster MCO1 (DmMCO1) affects iron homeostasis, DmMCO1 has ferroxidase activity, and DmMCO1 has predicted iron binding residues. In our current study, we expanded our focus to include MCO1 from Anopheles gambiae, Tribolium castaneum, and Manduca sexta. We verified that MCO1 orthologs have similar expression profiles, and that the MCO1 protein is located on the basal surface of cells where it is positioned to oxidize substrates in the hemolymph. In addition, we determined that RNAi-mediated knock down of MCO1 in A. gambiae affects iron homeostasis. To further characterize the enzymatic activity of MCO1 orthologs, we purified recombinant MCO1 from all four insect species and performed kinetic analyses using ferrous iron, ascorbate and two diphenols as substrates. We found that all of the MCO1 orthologs are much better at oxidizing ascorbate than they are at oxidizing ferrous iron or diphenols. This result is surprising because ascorbate oxidases are thought to be specific to plants and fungi. An analysis of three predicted iron binding residues in DmMCO1 revealed that they are not required for ferroxidase or laccase activity, but two of the residues (His374 and Asp380) influence oxidation of ascorbate. These two residues are conserved in MCO1 orthologs from insects and crustaceans; therefore, they are likely to be important for MCO1 function. The results of this study suggest that MCO1 orthologs function as ascorbate oxidases and influence iron homeostasis through an unknown mechanism.

Overview of chitin metabolism enzymes in Manduca sexta: Identification, domain organization, phylogenetic analysis and gene expression.

Chitin is one of the most abundant biomaterials in nature. The biosynthesis and degradation of chitin in insects are complex and dynamically regulated to cope with insect growth and development. Chitin metabolism in insects is known to involve numerous enzymes, including chitin synthases (synthesis of chitin), chitin deacetylases (modification of chitin by deacetylation) and chitinases (degradation of chitin by hydrolysis). In this study, we conducted a genome-wide search and analysis of genes encoding these chitin metabolism enzymes in Manduca sexta. Our analysis confirmed that only two chitin synthases are present in M. sexta as in most other arthropods. Eleven chitin deacetylases (encoded by nine genes) were identified, with at least one representative in each of the five phylogenetic groups that have been described for chitin deacetylases to date. Eleven genes encoding for family 18 chitinases (GH18) were found in the M. sexta genome. Based on the presence of conserved sequence motifs in the catalytic sequences and phylogenetic relationships, two of the M. sexta chitinases did not cluster with any of the current eight phylogenetic groups of chitinases: two new groups were created (groups IX and X) and their characteristics are described. The result of the analysis of the Lepidoptera-specific chitinase-h (group h) is consistent with its proposed bacterial origin. By analyzing chitinases from fourteen species that belong to seven different phylogenetic groups, we reveal that the chitinase genes appear to have evolved sequentially in the arthropod lineage to achieve the current high level of diversity observed in M. sexta. Based on the sequence conservation of the catalytic domains and on their developmental stage- and tissue-specific expression, we propose putative functions for each group in each category of enzymes.

Sequence conservation, phylogenetic relationships, and expression profiles of nondigestive serine proteases and serine protease homologs in Manduca sexta.

