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Cytolysin
Cytolysin refers to the substance secreted by microorganisms, plants or animals that is specifically toxic to individual cells,〔(Computer Retrieval of Information on Scientific Projects (CRISP) - Thesaurus - Cytolysin )〕〔("Cytolysin" entry from the American Heritage Medical Dictionary ), on TheFreeDictionary.com (Retrieved on January 22, 2009)〕 in many cases causing their dissolution through lysis. Cytolysins that have a specific action for certain cells are named accordingly. For instance, the cytolysins responsible for the destruction of red blood cells, thereby liberating hemoglobins, are named ''hemolysins'', and so on.〔("Hemolysin" entry ) on TheFreeDictionary.com (Retrieved on January 22, 2009)〕 Cytolysins may be involved in immunity as well as in venoms.
Hemolysin is also used by certain bacteria, such as ''Listeria monocytogenes'', to disrupt the phagosome membrane of macrophages and escape into the cytoplasm of the cell.
==History and background==
The term "Cytolysin" or "Cytolytic toxin" was first introduced by Alan Bernheimer to describe membrane damaging toxins (MDTs) that have cytolytic effects to cells.〔Bernheimer A W (1970) Cytolytic toxins of bacteria, vol I. In: Ajl S, Kadis S, Montie TC (eds) Microbial toxins. Academic, New York, pp 183-212〕 The first kind of cytolytic toxin discovered have hemolytic effects on erythrocytes of certain sensitive species, such as Human. For this reason "Hemolysin" was first used to describe any MDTs. In the 1960s certain MDTs were proved to be destructive on cells other than erythrocytes, such as leukocytes. The term "Cytolysin" is then introduced by Bernheimer to replace "Hemolysin". Cytolysins can destruct membranes without creating lysis to cells.〔Thelestam, M. and Mollby, R. (1975) Infect. Immun, 11, 640-648.〕 Therefore, "membrane damaging toxins" (MDTs) describes the essential actions of cytolysins. Cytolysins comprise more than 1/3 of all bacterial protein toxins. Bacterial protein toxins can be highly poisonous to human. For example, ''Botulinum'' is 3x105 more toxic than snake venom to human and its toxic dose is only 0.8x10−8 mg.〔http://textbookofbacteriology.net/proteintoxins.html〕 A wide variety of gram-positive and gram-negative bacteria use cytolysin as their primary weapon for creating diseases, such as ''Clostridium perfringens'' and ''Staphylococcus''.
A diverse range of studies has been done on Cytolysins. Since the 1970s, more than 40 new cytolysins have been discovered and grouped into different families.〔Alouf, J. E. "Pore-forming bacterial protein toxins: an overview." Pore-forming toxins. Springer Berlin Heidelberg, 2001. 1-14.〕 At genetic level, the genetic structures of about 70 Cytolysin proteins has been studied and published.〔Kaper J, Hacker J (2000) Pathogenicity islands and other mobile virulence elements. ASM, Washington D.C.〕 The detailed process of membrane damage has also been surveyed. Rossjohn et al. presents the crystal structure of ''perfringolysin'' O, a thiol-activated cytolysin, which creates membrane holes on eukaryotic cells. A detailed model of membrane channel formation that reveals membrane insertion mechanism is constructed.〔Rossjohn, Jamie, et al. "Structure of a cholesterol-binding, thiol-activated cytolysin and a model of its membrane form." Cell 89.5 (1997): 685-692.〕 Shatursky et al. studied the membrane insertion mechanism of Perfringolysin O (PFO), a cholesterol-dependent pore-forming cytolysin produced by pathogenic Clostridium perfringens. Instead of using a single amphipathic β hairpin per plypeptide, PFO monomer contains two amphipathic β hairpins, each spans the whole membrane. Larry et al. focused on the membrane penetrating models of RTX toxins, a family of MDT secreted by many gram-negative bacteria. The insertion and transport process of the protein from RTX to target lipid membrane was revealed.〔Lally, Edward T., et al. "The interaction between RTX toxins and target cells." Trends in microbiology 7.9 (1999): 356-361.〕
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