Overproduction of truncated subunit a of H+-ATPase causes growth inhibition of Escherichia coli

ATP Synthesis by Oxidative Phosphorylation

The F1F0-ATP synthase (EC 3.6.1.34) is a remarkable enzyme that functions as a rotary motor. It is found in the inner membranes of Escherichia coli and is responsible for the synthesis of ATP in response to an electrochemical proton gradient. Under some conditions, the enzyme functions reversibly and uses the energy of ATP hydrolysis to generate the gradient. The ATP synthase is composed of eight different polypeptide subunits in a stoichiometry of α3β3γδεab2c10. Traditionally they were divided into two physically separable units: an F1 that catalyzes ATP hydrolysis (α3β3γδε) and a membrane-bound F0 sector that transports protons (ab2c10). In terms of rotary function, the subunits can be divided into rotor subunits (γεc10) and stator subunits (α3β3δab2). The stator subunits include six nucleotide binding sites, three catalytic and three noncatalytic, formed primarily by the β and α subunits, respectively. The stator also includes a peripheral stalk composed of δ and b subunits, and part of the proton channel in subunit a. Among the rotor subunits, the c subunits form a ring in the membrane, and interact with subunit a to form the proton channel. Subunits γ and ε bind to the c-ring subunits, and also communicate with the catalytic sites through interactions with α and β subunits. The eight subunits are expressed from a single operon, and posttranscriptional processing and translational regulation ensure that the polypeptides are made at the proper stoichiometry. Recent studies, including those of other species, have elucidated many structural and rotary properties of this enzyme.

Time-delayed in vivo assembly of subunit a into preformed Escherichia coli FoF1 ATP synthase

Escherichia coli F(O)F(1) ATP synthase, a rotary nanomachine, is composed of eight different subunits in a α3β3γδεab2c10 stoichiometry. Whereas F(O)F(1) has been studied in detail with regard to its structure and function, much less is known about how this multisubunit enzyme complex is assembled. Single-subunit atp deletion mutants are known to be arrested in assembly, thus leading to formation of partially assembled subcomplexes. To determine whether those subcomplexes are preserved in a stable standby mode, a time-delayed in vivo assembly system was developed. To establish this approach, we targeted the time-delayed assembly of membrane-integrated subunit a into preformed F(O)F(1) lacking subunit a (F(O)F(1)-a) which is known to form stable subcomplexes in vitro. Two expression systems (araBADp and T7p-laco) were adjusted to provide compatible, mutually independent, and sufficiently stringent induction and repression regimens. In detail, all structural atp genes except atpB (encoding subunit a) were expressed under the control of araBADp and induced by arabinose. Following synthesis of F(O)F(1)-a during growth, expression was repressed by glucose/d-fucose, and degradation of atp mRNA controlled by real-time reverse transcription-PCR. A time-delayed expression of atpB under T7p-laco control was subsequently induced in trans by addition of isopropyl-β-d-thiogalactopyranoside. Formation of fully assembled, and functional, F(O)F(1) complexes was verified. This demonstrates that all subunits of F(O)F(1)-a remain in a stable preformed state capable to integrate subunit a as the last subunit. The results reveal that the approach presented here can be applied as a general method to study the assembly of heteromultimeric protein complexes in vivo.

FtsH-dependent degradation of phage shock protein C in Yersinia enterocolitica and Escherichia coli

The widely conserved phage shock protein (Psp) extracytoplasmic stress response has been studied extensively in Escherichia coli and Yersinia enterocolitica. Both species have the PspF, -A, -B, and -C proteins, which have been linked to robust phenotypes, including Y. enterocolitica virulence. PspB and PspC are cytoplasmic membrane proteins required for stress-dependent induction of psp gene expression and for bacterial survival during the mislocalization of outer membrane secretin proteins. Previously, we reported that Y. enterocolitica PspB functions to positively control the amount of PspC by an uncharacterized posttranscriptional mechanism. In this study, we have discovered that the cytoplasmic membrane protease FtsH is involved in this phenomenon. FtsH destabilizes PspC in Y. enterocolitica, but coproduction of PspC with its binding partner PspB was sufficient to prevent this destabilization. In contrast, FtsH did not affect any other core component of the Psp system. These data suggested that uncomplexed PspC might be particularly deleterious to the bacterial cell and that FtsH acts as an important quality control mechanism to remove it. This was supported by the observation that toxicity caused by PspC production was reduced either by coproduction of PspB or by increased synthesis of FtsH. We also found that the phenomenon of FtsH-dependent PspC destabilization is conserved between Y. enterocolitica and E. coli.

