Uptake across the cell envelope and insertion into the inner membrane of ion channel-forming colicins in E coli

Antibacterial toxin colicin N and phage protein G3p compete with TolB for a binding site on TolA

Most colicins kill Escherichia coli cells by membrane pore formation or nuclease activity and, superficially, the mechanisms are similar: receptor binding, translocon recruitment, periplasmic receptor binding and membrane insertion. However, in detail, they employ a wide variety of molecular interactions that reveal a high degree of evolutionary diversification. Group A colicins bind to members of the TolQRAB complex in the periplasm and heterotrimeric complexes of colicin–TolA–TolB have been observed for both ColA and ColE9. ColN, the smallest and simplest pore-forming colicin, binds only to TolA and we show here that it uses the binding site normally used by TolB, effectively preventing formation of the larger complex used by other colicins. ColN binding to TolA was by β-strand addition with a K_D of 1 µM compared with 40 µM for the TolA–TolB interaction. The β-strand addition and ColN activity could be abolished by single proline point mutations in TolA, which each removed one backbone hydrogen bond. By also blocking TolA–TolB binding these point mutations conferred a complete tol phenotype which destabilized the outer membrane, prevented both ColA and ColE9 activity, and abolished phage protein binding to TolA. These are the only point mutations known to have such pleiotropic effects and showed that the TolA–TolB β-strand addition is essential for Tol function. The formation of this simple binary ColN–TolA complex provided yet more evidence of a distinct translocation route for ColN and may help to explain the unique toxicity of its N-terminal domain.

Surface expression of O-specific lipopolysaccharide in Escherichia coli requires the function of the TolA protein

We investigated the involvement of Tol proteins in the surface expression of lipopolysaccharide (LPS). tolQ, -R, -A and -B mutants of Escherichia coli K-12, which do not form a complete LPS-containing O antigen, were transformed with the O7+ cosmid pJHCV32. The tolA and tolQ mutants showed reduced O7 LPS expression compared with the respective isogenic parent strains. No changes in O7 LPS expression were found in the other tol mutants. The O7-deficient phenotype in the tolQ and tolA mutants was complemented with a plasmid encoding the tolQRA operon, but not with a similar plasmid containing a frameshift mutation inactivating tolA. Therefore, the reduction in O7 LPS was attributed to the lack of a functional tolA gene, caused either by a direct mutation of this gene or by a polar effect on tolA gene expression exerted by the tolQ mutation. Reduced surface expression of O7 LPS was not caused by changes in lipid A-core structure or downregulation of the O7 LPS promoter. However, an abnormal accumulation of radiolabelled mannose was detected in the plasma membrane. As mannose is a sugar unique to the O7 subunit, this result suggested the presence of accumulated O7 LPS biosynthesis intermediates. Attempts to construct a tolA mutant in the E. coli O7 wild-type strain VW187 were unsuccessful, suggesting that this mutation is lethal. In contrast, a polar tolQ mutation affecting tolA expression in VW187 caused slow growth rate and serum sensitivity in addition to reduced O7 LPS production. VW187 tolQ cells showed an elongated morphology and became permeable to the membrane-impermeable dye propidium iodide. All these phenotypes were corrected upon complementation with cloned tol genes but were not restored by complementation with the tolQRA operon containing the frameshift mutation in tolA. Our results demonstrate that the TolA protein plays a critical role in the surface expression of O antigen subunits by an as yet uncharacterized involvement in the processing of O antigen.

Crystal structure of a colicin N fragment suggests a model for toxicity

BACKGROUND

Pore-forming colicins are water-soluble bacteriocins capable of binding to and translocating through the Escherichia coli cell envelope. They then undergo a transition to a transmembrane ion channel in the cytoplasmic membrane leading to bacterial death. Colicin N is the smallest pore-forming colicin known to date (40 kDa instead of the more usual 60 kDa) and the crystal structure of its membrane receptor, the porin OmpF, is already known. Structural knowledge of colicin N is therefore important for a molecular understanding of colicin toxicity and is relevant to toxic mechanisms in general.

RESULTS

The crystal structure of colicin N reveals a novel receptor-binding domain containing a six-stranded antiparallel beta sheet wrapped around the 63 A long N-terminal alpha helix of the pore-forming domain. The pore-forming domain adopts a ten alpha-helix bundle that has been observed previously in the pore-forming domains of colicin A, la and E1. The translocation domain, however, does not appear to adopt any regular structure. Models for receptor binding and translocation through the outer membrane are proposed based on the structure and biochemical data.

