AB toxin
Encyclopedia
The AB toxins are two-component protein
complexes secreted by a number of pathogenic
bacteria
. They can be classified as Type III toxins because they interfere with internal cell function. They are named AB toxins due to their components: the "A" component is usually the "active" portion, and the "B" component is usually the "binding" portion. The "A" subunit possesses enzyme
activity, and is transferred to the host cell
following a conformational change
in the membrane-bound transport
"B" subunit.
Among the toxins produced by certain Clostridium spp. are the binary exotoxins. These protein
s consist of two independent polypeptides, which correspond to the A/B subunit moieties. The enzyme component (A) enters the cell
through endosome
s produced by the oligomeric binding/translocation protein (B), and prevents actin
polymerisation through ADP-ribosylation of monomeric G-actin.
Members of the "A" binary toxin family include C. perfringens iota toxin Ia , C. botulinum C2 toxin CI, and Clostridium difficile
ADP-ribosyltransferase . Other homologous
proteins
have been found in Clostridium spiroforme.
Members of the "B" binary toxin family include the Bacillus anthracis
protective antigen
(PA) protein, most likely due to a common evolutionary
ancestor. B. anthracis, a large Gram-positive
spore-forming rod, is the causative agent of anthrax. Its two virulence factor
s are the poly-D-glutamate polypeptide capsule, and the actual anthrax exotoxin. The toxin comprises three factors: the protective antigen (PA); the oedema factor (EF); and the lethal
factor (LF). Each is a thermolabile protein of ~80kDa. PA forms the "B" part of the exotoxin and allows passage of the "A" moiety (consisting of EF and LF) into target cells
. PA protein forms the central part of the complete anthrax toxin, and [translocates the B moiety into host cell
s after assembling as a heptamer in the membrane
.
The AB5 toxin
s are usually considered a type of AB toxin, characterized by B pentamers. Less commonly, the term "AB toxin" is used to emphasize the monomeric character of the B component.
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
complexes secreted by a number of pathogenic
Pathogenicity
Pathogenicity is the ability of a pathogen to produce an infectious disease in an organism.It is often used interchangeably with the term "virulence", although virulence is used more specifically to describe the relative degree of damage done by a pathogen, or the degree of pathogenicity caused by...
bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. They can be classified as Type III toxins because they interfere with internal cell function. They are named AB toxins due to their components: the "A" component is usually the "active" portion, and the "B" component is usually the "binding" portion. The "A" subunit possesses enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
activity, and is transferred to the host cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
following a conformational change
Conformational change
A macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change...
in the membrane-bound transport
Transport
Transport or transportation is the movement of people, cattle, animals and goods from one location to another. Modes of transport include air, rail, road, water, cable, pipeline, and space. The field can be divided into infrastructure, vehicles, and operations...
"B" subunit.
Among the toxins produced by certain Clostridium spp. are the binary exotoxins. These protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s consist of two independent polypeptides, which correspond to the A/B subunit moieties. The enzyme component (A) enters the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
through endosome
Endosome
In biology, an endosome is a membrane-bound compartment inside eukaryotic cells. It is a compartment of the endocytic membrane transport pathway from the plasma membrane to the lysosome. Molecules internalized from the plasma membrane can follow this pathway all the way to lysosomes for...
s produced by the oligomeric binding/translocation protein (B), and prevents actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
polymerisation through ADP-ribosylation of monomeric G-actin.
Members of the "A" binary toxin family include C. perfringens iota toxin Ia , C. botulinum C2 toxin CI, and Clostridium difficile
Clostridium difficile
Clostridium difficile , also known as "CDF/cdf", or "C...
ADP-ribosyltransferase . Other homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
proteins
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
have been found in Clostridium spiroforme.
Members of the "B" binary toxin family include the Bacillus anthracis
Bacillus anthracis
Bacillus anthracis is the pathogen of the Anthrax acute disease. It is a Gram-positive, spore-forming, rod-shaped bacterium, with a width of 1-1.2µm and a length of 3-5µm. It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions.It is one of few bacteria known to...
protective antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
(PA) protein, most likely due to a common evolutionary
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ancestor. B. anthracis, a large Gram-positive
Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
spore-forming rod, is the causative agent of anthrax. Its two virulence factor
Virulence factor
Virulence factors are molecules expressed and secreted by pathogens that enable them to achieve the following:* colonization of a niche in the host...
s are the poly-D-glutamate polypeptide capsule, and the actual anthrax exotoxin. The toxin comprises three factors: the protective antigen (PA); the oedema factor (EF); and the lethal
Lethal
Lethal is an adjective meaning capable of causing death.Lethal may also refer to:* Lethal , an American heavy metal band* Leigh Matthews, Australian rules footballer nicknamed "Lethal" or "Lethal" Leigh...
factor (LF). Each is a thermolabile protein of ~80kDa. PA forms the "B" part of the exotoxin and allows passage of the "A" moiety (consisting of EF and LF) into target cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. PA protein forms the central part of the complete anthrax toxin, and [translocates the B moiety into host cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s after assembling as a heptamer in the membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
.
The AB5 toxin
AB5 toxin
The AB5 toxins are six-component protein complexes secreted by a number of pathogenic bacteria. All share a similar structure and mechanism for entering targeted host cells.- Structure and mechanism :...
s are usually considered a type of AB toxin, characterized by B pentamers. Less commonly, the term "AB toxin" is used to emphasize the monomeric character of the B component.