Serine protease (SP) and serine protease homolog (SPH) genes in insects encode a large family of proteins involved in digestion, development, immunity, and other processes. While 68 digestive SPs and their close homologs are reported in a companion paper (Kuwar et al., in preparation), we have identified 125 other SPs/SPHs in Manduca sexta and studied their structure, evolution, and expression. Fifty-two of them contain cystine-stabilized structures for molecular recognition, including clip, LDLa, Sushi, Wonton, TSP, CUB, Frizzle, and SR domains. There are nineteen groups of genes evolved from relatively recent gene duplication and sequence divergence. Thirty-five SPs and seven SPHs contain 1, 2 or 5 clip domains. Multiple sequence alignment and molecular modeling of the 54 clip domains have revealed structural diversity of these regulatory modules. Sequence comparison with their homologs in Drosophila melanogaster, Anopheles gambiae and Tribolium castaneum allows us to classify them into five subfamilies: A are SPHs with 1 or 5 group-3 clip domains, B are SPs with 1 or 2 group-2 clip domains, C, D1 and D2 are SPs with a single clip domain in group-1a, 1b and 1c, respectively. We have classified into six categories the 125 expression profiles of SP-related proteins in fat body, brain, midgut, Malpighian tubule, testis, and ovary at different stages, suggesting that they participate in various physiological processes. Through RNA-Seq-based gene annotation and expression profiling, as well as intragenomic sequence comparisons, we have established a framework of information for future biochemical research of nondigestive SPs and SPHs in this model species.

Understanding the apparent stator-rotor connections in the rotary ATPase family using coarse-grained computer modeling.

Advances in structural biology, such as cryo-electron microscopy (cryo-EM) have allowed for a number of sophisticated protein complexes to be characterized. However, often only a static snapshot of a protein complex is visualized despite the fact that conformational change is frequently inherent to biological function, as is the case for molecular motors. Computer simulations provide valuable insights into the different conformations available to a particular system that are not accessible using conventional structural techniques. For larger proteins and protein complexes, where a fully atomistic description would be computationally prohibitive, coarse-grained simulation techniques such as Elastic Network Modeling (ENM) are often employed, whereby each atom or group of atoms is linked by a set of springs whose properties can be customized according to the system of interest. Here we compare ENM with a recently proposed continuum model known as Fluctuating Finite Element Analysis (FFEA), which represents the biomolecule as a viscoelastic solid subject to thermal fluctuations. These two complementary computational techniques are used to answer a critical question in the rotary ATPase family; implicit within these motors is the need for a rotor axle and proton pump to rotate freely of the motor domain and stator structures. However, current single particle cryo-EM reconstructions have shown an apparent connection between the stators and rotor axle or pump region, hindering rotation. Both modeling approaches show a possible role for this connection and how it would significantly constrain the mobility of the rotary ATPase family.

Use of molecular and in silico bioinformatic tools to investigate pesticide binding to insect (Lepidoptera) phenoloxidases (PO): insights to toxicological aspects.

In the present work, the promising bioinformatic tools, based on structure-affinity analysis, allowed to screen several pesticides supposed to bind to the insect immune Phenoloxidases (PO). First, the binding of aminoparathion, a reference compound, to the PO was structurally (3D) validated in accordance with previous reports. Second, using the same docking conditions, a range of pesticides was checked for their binding ability to the crystal 3D structure (PDB 3HSS) of the insect Manduca sexta (Lepidoptera) PO. The obtained data showed that many of the tested pesticides were able to bind, in silico, to M. sexta PO. The combination of in vitro (chemical and biochemical) and in silico (automated docking) approaches was found advantageous to elucidate the behavior of phenolic pesticides as substrate-analogues when binding to the active site of insect POs. Our findings emphasize new ecotoxicological aspects of pesticide residues in the agro-chemical and environmental circles.

Manduca sexta proprophenoloxidase activating proteinase-3 (PAP3) stimulates melanization by activating proPAP3, proSPHs, and proPOs.

