Formation of ring-shaped structures on erythrocyte membranes after treatment with botulinolysin, a thiol-activated hemolysin from Clostridium botulinum

Isolation of botulinolysin, a thiol-activated hemolysin, from serotype D Clostridium botulinum: A species-specific gene duplication in Clostridia

Botulinolysin (BLY) is a toxin produced by Clostridium botulinum that belongs to a group of thiol-activated hemolysins. In this study, a protein exhibiting hemolytic activity was purified from the culture supernatant of C. botulinum serotype D strain 4947. The purified protein displayed a single band by sodium dodecyl sulfate polyacrylamide gel electrophoresis with a molecular mass of 55kDa, and its N-terminal and internal amino acid sequences exhibited high similarity to a group of thiol-activated hemolysins produced by gram-positive bacteria. Thus, the purified protein was identified as the BLY. Using the nucleotide sequences of previously cloned genes for hemolysins, two types of genes encoding BLY-like proteins were cloned unexpectedly. Molecular modeling analysis indicated that the products of both genes displayed very similar structures, despite the low sequence similarity. In silico screening revealed a specific duplication of the hemolysin gene restricted to serotypes C and D of C. botulinum and their related species among thiol-activated hemolysin-producing bacteria. Our findings provide important insights into the genetic characteristics of pathogenic bacteria.

Role of pore-forming toxins in bacterial infectious diseases

Pore-forming toxins (PFTs) are the most common bacterial cytotoxic proteins and are required for virulence in a large number of important pathogens, including Streptococcus pneumoniae, group A and B streptococci, Staphylococcus aureus, Escherichia coli, and Mycobacterium tuberculosis. PFTs generally disrupt host cell membranes, but they can have additional effects independent of pore formation. Substantial effort has been devoted to understanding the molecular mechanisms underlying the functions of certain model PFTs. Likewise, specific host pathways mediating survival and immune responses in the face of toxin-mediated cellular damage have been delineated. However, less is known about the overall functions of PFTs during infection in vivo. This review focuses on common themes in the area of PFT biology, with an emphasis on studies addressing the roles of PFTs in in vivo and ex vivo models of colonization or infection. Common functions of PFTs include disruption of epithelial barrier function and evasion of host immune responses, which contribute to bacterial growth and spreading. The widespread nature of PFTs make this group of toxins an attractive target for the development of new virulence-targeted therapies that may have broad activity against human pathogens.

Immunisation with anthrolysin O or a genetic toxoid protects against challenge with the toxin but not against Bacillus anthracis

Anthrolysin O (ALO) is a toxin produced by Bacillus anthracis, the causative agent of anthrax. It is a member of the cholesterol-dependent cytolysin (CDC) group of toxins, many of which are potential vaccine candidates that protect against their producing organisms. Pore formation by ALO was studied by transmission electron microscopy and pores were found to be consistent with those formed by other members of this toxin family. We constructed and characterised a novel genetic toxoid of anthrolysin O, Delta6mALO, which was able to bind to cells but was incapable of pore-formation or haemolysis. The capacity of the haemolytic and non-haemolytic forms of ALO to protect against challenge with the toxin or B. anthracis was determined. Immunisation with both active and non-haemolytic forms of ALO elicited protection against lethal i.v. challenge with ALO but neither was protective against B. anthracis in a murine i.p. challenge model. Immunisation with another CDC, pneumolysin, did not confer cross-protection against challenge with ALO. Histopathological investigation following lethal i.v. challenge with ALO revealed acute pathology in the lungs with occlusion of alveolar vessels by fibrin deposits.

Ultrastructural analysis of the membrane insertion of domain 3 of streptolysin O

Streptolysin O (SLO) is a membrane-damaging toxic protein produced by group A streptococci. We performed an ultrastructural analysis of pore formation and the mechanism of hemolysis by SLO, using a mutant form of SLO [SLO(C/A)-SS] and native SLO. SLO(C/A)-SS was unable to penetrate the erythrocyte membrane as a consequence of immobilization that was due to a disulfide bond between domains. The SLO(C/A)-SS molecules that bound to membranes formed numerous single-layered ring-shaped structures that did not result in pores on the membranes. These structures were similar to the structures formed by native SLO at 0 degrees C. After treatment with dithiothreitol, SLO(C/A)-SS that had bound to membranes formed double-layered rings with pores on the membranes, as does native SLO at room temperature. Our morphological evidence demonstrates that an increase in temperature is necessary for the occurrence of conformational changes and for the formation of double-layered rings after the insertion of domain 3 into the host cell membrane. On the basis of a model of the oligomeric structure of SLO, we propose some new details of the mechanism of hemolysis by SLO.

Cholesterol-binding cytolytic protein toxins

Cholesterol-binding cytolysins (CBCs) are a large family of 50- to 60-kDa single-chain proteins produced by 23 taxonomically different species of Gram-positive bacteria from the genera Streptococcus, Bacillus, Clostridium, Listeria and Arcanobacterium. Apart pneumolysin, which is an intracytoplasmic toxin, all the other toxins are secreted in the extracellular medium. Among the species producing CBCs, only L. monocytogenes and L. ivanovii are intracellular pathogens which grow and release their toxins in the phagocytic cells of the host. CBCs are lethal to animals and highly lytic toward eukaryotic cells, including erythrocytes. Their lytic and lethal properties are suppressed by sulfhydryl-group-blocking agents and reversibly restored by thiols or other reducing agents. These properties are irreversibly abrogated by very low concentrations of cholesterol and other 3beta-hydroxysterols. Membrane cholesterol is thought to be the toxin-binding site at the surface of eukaryotic cells. Toxins molecules bind as monomers to the membrane surface with subsequent oligomerization into arc-and ring-shaped structures surrounding large pores generated by this process. Thirteen structural genes of the toxins (all chromosomal) have been cloned and sequenced to date. The deduced primary structure of the proteins shows obvious sequence homology particularly in the C-terminal part and a characteristic common consensus sequence containing a unique Cys residue (ECTGLAWEWWR) near the C-terminus of the molecules (except pyolysin and intermedilysin). However, another Cys residue outside this undecapeptide and closer to the C-terminus occurs in ivanolysin. Genetic replacement of the Cys residue in the consensus undecapeptide by certain amino acids demonstrated that this residue was not essential for toxin function. Other residues in the undecapeptide have been mutagenized, particularly the Trp residues. One of these Trp appeared critical for lytic activity. The recent elucidation of the 3-D structure of perfringolysin O provided interesting information on the structure-activity relationship. The molecule was divided into four domains. Three domains are arranged in a row, giving an elongated shape. Domain 3 is covalently connected to the N-terminal domain 1 and packed laterally against domain 2. Membrane interaction of the monomer appears to be mediated by domain 4, while, oligomerization involves several sites scattered throughout the sequence. The Trp-rich region around the conserved Cys residue within domain 4 is assumed to conformationally adapt to cholesterol, and domain 3 is envisaged to move across the "hinge" by which it is connected to domain 1.