Subunit a of the F(1)F(0) ATP synthase requires YidC and SecYEG for membrane insertion

The insertion of inner membrane proteins in Escherichia coli occurs almost exclusively via the SecYEG pathway, while some membrane proteins require the membrane protein insertase YidC. In vitro analysis demonstrates that subunit a of the F(1)F(0) ATP synthase (F(0)a) is strictly dependent on Ffh, SecYEG and YidC for its membrane insertion but independent of the proton motive force. The insertion of the first transmembrane segment of F(0)a also depends on Ffh and SecYEG but not on YidC, whereas the insertion is strongly dependent on the proton motive force, unlike the full-length F(0)a protein. These data demonstrate an extensive role of YidC in the assembly of the F(0) sector of the F(1)F(0) ATP synthase.

The C-terminally truncated NuoL subunit (ND5 homologue) of the Na+-dependent complex I from Escherichia coli transports Na+

The NADH:quinone oxidoreductase (complex I) from Escherichia coli acts as a primary Na+ pump. Expression of a C-terminally truncated version of the hydrophobic NuoL subunit (ND5 homologue) from E. coli complex I resulted in Na+-dependent growth inhibition of the E. coli host cells. Membrane vesicles containing the truncated NuoL subunit (NuoLN) exhibited 2-4-fold higher Na+ uptake activity than control vesicles without NuoLN. Respiratory proton transport into inverted vesicles containing NuoLN decreased upon addition of Na+, but was not affected by K+, indicating a Na+-dependent increase of proton permeability of membranes in the presence of NuoLN. The His-tagged NuoLN protein was solubilized, enriched by affinity chromatography, and reconstituted into proteoliposomes. Reconstituted His6-NuoLN facilitated the uptake of Na+ into the proteoliposomes along a concentration gradient. This Na+ uptake was prevented by EIPA (5-(N-ethyl-N-isopropyl)-amiloride), which acts as inhibitor against Na+/H+ antiporters.

Over-expression of Escherichia coli F1F(o)-ATPase subunit a is inhibited by instability of the uncB gene transcript

Little is known about the stability of transcripts encoding membrane proteins in strong expression systems and its effect on membrane protein over-production. We have expressed all the genes encoding subunits of the membrane domain F(o) of the ATP synthase in a T7 RNA polymerase-based system. All of them but uncB (subunit a) were expressed separately at very high levels in the bacterial hosts Escherichia coli C41(DE3) and C43(DE3). However, expression of uncB was extremely toxic to the bacteria. Northern blot analysis showed that the level of accumulation of the mRNA from uncB was very low. Deletion of uncB in combination with gene fusion experiments demonstrated that the middle region of the gene, encoding amino acids 92-171, exhibited a dominant toxic phenotype associated with a very poor level of expression. Green fluorescent protein fusions with N- and C-ends of uncB helped to stabilize the mRNA and to obtain high yields of protein.

A model for the structure of subunit a of the Escherichia coli ATP synthase and its role in proton translocation

Most of what is known about the structure and function of subunit a, of the ATP synthase, has come from the construction and isolation of mutations, and their analysis in the context of the ATP synthase complex. Three classes of mutants will be considered in this review. (1) Cys substitutions have been used for structural analysis of subunit a, and its interactions with subunit c. (2) Functional residues have been identified by extensive mutagenesis. These studies have included the identification of second-site suppressors within subunit a. (3) Disruptive mutations include deletions at both termini, internal deletions, and single amino acid insertions. The results of these studies, in conjunction with information about subunits b and c, can be incorporated into a model for the mechanism of proton translocation in the Escherichia coli ATP synthase.

