Specific cleavage of colicin A by outer membrane proteases from sensitive and insensitive strains ofE. coli

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

Pore-forming colicins exert their lethal effect on E coli through formation of a voltage-dependent channel in the inner (cytoplasmic-membrane) thus destroying the energy potential of sensitive cells. Their mode of action appears to involve 3 steps: i) binding to a specific receptor located in the outer membrane; ii) translocation across this membrane; iii) insertion into the inner membrane. Colicin A has been used as a prototype of pore-forming colicins. In this review, the 3 functional domains of colicin A respectively involved in receptor binding, translocation and pore formation, are defined. The components of sensitive cells implicated in colicin uptake and their interactions with the various colicin A domains are described. The 3-dimensional structure of the pore-forming domain of colicin A has been determined recently. This structure suggests a model of insertion into the cytoplasmic membrane which is supported by model membrane studies. The role of the membrane potential in channel functioning is also discussed.

Colicin cleavage by OmpT protease during both entry into and release from Escherichia coli cells

Proteolysis of colicins A, E1, E2, and E3 was observed after they were added to whole cells carrying a functional ompT gene. Recombinant plasmid pML19 containing the ompT gene enabled two mutant strains to cleave the added colicins. On the other hand, two colicin A recombinants were split after release from the wild-type bacteria that produced them but not from ompT mutant cells.

Interactions of colicin A domains with phospholipid monolayers and liposomes: relevance to the mechanism of action

The colicin A polypeptide chain (592 amino acid residues) contains three domains which are linearly organized and participate in the sequential steps involved in colicin action. We have compared the penetrating ability in phospholipid monolayers and the ability to promote vesicle fusion at acidic pH of colicin A and of protein derivatives containing various combinations of its domains. The NH2-terminal domain (171 amino acid residues), required for translocation across the outer membrane, has little affinity for dilauroylphosphatidylglycerol (DLPG) monolayers at all pHs tested. The central domain has a pH-dependent affinity, although lower than that of the entire colicin A. The COOH-terminal domain contains a high-affinity lipid binding site, but in addition an electrostatic interaction is required as a first step in the process of penetration into negatively charged DLPG films. In contrast to the constructs containing the ionophoric domain, the NH2-terminal domain alone has no fusogenic activity for liposomes. These results are discussed with regard to the mechanism of entry and action of colicin A in sensitive cells. Our results suggest the existence of a pH-dependent interaction between the receptor binding domain (amino acid residues 172-388) and the pore-forming domain of colicin A (amino acid residues 389-592).

Conformation of colicin A: apparent difference between cytoplasmic and extracellular polypeptide chain

Cytoplasmic colicin A has the ability to bind to membranes and to form stable dimers. This form remains stable even in the presence of 1% SDS at 25 degrees C. Both of these properties were not observed for extracellular colicin A suggesting a possible difference in the conformation between cytoplasmic and extracellular colicin A.

Localization of the streptococcal C5a peptidase to the surface of group A streptococci

Immunofluorescent staining was used to determine that the streptococcal C5a peptidase (SCP) exists as a cell surface antigen on group A streptococci. The ability of hyperimmune serum to neutralize cell-associated SCP activity provided further evidence for the location of SCP. Quantification of SCP during growth in vitro by indirect enzyme-linked immunosorbent assay showed that approximately 90% of the measurable antigen is cell bound.

Uptake of cloacin DF13 by susceptible cells: removal of immunity protein and fragmentation of cloacin molecules

Monoclonal antibodies (MAb) directed against different epitopes on the equimolar complex of cloacin and immunity protein (cloacin DF13) were isolated, characterized, and used to study the uptake of cloacin DF13 by susceptible cells. Four MAbs recognized the amino-terminal part, one MAb recognized the central part, and three MAbs recognized the carboxyl-terminal part of the cloacin molecule. Three MAbs reacted with the immunity protein. Five MAbs inhibited the lethal action of cloacin DF13, but none of the MAbs inhibited the binding of cloacin DF13 to its purified outer membrane receptor protein or the in vitro inactivation of ribosomes. Binding of cloacin DF13 to susceptible cells cultured in broth resulted in a specific, time-dependent dissociation of the complex and a fragmentation of the cloacin molecules. Increasing amounts of immunity protein were detected in the culture medium from about 20 min after the addition of cloacin DF13. Cloacin was fragmented into two carboxyl-terminal fragments with relative molecular masses of 50,000 and 10,000. The larger fragment was detected 5 min after the binding of the bacteriocin complex to the cells. The smaller fragment was detected after 10 min. Both fragments were associated with the cells and could not be detected in the culture supernatant fraction. Cells grown in brain heart infusion were much less susceptible to cloacin DF13 than cells grown in broth, although they possessed a similar number of outer membrane receptor molecules. This decreased susceptibility correlated with a decreased translocation, dissociation, and fragmentation of cloacin DF13.

A molecular, genetic and immunological approach to the functioning of colicin A, a pore-forming protein

We have constructed, by recombinant DNA techniques, one hybrid protein, colicin A-beta-lactamase (P24), and two modified colicin As, one (P44) lacking a large central domain and the other (PX-345) with a different C-terminal region. The regulation of synthesis, the release into the medium and the properties of these proteins were studied. Only P44 was released into the medium. This suggests that both ends of the colicin A polypeptide chain might be required for colicin release. None of the three proteins was active on sensitive cells in an assay in vivo. However, P44 was able to form voltage-dependent channels in phospholipid planar bilayers. Its lack of activity in vivo is therefore probably caused by the inability to bind to the receptor in the outer membrane. PX-345 is a colicin in which the last 43 amino acids of colicin A have been replaced by 27 amino acids encoded by another reading frame in the same region of the colicin A structural gene; it was totally unable to form pores in planar bilayers at neutral pH but showed a very slight activity at acidic pH. These results confirm that the C-terminal domain of colicin A is involved in pore formation and indicate that at least the 43 C-terminal amino acid residues of this domain play a significant role in pore formation or pore function. Fifteen monoclonal antibodies directed against colicin A have been isolated by using conventional techniques. Five out of the 15 monoclonal antibodies could preferentially recognize wild-type colicin A. In addition, the altered forms of the colicin A polypeptide were used to map the epitopes of ten monoclonal antibodies reacting specifically with colicin A. Some of the antibodies did not bind to colicin A when it was pre-incubated at acidic pH suggesting that colicin A undergoes conformational change at about pH 4. The effects of monoclonal antibodies on activity in vivo of colicin A were investigated. The degree of inhibition observed was related to the location of the epitopes, with monoclonal antibodies reacting with the N terminus giving greater inhibition. The monoclonal antibodies directed against the C-terminal region promoted an apparent activation of colicin activity in vivo.

A protease as a possible sensor of environmental conditions in E. coli outer membrane

A mutant strain devoid of the colicin A protease activity (cpr) located at the external face of the outer membrane has been isolated. The location of the mutation (about 74 min on the genetic map), its pleiotropic effects concerning mainly the protein composition of the outer membrane and sensitivity to phages and colicins, are very similar in cpr and in tpo, envZ, perA strains that are affected in the ompB locus. Conditions resulting in inhibition of the colicin A protease activity also result in transcriptional regulation of OmpF, OmpC, and LamB protein synthesis. The possibility for this protease as an osmosensor of the cell's external environment is discussed.