Structural Highlights
Shiga toxin (stx) has multiple classifications. The shiga toxin produced by Shigella dysenteriae is classified as stx, while shiga toxin produced by Escherichia coli can be classified as either stx1 or stx2. All three classifications very slightly in structure but have identical functions. The A subunit of stx1 differs from stx by only one residue: a serine in position 45 as opposed to threonine[1]. Stx is a 70 kDa AB5 protein, meaning that it is composed of an A subunit bound to a B subunit pentamer. A noncovalent interaction causes the association between the A and B subunits, as the carboxy terminal tail of the A subunit is surrounded by the B pentamer. The A subunit is 293 amino acids long with its active site being glutamic acid 167. Though the active site is a single residue, studies have shown that the first 239 residues are essential for the enzymatic activity of the A subunit. Residues 240-251 are essential for the A subunit’s translocation from the endoplasmic reticulum of an infected cell to the cytosol. The A subunit can be broken down into A1 and A2 units. This break down occurs when a trypsin sensitive region, residues 248-251 is cleaved. This cleavage results in the only remaining link being a disulfide bridge between cysteine 241 and cysteine 260. If the disulfide bridge is reduced, the A1 and A2 subunits can completely separate. Interestingly, the disulfide bridge blocks the active site of the A subunit, so the A subunit is not enzymatically active unless it has been cleaved.
The B subunit is a symmetrical pentamer that resembles a star and can be divided into five identical subunits. Each of the subunits is 69 amino acids in length. The purpose of the B subunit is to bind to globotriaosylceramide (GB3) which is a glycosphingolipid that is found on the lipid rafts of endothelial cells. Each monomer of the B subunit has three binding sites for GB3, binding to the carbohydrate portion of GB3. The affinity between the B subunit and GB3 is incredibly low, and is actually one of the lowest recorded affinities for carbohydrate-protein interaction. Some studies have shown that the level of saturation and length of the fatty acid on GB3 affects the strength of its interaction with the B subunit. Since the B subunit of stx has fifteen binding sites though, it has an incredibly high avidity (Melton-Celsa 2013).
Function
The pathway of stx entering a cell begins with the B subunit’s binding to GB3. Once this occurs, the A subunit disconnects from the B subunit and enters the cell through endocytosis. Using retrograde transport the A subunit passes through the Golgi apparatus and the rough endoplasmic reticulum. In the rough endoplasmic reticulum, the A subunit is split into two parts called A1 and A2 through the cleavage of trypsin sensitive residues and the reduction of a disulfide bridge. A2 is degraded, but A1 freely enters the cytosol (Sandvig 2000). Once in the cytosol, A1 acts as an N-glycosidase, which is an enzyme that hydrolyzes bonds that link sugars. With this enzymatic activity, A1 removes an adenine from the alpha-sarcin loop in the 28S RNA of the 60S ribosomal subunit (Melton-Celsa 2013). The removal of the adenine prevents elongation factors from associating with the ribosomal subunit. Without elongation factors, the ribosome can no longer synthesize proteins, leading to cell death.
Disease and It's Pathogenesis
The shiga toxin is produced by Shigella dysenteriae and certain strains of Escherichia coli. S. dysenteriae is a gram negative bacterium that causes disease in its host by entering into intestinal epithelial cells (Zaidi 2014). It has a very low infectious dose of only ten to one hundred cells, and is contracted through the consumption of contaminated water or food. One it begins replicating in the intestines, its proliferation and secretion of stx erodes the intestinal lining and leads to dysentery. Those with dysentery can simply have diarrhea as a symptom, but if the infection progresses, it can lead to hemorrhagic colitis (bloody stool) or hemolytic uremic syndrome (HUS). HUS is characterized as acute renal failure, thrombocytopenia, and microangiopathic anemia (Kaper). As stated previously, stx enters cells through its binding to GB3, but curiously many intestinal cells do not express GB3. Some figure it enters the cells through macropinocyosis, but when macropinocytotic function is removed from cells, stx still manages to enter through an undefined mechanism.
The genes necessary to produce the toxin are interestingly not native to S. dysenteriae. The genes for stx are carried by prophages: bacteria that have had viral DNA inserted into their genome. During certain conditions, the phage genes are expressed leading to the production of more bacteriophages as well as the shiga toxin. When the bacteria undergo lysis, the bacteriophages and stx are released. It may seem as if there is no benefit for S. dysenteriae to carry the viral DNA, but studies have shown that while it is in the lysogenic cycle S. dysenteriae has increased acid tolerance and motility (Krüger 2014).
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