The role of electrostatics in colicin nuclease domain translocation into bacterial cells

The mechanism(s) by which nuclease colicins translocate distinct cytotoxic enzymes (DNases, rRNases, and tRNases) to the cytoplasm of Escherichia coli is unknown. Previous in vitro investigations on isolated colicin nuclease domains have shown that they have a strong propensity to associate with anionic phospholipid vesicles, implying that electrostatic interactions with biological membranes play a role in their import. In the present work we set out to test this hypothesis in vivo. We show that cell killing by the DNase toxin colicin E9 of E. coli HDL11, a strain in which the level of anionic phospholipid and hence inner membrane charge is regulated by isopropyl beta-D-thiogalactopyranoside induction, is critically dependent on the level of inducer, whereas this is not the case for pore-forming colicins that take the same basic route into the periplasm. Moreover, there is a strong correlation between the level and rate of HDL11 cell killing and the net positive charge on a colicin DNase, with similar effects seen for wild type E. coli cells, data that are consistent with a direct, electrostatically mediated interaction between colicin nucleases and the bacterial inner membrane. We next sought to identify how membrane-associated colicin nucleases might be translocated into the cell. We show that neither the Sec or Tat systems are involved in nuclease colicin uptake but that nuclease colicin toxicity is instead dependent on functional FtsH, an inner membrane AAA(+) ATPase and protease that dislocates misfolded membrane proteins to the cytoplasm for destruction.     

Essential role of the N-terminal domain in the regulation of RIG-I ATPase activity

Retinoic acid-inducible gene I (RIG-I) is a cytosolic receptor that recognizes viral RNA and activates the interferon-mediated innate antiviral response. To understand the mechanism of signal activation at the receptor level, we cloned, expressed, and purified human RIG-I containing the two caspase activation and recruitment domains (CARDs) followed by the C-terminal helicase domain. We found that recombinant RIG-I is a functional protein that interacts with double-stranded RNA with substantially higher affinity as compared with single-stranded RNA structures unless they contain a 5'-triphosphate group. Viral RNA binding to RIG-I stimulates the velocity of ATP hydrolysis by 33-fold, which at the cellular level translates into a 43-fold increase of interferon-beta expression. In contrast, the isolated ATPase/helicase domain is constitutively activated while also retaining its RNA ligand binding properties. These results support the recent model by which RIG-I signaling is autoinhibited in the absence of RNA by intra-molecular interactions between the CARDs and the C terminus. Based on pH profile and metal ion dependence experiments, we propose that the active site of RIG-I cannot efficiently accommodate divalent cations under the RNA-free repressed conformation. Overall, these results show a direct correlation between RNA binding and ATPase enzymatic function leading to signal transduction and suggest that a tight control of ATPase activity by the CARDs prevents RIG-I signaling in the absence of viral RNA.     

Proton translocation by ATPase and bacteriorhodopsin.

Stable membrane proteins and lipids are convenient to study biomembranes. Two stable proton translocating proteins were purified and reconstituted into vesicles capable of proton translocation. One was a thermostable ATPase (TF0-F1) of thermophilic bacterium PS3 and the other was rhodopsin of Halobacterium halobium. TF0-F1 was composed of a proton pump moiety (TF1) and a proton channel moiety (TF0). TF1 was the first membrane ATPase which was crystallized and reconstituted from its five polypeptides. Like TF0 and TF1, the rhodopsin in purple membrane was highly stable against dissociating agents, acids and alkali. Phospholipids of these biomembranes were also stable and contained no unsaturated fatty acyl groups. The molecular species of the phospholipids of PS3 were determined by mass chromatography. Measurements were made of the difference in electrochemical potential of protons (deltamicronH+) across the membrane of the reconstituted vesicles. The deltamicronH+ attained was 312 mV in TF0-F1 vesciles and was 230 mV in the rhodopsin vesicles. To conclude that electron transport components are not necessary for ATP synthesis in energy yielding biomembranes, two experiments were performed: The ATP synthesis was observed i) on acid-base treatment of TF0-F1 vesicles, and ii) on illumination of the rhodopsin-TF0-F1 vesicles.     

The ATPase domain of SecA can form a tetramer in solution.

