Cholesterol-binding cytolytic protein toxins

Cholesterol-binding cytolysins (CBCs) are a large family of 50- to 60-kDa s ingle-chain protein produced by 23 taxonomically different species of Gram- positive bacteria from the genera Streptococcus, Bacillus, Clostridium, Lis teria 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 intr acellular pathogens which grow and release their toxins in the phagocytic c ells of the host. CBCs are lethal to animals and highly lyric toward eukary otic cells, including erythrocytes. Their lyric and lethal properties are s uppressed by sulhydryl-group-blocking agents and reversibly restored by thi ols or other reducing agents. These properties are irreversibly abrogated b y very low concentrations of cholesterol and other 3 beta -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 sur face with subsequent oligomerization into are-and ring-shaped structures su rrounding 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 s equence containing a unique Cys residue (ECTGLAWEWWR) near the C-terminus o f the molecules (except pyolysin and intermedilysin). However, another Cys residue outside this undecapeptide and closer to the C-terminus occurs in i vanolysin. Genetic replacement of the Cys residue in the consensus undecapeptide by ce rtain amino acids demonstrated that this residue was not essential for toxi n function. Other residues in the undecapeptide have been mutagenized, part icularly the Trp residues. One of these Trp appeared critical for lyric act ivity The recent elucidation of the 3-D structure of perfringolysin O provided in teresting 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 doma in 1 and packed laterally against domain 2. Membrane interaction of the mon omer appears to be mediated by domain 4, while, oligomerization involves se veral sites scattered throughout the sequence. The Trp-rich region around t he conserved Cys residue within domain 4 is assumed to conformationally ada pt to cholesterol, and domain 3 is envisaged to move across the "hinge" by which it is connected to domain 1.