Subunit a of proton ATPase F0 sector is a substrate of the FtsH protease in Escherichia coli

Escherichia coli FtsH is a membrane-bound ATPase with a proteolytic activity against the SecY subunit of protein translocase. We now report that subunit a of the membrane-embedded Fo part of H+-ATPase is another substrate of FtsH. Pulse-chase experiments showed that subunit a is unstable when it alone (without Fo subunits b and c) was oversynthesized and that it is stabilized in the ftsH mutants. Selective and ATP-dependent degradation of subunit a by purified FtsH protein was demonstrated in vitro. These results suggest that FtsH serves as a quality-control mechanism to avoid potentially harmful accumulation of free subunit a in the membrane.

Assembly of F0 sector of Escherichia coli H+ ATP synthase. Interdependence of subunit insertion into the membrane

The F0 sector of the Escherichia coli H+ transporting ATP synthase is composed of a complex of three subunits, each of which traverses the inner membrane. We have studied the interdependence of subunit insertion into the membrane in a series of chromosomal mutants in which the primary mutation prevented insertion of one of the F0 subunits. Subunit insertion was assessed using Western blots of mutant membrane preparations. Subunit b and subunit c were found to insert into the membrane independently of the other two F0 subunits. On the other hand, subunit a was not inserted into membranes that lacked either subunit b or subunit c. The conclusion that subunit a insertion is dependent upon the co-insertion of subunits b and c differs from the conclusion of several studies, where subunits were expressed from multicopy plasmids.

Molecular organization of the Escherichia coli gab cluster: nucleotide sequence of the structural genes gabD and gabP and expression of the GABA permease gene

We have determined the nucleotide sequences of two structural genes of the Escherichia coli gab cluster, which encodes the enzymes of the 4-aminobutyrate degradation pathway: gabD, coding for succinic semialdehyde dehydrogenase (SSDH, EC 1.2.1.16) and gabP, coding for the 4-aminobutyrate (GABA) transport carrier (GABA permease). We have previously reported the nucleotide sequence of the third structural gene of the cluster, gabT, coding for glutamate: succinic semialdehyde transaminase (EC 2.6.1.19). All three gab genes are transcribed unidirectionally and their orientation within the cluster is 5'-gabD-gabT-gabP-3'. gabT and gabP are separated by an intergenic region of 234-bp, which contains three repetitive extragenic palindromic (REP) sequences. The gabD gene consists of 1,449 nucleotides specifying a protein of 482 amino acids with a molecular mass of 51.7 kDa. The protein shows significant homologies to the NAD(+)-dependent aldehyde dehydrogenase (EC 1.2.1.3) from Aspergillus nidulans and several mammals, and to the tumor associated NADP(+)-dependent aldehyde dehydrogenase (EC 1.2.1.4) from rat. The permease gene gabP comprises 1,401 nucleotides coding a highly hydrophobic protein of 466 amino acids with a molecular mass of 51.1 kDa. The GABA permease shows features typical for an integral membrane protein and is highly homologous to the aromatic acid carrier from E. coli, the proline, arginine and histidine permeases from Saccharomyces cerevisiae and the proline transport protein from A. nidulans. Uptake of GABA was increased ca. 5-fold in transformants of E. coli containing gabP plasmids.(ABSTRACT TRUNCATED AT 250 WORDS)

RNase E-dependent cleavages in the 5' and 3' regions of the Escherichia coli unc mRNA

The endonucleolytic processing of the unc mRNA encoding the eight subunits of the Escherichia coli F1F0-ATPase was studied. Northern (RNA) blots of mRNA expressed from a plasmid which contained the 3'-terminal portion of the operon including the uncDC sequences revealed, in addition to the expected 2-kb mRNA, a 0.5-kb RNA species which hybridized to an uncC antisense RNA probe. An uncD antisense RNA probe hybridized to only the 2-kb mRNA, implying that the upstream 1.5-kb fragment is rapidly degraded. The 5' end of the 0.5-kb fragment was determined by primer extension analysis to be 11 bases into the coding region of the uncC gene. In RNase E-deficient strains, the amount of the 0.5-kb product was strongly reduced while the levels of the precursor uncDC transcript remained high. Similar RNase E-dependent processing was found in the chromosomally encoded unc mRNA. As this RNase E-dependent cleavage directly inactivates uncC and appears to leave uncD susceptible to degradation, it seems unlikely to play a role in differential expression of the gene products but may be an important event in unc mRNA degradation. RNase E mutants also showed altered processing of the chromosomally encoded unc mRNA in the uncB region near the 5' end. The expected full-length (7-kb) transcript was recognized when RNA from the RNase E-deficient strain was subjected to Northern blot analysis with uncB- and uncC-specific probes. RNA from strains with functional RNase E lacked the 7-kb transcript but had a 6.2-kb mRNA detectable with the uncC but not the uncB probe. RNase E is therefore implicated in multiple cleavages of the unc mRNA.