Preprotein translocase is a general and essential system for bacterial protein export, the minimal components of which are SecA and SecYEG. SecA is a peripheral ATPase that associates with nucleotide, preprotein, and the membrane integral SecYEG to form a translocation-competent complex. SecA can be separated into two domains: an N-terminal 68 kDa ATPase domain (N68) that binds preprotein and catalyzes ATP hydrolysis, and a 34 kDa C-terminal domain that regulates the ATPase activity of N68 and mediates dimerization. We have carried out gel filtration chromatography, analytical ultracentrifugation, and small-angle X-ray scattering (SAXS) to demonstrate that isolated N68 self-associates to form a tetramer in solution, indicating that removal of the C-terminal domain facilitates the formation of a higher-order SecA structure. The associative process is best modelled as a monomer-tetramer equilibrium, with a K(D) value of 63 microM(3) (where K(D)=[monomer](4)/[tetramer]) so that at moderate concentrations (10 microM and above), the tetramer is the major species in solution. Hydrodynamic properties of the N68 monomer indicate that it is almost globular in shape, but the N68 tetramer has a more ellipsoidal structure. Analysis of SAXS data indicates that the N68 tetramer is a flattened, bi-lobed structure with dimensions of approximately 13.5 nm x 9.0 nm x 6.5 nm, that appears to contain a central pore.     

In vitro translocation of secretory proteins possessing no charges at the mature domain takes place efficiently in a protonmotive force-dependent manner.

The effect of charges existing on the mature domain of secretory proteins on the efficiency and protonmotive force dependence of translocation into everted membrane vesicles of Escherichia coli was studied. Model secretory proteins devoid of charges on the mature domain were constructed at the DNA level using proOmpF-Lpp as the starting protein. The chargeless presecretory proteins thus constructed were translocated and processed for the signal peptide much faster than proOmpF-Lpp and the rate of translocation was appreciably enhanced by imposition of the protonmotive force. Not only the membrane potential but also delta pH were effective in stimulating the rate of translocation of the chargeless proteins. The results indicate that the mature domain does not have to be charged for the secretory translocation and that the major requirement of the protonmotive force for the secretory translocation is not for the movement, including an electrophoretic one, of charged regions of the mature domain. All of the proOmpF-Lpp derivatives thus constructed were translocated efficiently into everted membrane vesicles in a SecA-dependent manner, irrespective of their size. The mature domain of the smallest one was 45 amino acid residues in length. Contrary to the views previously presented by other workers, these results suggest that there is no sharp boundary at the reported regions for the translocation of presecretory proteins across the cytoplasmic membrane or for the requirement of SecA.     

The DNA translocation and ATPase activities of restriction-deficient mutants of Eco KI.

Eco KI, a type I restriction enzyme, specifies DNA methyltransferase, ATPase, endonuclease and DNA translocation activities. One subunit (HsdR) of the oligomeric enzyme contributes to those activities essential for restriction. These activities involve ATP-dependent DNA translocation and DNA cleavage. Mutations that change amino acids within recognisable motifs in HsdR impair restriction. We have used an in vivo assay to monitor the effect of these mutations on DNA translocation. The assay follows the Eco KI-dependent entry of phage T7 DNA from the phage particle into the host cell. Earlier experiments have shown that mutations within the seven motifs characteristic of the DEAD-box family of proteins that comprise known or putative helicases severely impair the ATPase activity of purified enzymes. We find that the mutations abolish DNA translocation in vivo. This provides evidence that these motifs are relevant to the coupling of ATP hydrolysis to DNA translocation. Mutations that identify an endonuclease motif similar to that found at the active site of type II restriction enzymes and other nucleases have been shown to abolish DNA nicking activity. When conservative changes are made at these residues, the enzymes lack nuclease activity but retain the ability to hydrolyse ATP and to translocate DNA at wild-type levels. It has been speculated that nicking may be necessary to resolve the topological problems associated with DNA translocation by type I restriction and modification systems. Our experiments show that loss of the nicking activity associated with the endonuclease motif of Eco KI has no effect on ATPase activity in vitro or DNA translocation of the T7 genome in vivo.     

F-actin in guinea pig spermatozoa: its role in calmodulin translocation during acrosome reaction.