CONCLUSIONS

The colicin N-ompF system is now the structurally best-defined translocation pathway. Knowledge of the colicin N structure, coupled with the structure of its receptor, OmpF, and previously published biochemical data, limits the numerous possibilities of translocation and leads to a model in which the translocation domain inserts itself through the porin pore, the receptor-binding domain stays outside and the pore-forming domain translocates along the outer wall of the trimeric porin channel.

Ligand-specific opening of a gated-porin channel in the outer membrane of living bacteria

Ligand-gated membrane channels selectively facilitate the entry of iron into prokaryotic cells. The essential role of iron in metabolism makes its acquisition a determinant of bacterial pathogenesis and a target for therapeutic strategies. In Gram-negative bacteria, TonB-dependent outer membrane proteins form energized, gated pores that bind iron chelates (siderophores) and internalize them. The time-resolved operation of the Escherichia coli ferric enterobactin receptor FepA was observed in vivo with electron spin resonance spectroscopy by monitoring the mobility of covalently bound nitroxide spin labels. A ligand-binding surface loop of FepA, which normally closes its transmembrane channel, exhibited energy-dependent structural changes during iron and toxin (colicin) transport. These changes were not merely associated with ligand binding, but occurred during ligand uptake through the outer membrane bilayer. The results demonstrate by a physical method that gated-porin channels open and close during membrane transport in vivo.

Novel colicin 10: assignment of four domains to TonB- and TolC-dependent uptake via the Tsx receptor and to pore formation

Uptake of a new colicin, colicin 10 (Col10), into cells of Escherichia coli required TonB, ExbBD (Ton system), but its cognate receptor, Tsx, functioned independently of Ton and TolQRAB (Tol system). Uptake of Col10 also required TolC which is unique for a Ton-coupled translocation through the outer membrane. A 2470 bp DNA fragment from the natural plasmid pCol10 encoding the Col10 activity (cta), immunity (cti) and lysis (ctl) genes was sequenced. The Cta, Cti and Ctl proteins, as deduced from the nucleotide sequences, consisted of 490 (M(r) 53,342), 96 (M(r) 11,586) and 43 (M(r) 4484) amino acid residues, respectively. Col10 (Cta) was highly homologous to colicin E1 in two regions which determined the common TolC requirement for uptake and the pore-forming activity. Col10 and E1 differed entirely in the regions which are predicted to determine the Ton dependence of Col10 and the Tol dependence of E1, and binding to the receptors Tsx and BtuB, respectively. The region responsible for the Ton-dependent uptake of Col10 was localized in the sequence ranging from residues 1 to 43 (Ton region), and the region responsible for the Tol-dependent uptake of colicin E1 extended from residues 1 to 34 (Tol region). Each Tol-dependent colicin contained a pentapeptide homologous to the sequence DGSGS in the Tol region of E1 which is proposed to be implicated in Tol-dependent uptake (TolA box). After the exchange of the Ton and the Tol regions between Col10 and E1, the Col10-E1 fusion protein was carried into cells via the Ton system and BtuB, whereas the E1-Col10 fusion protein was imported via the Tol system and Tsx. Although the immunity proteins of Col10 and E1 displayed a low homology, Cti conferred full immunity to E1, in contrast to the immunity protein of E1 which did not protect cells against Col10. It is proposed that Col10 belongs to the colicin E1, Ia, Ib group as opposed to the colicin A, B, N group of pore-forming colicins. Col10 consists of 4 domains of which two are very similar and two are very different to E1, supporting our previous proposal that colicins evolved by recombination of DNA fragments which encode uptake and activity domains.