Melanization participates in various insect physiological processes including antimicrobial immune responses. Phenoloxidase (PO), a critical component of the enzyme system catalyzing melanin formation, is produced as an inactive precursor prophenoloxidase (proPO) and becomes active via specific proteolytic cleavage by proPO activating proteinase (PAP). In Manduca sexta, three PAPs can activate proPOs in the presence of two serine proteinase homologs (SPH1 and SPH2). While the hemolymph proteinases (HPs) that generate the active PAPs are known, it is unclear how the proSPHs (especially proSPH1) are activated. In this study, we isolated from plasma of bar-stage M. sexta larvae an Ile-Glu-Ala-Arg-p-nitroanilide hydrolyzing enzyme that cleaved the proSPHs. This proteinase, PAP3, generated active SPH1 and SPH2, which function as cofactors for PAP3 in proPO activation. Cleavage of the purified recombinant proSPHs by PAP3 yielded 38 kDa bands similar in mobility to the SPHs formed in vivo. Surprisingly, PAP3 also can activate proPAP3 to stimulate melanization in a direct positive feedback loop. The enhanced proPO activation concurred with the cleavage activation of proHP6, proHP8, proPAP1, proPAP3, proSPH1, proSPH2, proPOs, but not proHP14 or proHP21. These results indicate that PAP3, like PAP1, is a key factor of the self-reinforcing mechanism in the proPO activation system, which is linked to other immune responses in M. sexta.

YC-1 binding to the ß subunit of soluble guanylyl cyclase overcomes allosteric inhibition by the α subunit.

Soluble guanylate cyclase (sGC) is a heterodimeric heme protein and the primary nitric oxide receptor. NO binding stimulates cyclase activity, leading to regulation of cardiovascular physiology and making sGC an attractive target for drug discovery. YC-1 and related compounds stimulate sGC both independently and synergistically with NO and CO binding; however, where the compounds bind and how they work remain unknown. Using linked equilibrium binding measurements, surface plasmon resonance, and domain truncations in Manduca sexta and bovine sGC, we demonstrate that YC-1 binds near or directly to the heme-containing domain of the ß subunit. In the absence of CO, YC-1 binds with a Kd of 9-21 µM, depending on the construct. In the presence of CO, these values decrease to 0.6-1.1 µM. Pfizer compound 25 bound ∼10-fold weaker than YC-1 in the absence of CO, whereas compound BAY 41-2272 bound particularly tightly in the presence of CO (Kd = 30-90 nM). Additionally, we found that CO binds much more weakly to heterodimeric sGC proteins (Kd = 50-100 µM) than to the isolated heme domain (Kd = 0.2 µM for Manduca ß H-NOX/PAS). YC-1 greatly enhanced binding of CO to heterodimeric sGC, as expected (Kd ∼ 1 µM). These data indicate the α subunit induces a heme pocket conformation with a lower affinity for CO and NO. YC-1 family compounds bind near the heme domain, overcoming the α subunit effect and inducing a heme pocket conformation with high affinity. We propose this high-affinity conformation is required for the full-length protein to achieve high catalytic activity.

Subunit positioning and stator filament stiffness in regulation and power transmission in the V1 motor of the Manduca sexta V-ATPase.

The vacuolar H(+)-ATPase (V-ATPase) is an ATP-driven proton pump essential to the function of eukaryotic cells. Its cytoplasmic V1 domain is an ATPase, normally coupled to membrane-bound proton pump Vo via a rotary mechanism. How these asymmetric motors are coupled remains poorly understood. Low energy status can trigger release of V1 from the membrane and curtail ATP hydrolysis. To investigate the molecular basis for these processes, we have carried out cryo-electron microscopy three-dimensional reconstruction of deactivated V1 from Manduca sexta. In the resulting model, three peripheral stalks that are parts of the mechanical stator of the V-ATPase are clearly resolved as unsupported filaments in the same conformations as in the holoenzyme. They are likely therefore to have inherent stiffness consistent with a role as flexible rods in buffering elastic power transmission between the domains of the V-ATPase. Inactivated V1 adopted a homogeneous resting state with one open active site adjacent to the stator filament normally linked to the H subunit. Although present at 1:1 stoichiometry with V1, both recombinant subunit C reconstituted with V1 and its endogenous subunit H were poorly resolved in three-dimensional reconstructions, suggesting structural heterogeneity in the region at the base of V1 that could indicate positional variability. If the position of H can vary, existing mechanistic models of deactivation in which it binds to and locks the axle of the V-ATPase rotary motor would need to be re-evaluated.