The presence of actin has been determined in mammalian spermatozoa. However, its function in these cells is still almost unknown. Only in boar spermatozoa has evidence for F-actin and a possible function for it been presented. In this work, actin distribution and F-actin were determined in uncapacitated, capacitated, and acrosomal-reacted guinea pig spermatozoa, by means of monoclonal and polyclonal antibodies, using an indirect immunoperoxidase technique, and by the use of rhodamine-phalloidin. With the last probe we found filamentous actin in these cells. By both techniques, actin was detected in the acrosome and in the entire tail. In some cells with acrosomal reaction, actin was also detected in the equatorial and in the postacrosomal regions. SDS-PAGE and Western blots immunostained with monoclonal and polyclonal anti-actin antibodies confirmed the presence of actin in extracts of guinea pig spermatozoa. Actin was also detected in preparations of Percoll-purified spermatozoa. We have communicated that guinea pig spermatozoa show a change on calmodulin location during the acrosome reaction. They present it first in the equatorial region and later in the postacrosomal region. To determine if F-actin participates in this calmodulin translocation, we studied the effect of cytochalasin D. It was found that the number of cells with calmodulin in the equatorial region increased in the presence of cytochalasin D while the number of cells with calmodulin in the postacrosomal region decreased. We also found that after cytochalasin D treatment acrosome loss was increased and sperm motility was slightly inhibited. Our results suggest that actin participate in calmodulin translocation to the postacrosomal region during acrosome reaction, in maintaining the acrosome structure, and perhaps also in sperm motility.     

The role of the polar, carboxyl-terminal domain of Escherichia coli leader peptidase in its translocation across the plasma membrane

Leader peptidase, an integral membrane protein of Escherichia coli, is made without a cleavable leader sequence. It has 323 amino acid residues and spans the plasma membrane with a small amino-terminal domain exposed to the cytoplasm and a large, carboxyl-terminal domain exposed to the periplasm. We have investigated which regions of leader peptidase are necessary for its assembly across the membrane. Deletions were made in the carboxyl-terminal domain of leader peptidase, removing residues 141-222, 142-323, or 222-323. Protease accessibility was used to determine whether the polar, carboxyl-terminal domains of these truncated leader peptidases were translocated across the membrane. The removal of either residues 222-323 (the extreme carboxyl terminus) or residues 141-222 does not prevent leader peptidase membrane assembly. However, leader peptidase lacking both regions, i.e. amino acid residues 142-323, cannot translocate the remaining portion of its carboxyl terminus across the membrane. Our data suggest that the polar, periplasmic domain of leader peptidase contains information which is needed for membrane assembly.     

Energy transduction in Escherichia coli. The role of the Mg2+ATPase.

Inverted membrane vesicles from strain 7, a wild type Escherichia coli K12 strain, actively transport calcium with energy supplied either by respiration or by ATP. These vesicles also have energy-linked quenching of quinacrine fluorescence. Membranes of strain 7, depleted of Mg2+ATPase by EDTA treatment, lack both activities. Membrane vesicles from strain NR70, a mutant lacking the Mg2+ATPase, show neither calcium transport nor energy-linked fluorescence quenching. Neither EDTA treatment nor genetic loss of the Mg2+atpase causes a reduction in respiration. Purified Mg2+ATPase from strain 7 can bind to EDTA-treated membrane vesicles from either strain 7 or NR70. This binding restored both calcium transport and fluorescence quenching, driven either by respiration or by ATP. Dicyclohexylcarbodiimide treatment mimics the effect of the Mg2+ATPase in the case of respiration-driven reactions. Treatment with EDTA, while not essential for the binding of the Mg2+ATPase to membrane vesicles of NR70, produced better restoration of both activities. The rate of restoration of fluorescence quenching showed a time lag which may indicate that binding of the Mg2+ATPase is a relatively slow process. Antiserum prepared against the Mg2+ATPase inhibited the quenching of quinacrine fluorescence when driven by ATP but not when driven by respiration. Addition of antiserum prior to addition of Mg2+ATPase prevented the restoration of fluorescence quenching, whether driven by respiration or ATP. These results clearly show that MG2+ATPase has an important role not only in catalyzing ATP synthesis and hydrolysis but also in maintaining the energized membrane state.     

The bacterial ATPase SecA functions as a monomer in protein translocation

The ATPase SecA drives the post-translational translocation of proteins through the SecY channel in the bacterial inner membrane. SecA is a dimer that can dissociate into monomers under certain conditions. To address the functional importance of the monomeric state, we generated an Escherichia coli SecA mutant that is almost completely monomeric (>99%), consistent with predictions from the crystal structure of Bacillus subtilis SecA. In vitro, the monomeric derivative retained significant activity in various assays, and in vivo, it sustained 85% of the growth rate of wild type cells and reduced the accumulation of precursor proteins in the cytoplasm. Disulfide cross-linking in intact cells showed that mutant SecA is monomeric and that even its parental dimeric form is dissociated. Our results suggest that SecA functions as a monomer during protein translocation in vivo.     

