Structural Biochemistry/AB5 Toxins

Background
The AB5 toxins are vital virulence factors for several bacterial pathogens. AB 5 toxins are one of many virulence factors deployed by major bacterial pathogens, which collectively kill over a million people each year. AB5 toxins have recently become of interest for disease pathogenesis, due to the widespread and severe bacterial infections that have resulted from the action of AB5 toxins.

Abstract
The AB5 toxin is a group of polypeptide chains, also known as a protein complex that pathogenic bacteria secrete in order to assist in overtaking a host. The protein complex is composed of six components; five "B" Subunits (binds to the glycan receptors on the host cell) and one "A" subunit (the toxic subportion that disrupts host functions). The B subunits of the protein complex form a ring that the A subunit is attached to. By doing so, the protein complex is allowed to function; the B subunits attach to the cell while the A subunit employs its toxicity.

This Toxin is important because of its presence in many common or important pathogens including, but not limited to:

-Various forms of Escherichia Coli; Heat-Labile Enterotoxins (Diharrhea)

-Bordetella Pertussis; Pertussis Toxin (Causes Whooping Cough)

-Shigella Dysenteriae; Shiga Toxin (Dysentery)

-Vibrio Cholerae; Cholera Toxin (Cholera)

-Campylobacter jejuni; Campylobacter Jejuni Enterotoxin

The study of this toxin is interesting as it provides researchers with valuable insight on how bacteria and cells function. In fact, researchers are investigating ways to incorporate the concept behind this toxin into treatment for different diseases.

Structure
In the past two decades, close to 30 AB5 crystal structures have been depicted. These depictions have proven to provide significant structural insights into the biological function and catalytic activity of the holotoxins. Based on sequence homology and catalytic activity, AB5 toxins have been classified into four separate families. Although the toxins share similar structural aspects, they still differ in their host cell surface receptor specificity, catalytic activity and intracellular trafficking.

Risk
Millions of people die each year due to bacterial infections. Many of these infections are caused by AB5 toxins that are released by bacteria that humans come into contact on a regular basis. For example:

E. Coli: People that reside in or are visiting developing countries are at a high risk for exposure to E. Coli and the symptoms it causes.

Vibrio Cholerae: Epidemic cholera outbreaks all over the world that result in the death of anywhere from dozens to thousands.

Shigella Dysenteriae: Similar to E. Coli, dysentery causes gastrointestinal issues that sometimes lead to life-threatening conditions such as systemic sequelae and haemolytic uraemic syndrome. Suprisingly enough, haemolytic uraemic syndrome caused by dysentery has a higher mortality rate in adults then in children.

A-Subunit
The A-Subunit of the AB5 Toxin is the part that actually conducts the attack on the target cell. The subunit itself is broken down further into two parts linked through a disulfide bond that forms a polypeptide.

A1: The first part of the A-subunit is the part that actually contains the toxins that disrupts host cell activities

A2: The second part of the A-subunit connects the A1 part with the B-Subunits, linking all of it together and allowing it to function as one cohesive unit.

Furthermore, this subunit is divided into different families according to their catalytic activity and their homology. Each family contains different forms of the same toxin. These different forms share a very similar amino acid composition with the other forms in the same family; toxins from different familys may share some similarities in amino acid composition but the percentage is significantly lower. Some toxins utilize similar methods of attacks even though they derive from different families. For example, both the toxins for Cholera and Whooping cough disrupt the "G-Protein Signal Transduction Pathways" which leads to a failure in the ion transporting system in the cell. Although they result in similar failures, these two toxins ultimately cause different symptoms to be expressed. The toxins in the STX family in which dysentery derives from attacks the cell by disrupting the way the cell synthesizes proteins. It does this by causing an abnormality in the nucleotide sequence in the rRNA of these cells. In doing so, it causes cell death by denying the cell the ability to synthesize proteins.The last family of AB5 toxins is classified as the SubAB toxins due to their subtilase like cytotoxin. This family of AB5 toxins targets Binding immunogloblin protein (BiP) and acts as a protease dismantling BiP. This disrupts the protein folding process in the cell and therefore eventually causes the fatality of the cell.

B-Subunit
This subunit is composed of five monomers arranged in a pentameric structure.The B-Subunit is tasked with both identifying the host cell and transporting the toxic A-Subunit to and into the cell. For this reason, the identification of the correct cell is very important for the toxin. The CTX, STX, and SubAB families share a similar structure while the PTX family is composed differently. However, each family responds to and seeks out different glycan cell receptors. They also have a varying amount of binding sites on each monomer of the B-Subunit; varying from one on the SubAB and CTX family to three on each monomer of the STX family. These toxins look to bind to different glycolipids and glycoproteins on the surface of cells and sometimes even use the antigens present on blood as receptors. The toxins that utilize blood as a host cell may recognize certain types of blood cells better than others causing humans with certain blood types to be more susceptible to cholera and e. coli attacks. An interesting case to observe is the inability of humans to synthesize the sialic acid Neu5Gc. Humans still produce Neu5Ac, a similar sialic acid; however, it is believed that humans stopped producing Neu5Gc as a method of avoiding infection due to AB5 toxins.

AB5 toxins as Cellular Tools and Novel Therapeutics
AB5 toxins have been used to specifically manipulate associated signaling pathways in cells. Studying the B-Subunits of AB5 Toxins has helped scientists to understand receptor pathways better by allowing them to view the steps that the toxin takes. AB5 toxin B subunits have been utilized to help counter certain allergens. In essence, by isolating only the non-toxic portion of the AB5 toxin, it is possible to utilize them in ways that may be beneficial. An example of this would be the B-Subunit of the CTX family which has been used in mice to suppress certain allergic reactions. Also, the B-Subunit of the STX family may provide the basis for an anti-tumor vaccine. Both the CTX family and SubAB family may quite possibly have a large effect on inflammatory responses of cells; they have potential as immunomodulatory agents.

In addition to their potential use as immunomodulatory agents, AB5 toxins can be used as therapeutic agents against a range of diseases. AB5 Toxin research is very promising in the fight against cancer. The B-Subunit of these toxins can bind to the glycan sites that many cancer and tumor cells exhibit. The A-Subunit possess the ability to kill the cancer cell by obstructing its protein synthesis ability. One of the issues is how to target only cancer cells since the toxins do exhibit the tendency to target normal cells as well as cancer cells. The Sub-AB shows the most promise as it prefers to bind to glycans that humans cannot synthesize (but uptake through diet).