Charged amino-terminal amino acids affect the lethal capacity of Lambda lysis proteins S107 and S105

The lysis inhibitor protein S107 and the lysis effector protein S105 start at Met codons 1 and 3 of the Lambda S gene, respectively. The antagonistic action of both proteins precisely schedules lysis by formation of a non-specific lesion in the inner membrane through which the Lambda-encoded murein transglycosylase can pass. Here, we show that the main difference between lysis-effector and lysis-inhibitor is the degree by which an energized membrane inhibits either protein from hole formation. To dissect the structural parameters responsible for intrinsic inhibition of both proteins, charged amino acids were replaced proximal to the first putative membrane-spanning region in both S proteins. Our results show that the distribution of amino-terminal charged amino acids as well as the total amino-terminal net charge of S107 and S105 influence their lethal potential. The data are interpreted in terms of a model in which the electrostatic status of the amino-terminus of both S107 and S105 is an important feature affecting their conformational change required for formation of the S-dependent hole.

Membrane topography of ColE1 gene products: the immunity protein

The topography of the colicin E1 immunity (Imm) protein was determined from the positions of TnphoA and complementary lacZ fusions relative to the three long hydrophobic segments of the protein and site-directed substitution of charged for nonpolar residues in the proposed membrane-spanning segments. Inactivation of the Imm protein function required substitution and insertion of two such charges. It was concluded that the 113-residue colicin E1 Imm protein folds in the membrane as three trans-membrane alpha-helices, with the NH2 and COOH termini on the cytoplasmic and periplasmic sides of the membrane, respectively. The approximate spans of the three helices are Asn-9 to Ser-28, Ile-43 to Phe-62, and Leu-84 to Leu-104. An extrinsic highly charged segment, Lys-66 to Lys-74, containing seven charges in nine residues, extends into the cytoplasmic domain. The specificity of the colicin E1 Imm protein for interaction with the translocation apparatus and the colicin E1 ion channel is proposed to reside in its peripheral segments exposed on the surface of the inner membrane. These regions include the highly charged segment Lys-66 to Lys-83 (loop 2) and the short (approximately eight-residue) NH2 terminus on the cytoplasmic side, and Glu-29 to Val-44 (loop 1) and the COOH-terminal segment Gly-105 to Asn-113 on the periplasmic side.

The tol gene products and the import of macromolecules into Escherichia coli

Genetic studies have identified a number of genes whose products appear to be required for the transport of the group A colicins and the single-stranded DNA of certain filamentous bacteriophages into Escherichia coli. Mutations in these genes allow normal binding of the colicins to their outer-membrane receptors and of the bacteriophage of the tip of specific conjugative pili, but do not allow translocation of the macromolecules to their target. These mutations have been designed 'tolerant' (tol) mutations and the protein products specified by these genes appear to comprise part of a transport system known as the Tol import system. Some of these genes have been isolated, sequenced and their protein products localized to the membranes or periplasm of E. coli. Information is also available regarding the domains of the colicins or phage proteins which interact with the Tol proteins. A preliminary model of the location and possible interactions of the Tol proteins is presented.

Interfaces of the yeast killer phenomenon

A new prophylactic and therapeutic antimicrobial strategy based on a specific physiological target that is effectively used by killer yeasts in their natural ecological competition is theorized. The natural system exploited is the yeast killer phenomenon previously adopted as an epidemiological marker for intraspecific differentiation of opportunistic yeasts, hyphomycetes, and bacteria. Pathogenic microorganisms (Candida albicans) may be susceptible to the activity of yeast killer toxins due to the presence of specific cell wall receptors. On the basis of the idiotypic network, we report that antiidiotypic antibodies, produced against a monoclonal antibody bearing the receptor-like idiotype, are in vivo protecting animals immunized through idiotypic vaccination and in vitro mimicking the antimicrobial activity of yeast killer toxins, thus acting as antibiotics.

Insertion and translocation of proteins into and through membranes

In prokaryotic and eukaryotic organisms, proteins are efficiently sorted to reach their final destinations in a whole range of subcellular compartments. Targeting is mediated by hydrophobic signal sequences or hydrophilic targeting sequences depending upon the compartment, these sequences being often processed. Proteins cannot be translocated through a membrane in a tightly folded stage, they must have a loose conformation, the so-called 'translocation competent state', which is usually kept through interactions with chaperones. In addition to these cytosolic receptor-like components, receptors are also present on the target membranes. Depending upon the organelles and organisms, two different energy sources have been identified, energy rich phosphate bonds (ATP and GTP) and a potential across the target membrane. Besides the signal peptides, various classes of signals have been identified to account for topologies of membrane proteins. Protein secretion in bacterial organisms has been extensively studied. Various classes of proteins use different strategies, some of these may also be used in eukaryotic cells.