The catalytic cycle of the escherichia coli SecA ATPase comprises two distinct preprotein translocation events.

SecA is the ATP-dependent force generator in the Escherichia coli precursor protein translocation cascade, and is bound at the membrane surface to the integral membrane domain of the preprotein translocase. Preproteins are thought to be translocated in a stepwise manner by nucleotide-dependent cycles of SecA membrane insertion and de-insertion, or as large polypeptide segments by the protonmotive force (Deltap) in the absence of SecA. To determine the step size of a complete ATP- and SecA-dependent catalytic cycle, translocation intermediates of the preprotein proOmpA were generated at limiting SecA translocation ATPase activity. Distinct intermediates were formed, spaced by intervals of approximately 5 kDa. Inhibition of the SecA ATPase by azide trapped SecA in a membrane-inserted state and shifted the step size to 2-2.5 kDa. The latter corresponds to the translocation elicited by binding of non-hydrolysable ATP analogues to SecA, or by the re-binding of partially translocated polypeptide chains by SecA. Therefore, a complete catalytic cycle of the preprotein translocase permits the stepwise translocation of 5 kDa polypeptide segments by two consecutive events, i.e. approximately 2.5 kDa upon binding of the polypeptide by SecA, and another 2.5 kDa upon binding of ATP to SecA.     

ATPase in mature and differentiating phloem and xylem

A cytochemical study using a lead precipitation technique has been made of the distribution of adenosine triphosphatase (ATPase) in mature and differentiating phloem and xylem cells of Nicotiana tabacum and Pisum sativum. The sites of ATPase localization in tobacco phloem were the plasma membrane, endoplasmic reticulum, mitochondria, dictyosomes, plasmodesmata, and the dispersed P proteins of mature sieve elements. In pea phloem sieve elements ATPase was localized in the endoplasmic reticulum, but was not associated with the P proteins or plasma membranes at any stage of their differentiation. In pea transfer cells ATPase activity was associated with the endoplasmic reticulum at all stages of their differentiation and with the plasma membrane of transfer cells that had formed wall ingrowths. In xylem cells of both tobacco and pea the patterns of ATPase activity was similar. At early stages of differentiation ATPase activity was associated with the plasma membrane and the endoplasmic reticulum. At intermediate stages of differentiation ATPase activity continued to be associated with the endoplasmic reticulum, but was no longer associated with the plasma membrane. At later stages of xylem element differentiation ATPase activity was associated with disintegrating organelles and with the hydrolyzing cell walls.     

The Hsp70 peptide-binding domain determines the interaction of the ATPase domain with Tim44 in mitochondria

Ssc1, a molecular chaperone of the Hsp70 family, drives preprotein import into the mitochondrial matrix by a specific interaction with the translocase component Tim44. Two other mitochondrial Hsp70s, Ssc3 (Ecm10) and Ssq1, show high sequence homology to Ssc1 but fail to replace Ssc1 in vivo, possibly due to their inability to interact with Tim44. We analyzed the structural basis of the Tim44 interaction by the construction of chimeric Hsp70 proteins. The ATPase domains of all three mitochondrial Hsp70s were shown to bind to Tim44, supporting the active motor model for the Hsp70 mechanism during preprotein translocation. The peptide-binding domain of Ssc1 sustained binding of Tim44, while the peptide-binding domains of Ssc3 and Ssq1 exerted a negative effect on the interaction of the ATPase domains with Tim44. A mutation in the peptide-binding domain of Ssc1 resulted in a similar negative effect not only on the ATPase domain of Ssc1, but also of Ssq1 and Ssc3. Hence, the determination of a crucial Hsp70 function via the peptide-binding domain suggests a new regulatory principle for Hsp70 domain cooperation.     

The role of the mature part of secretory proteins in translocation across the plasma membrane and in regulation of their synthesis in Escherichia coli

Presently available data are reviewed which concern the role of the mature parts of secretory precursor proteins in translocation across the plasma membrane of Escherichia coli. The following conclusions can be drawn; i) signals, acting in a positive fashion and required for translocation do not appear to exist in the mature polypeptides; ii) a number of features have been identified which either affect the efficiency of translocation or cause export incompatibility. These are: alpha) protein folding prior to translocation; beta) restrictions regarding the structure of N-terminus; gamma) presence of lipophilic anchors; delta) too low a size of the precursor. Efficiency of translocation is also enhanced by binding of chaperonins (SecB, trigger factor, GroEL) to precursors. Binding sites for chaperonins appear to exist within the mature parts of the precursors but the nature of these sites has remained rather mysterious. Mutant periplasmic proteins with a block in release from the plasma membrane have been described, the mechanism of this block is not known. The mature parts of secretory proteins can also be involved in the regulation of their synthesis. It appears that exported proteins are already recognized as such before they are channelled into the export pathway and that their synthesis can be feed-back inhibited at the translational level.     