Characterization of the Escherichia coli K12 gltS glutamate permease gene

The gltS gene is known to encode a sodium-dependent, glutamate-specific permease. We have localized the Escherichia coli K12 gltS gene with respect to the spoT gene, sequenced it, and recombined a null insertion-deletion allele into the chromosome without loss of viability. The gltS null allele gives a Glt- phenotype, i.e. it abolishes the ability of a gltCc host to grow on glutamate as sole carbon and nitrogen source and also confers alpha-methylglutamate resistance. A multicopy plasmid expressing the gltS gene can reverse the Glt- phenotype of gltS- or wild-type strains while other plasmids show host-dependent complementation patterns. Induction of gltS gene overexpression under control of isopropyl-beta-D-thiogalactoside (IPTG)-inducible promoters severely inhibits growth. The GltS protein is deduced to be a 42425 dalton hydrophobic protein with 2 sets of 5 possible integral protein domains, each flanking a central hydrophilic, flexible region.

Role of the carboxyl terminal region of H(+)-ATPase (F0F1) a subunit from Escherichia coli

The effects of amino acid substitutions in the carboxyl terminal region of the H(+)-ATPase a subunit (271 amino acid residues) of Escherichia coli were studied using a defined expression system for uncB genes coded by recombinant plasmids. The a subunits with the mutations, Tyr-263----end, Trp-231----end, Glu-219----Gln, and Arg-210----Lys (or Gln) were fully defective in ATP-dependent proton translocation, and those with Gln-252----Glu (or Leu), His-245----Glu, Pro-230----Leu, and Glu-219----His were partially defective. On the other hand, the phenotypes of the Glu-269----end, Ser-265----Ala (or end), and Tyr-263----Phe mutants were essentially similar to that of the wild-type. These results suggested that seven amino acid residues between Ser-265 and the carboxyl terminus were not required for the functional proton pathway but that all the other residues except Arg-210, Glu-219, and His-245 were required for maintaining the correct conformation of the proton pathway. The results were consistent with a report that Arg-210 is directly involved in proton translocation.

Overproduction and purification of the Tn10-specified inner membrane tetracycline resistance protein Tet using fusions to beta-galactosidase

Tetracycline resistance in the Enterobacteriaceae is mediated by a number of genetically related, usually plasmid-borne, determinants which specify an efflux system involving an inner membrane protein, Tet. Attempts to overproduce the Tn10 (Class B)-encoded Tet in Escherichia coli by cloning the structural gene tet downstream of the lambda PL promoter under regulation by temperature-sensitive lambda repressor cI857 were unsuccessful; induction at 42 degrees C resulted in filamentous, non-viable cells containing little detectable overproduction of the protein. However, cells containing tet fused to lacZ were resistant to tetracycline at 30 degrees C and synthesized modest amounts of a large fusion protein when induced at 42 degrees C. Fusion of the N-terminal half or the first 38 amino acids of tet to lacZ did lead to increased production of fusion proteins. Fusions could be purified by size or by LacZ immunoaffinity or substrate-affinity chromatography. In the latter method, selected detergents were required to counteract nonspecific binding of Tet to the adsorbant. Amino acid sequencing of the N-terminus of Tet-LacZ fusion proteins indicated that most molecules were blocked at this terminus. The sequence of an unblocked subpopulation was consistent with that expected from the nucleotide sequence. A collagen peptide linker, genetically placed between tet and lacZ, allowed recovery of purified Tet protein after collagenase treatment of the purified fusion protein.