The role of reductants in the tyrosine hydroxylase reaction.

The role of several reducing systems in the tyrosine hydroxylase reaction has been studied. A significant dependence upon the reducing systems beyond that required to regenerate the oxidized cofactor has been observed. 2-Mercaptoethanol, NADPH, and ascorbate are each effective at reducing the cofactor, but their abilities to stimulate tyrosine hydroxylase vary over a threefold range. NADPH is a suitable reductant for the tyrosine hydroxylase reaction, even in the absence of pteridine reductase. A reducing system containing ascorbate, ferrous ion, and catalase gives unusually high enzyme activity and low blanks. This ascorbate system, in addition to being useful for in vitro enzyme assays, may serve as a model for the in vivo reaction. Ascorbate may play an important role in the hydroxylation of tyrosine in catecholaminergic tissues. This study demonstrates that an efficient reductant for the tyrosine hydroxylase reaction must, in addition to reducing the pterin cofactor, also interact effectively with the enzyme itself.     

The role of ascorbate in the prolyl hydroxylase reaction.

Binding isotherms of some anthracyclic derivatives with DNA have been determined by modern electrochemical techniques(Voltammetry - ac polarography). These techniques making use of the difference between the diffusion coefficients of DNA-drug complex and of free molecules of drug in solution allow the direct determination of the later ones. No striking differences can be noticed between the tested anthracyclines. No correlation can be established between the affinity of daunorubicin and some of its analogs for DNA and their more or less antitumoral activity.     

The role of bacterial pili in protein and DNA translocation.

Gram-negative bacteria have surface appendages that assemble via different secretion machineries. Recently, new experimental approaches have contributed to a better understanding of the molecular mechanisms of flagellar and pilus assembly, and protein secretion. These findings can be applied to plant pathogenic bacteria, which probably transfer effector proteins directly into their eukaryotic host cells. Here, it is suggested that assembly of Hrp pili occurs in the periplasm and that unfolded effector proteins attach to pilins within the pili, thus effecting protein translocation. A two-domain structure for the HrpA pilin from Pseudomonas syringae is also predicted.     

The role of phospholipid acyl chains in the activation of mitochondrial ATPase complex.

1. The role of length and unsaturation of phospholipid acyl chains in the activation of ATPase complex was studied with synthetic phosphatidylcholines and a phospholipid-dependent preparation obtained after cholate-extraction of submitochondrial particles (Kagawa, Y. and Racker, E. (1966) J. Biol. Chem. 241, 2467--2474). 2. Micelle-forming, short-chain phosphatidylcholines produced activation only at critical micellar concentration. The reactivated complex was cold-stable but the oligomycin sensitivity was low. 3. Bilayer-forming saturated phosphatidylcholines produced activation which was maximal at 9 carbon atoms in each chain but decreased sharply as the chain-length was increased and essentially disappeared at 14 carbon atoms. By contrast the oligomycin-sensitivity increased with the increase in chain length. 4. Activation of ATPase complex reappeared when bilayers were formed with long-chain unsaturated phosphatidylcholines. The activity was oligomycin sensitive. Significant inhibition of activity was observed also after incorporation of cholesterol into the bilayers. 5. By contrast the activation induced by negatively charged liposomes of diacylphosphatidylglycerol was independent on acyl-chain composition and occurred at very low amounts of phospholipid. 6. The discontinuity in the Arrhenius plot of activity of the ATPase complex reactivated with saturated phospholipids was found at temperatures close to the gel-to-liquid crystalline transition of the lipid showing that the activity of ATPase complex was sensitive to the physical state of membrane phospholipids. 7. It is concluded that (a) reactivation of ATPase complex by isoelectric phospholipids is an interfacial activation, the minimum requirement for the lipid effect being micelle formation. (b) In order to gain the properties of the native complex a stable lamellar phase is needed. Both activity and oligomycin sensitivity are regulated by the chain length and degree of unsaturation of phospholipid acyl chains.