Shiga toxin (Stx) is a potent bacterial product, originally identified in Shigella dysenteriae and also produced by Shiga toxin-producing E. coli (STEC). These toxins drive the severity of diseases ranging from severe diarrhea to life-threatening conditions. Although released in the gut, the toxin travels through the bloodstream to reach susceptible cells throughout the body. The danger of Stx lies in its ability to hijack and dismantle the internal machinery of a host cell. The disease process is a direct consequence of the toxin attacking the cell’s most fundamental function: the creation of new proteins.
Toxin Structure and Initial Cellular Binding
Shiga toxin is classified as an AB5 toxin, a common structural motif among bacterial toxins. This structure consists of six total protein subunits: one A subunit, which is the enzymatically active component, and a ring of five identical B subunits. The B subunits are noncovalently associated with the single A subunit, forming the complete toxin complex, also known as the holotoxin.
The pentamer of B subunits serves as the binding apparatus. This B pentamer specifically recognizes and attaches to globotriaosylceramide (Gb3), a glycolipid molecule on the surface of host cells. Each B pentamer has 15 sites for binding to Gb3, ensuring strong attachment to the cell membrane. The presence of the Gb3 receptor determines which tissues are susceptible to the toxin, making cells like renal endothelial cells particularly vulnerable.
Once the B subunits bind to the Gb3 receptors, the entire toxin-receptor complex is drawn into the host cell. This entry is initiated through receptor-mediated endocytosis, where the cell membrane folds inward to create an endosome. The toxin is internalized within this early endosome, beginning its journey toward the cell’s interior.
Retrograde Transport Through Cellular Organelles
After internalization via endocytosis, Shiga toxin must navigate intracellular compartments to reach its final destination. Unlike most substances routed to lysosomes for degradation, Shiga toxin avoids this fate. It instead employs a unique process known as retrograde transport, moving backward through the cell’s normal secretory pathway.
The toxin-containing endosome first travels from the cell periphery to the Golgi apparatus. This direct transport from the early endosome allows the toxin to bypass the degradative late endosomes. The Golgi apparatus acts as a central sorting station, and the toxin complex exploits its internal transport machinery.
From the Golgi, the toxin continues its retrograde journey to the endoplasmic reticulum (ER). The ER is a network of membranes responsible for protein folding and modification, and its internal environment is where the toxin must be activated.
Catalytic Activity and Ribosomal Damage
The arrival of the Shiga toxin holotoxin in the ER lumen triggers a series of events that culminate in the destruction of the cell’s protein synthesis capacity. While in the ER, the single A subunit undergoes structural modification. Host cell proteases, specifically the enzyme furin, cleave the A subunit into two fragments: the A1 fragment and the A2 fragment.
The resulting A1 and A2 fragments remain temporarily linked by a disulfide bond. To become fully active, the A1 fragment must be released from the complex. This release occurs when the disulfide bond is reduced, facilitated by the reducing environment within the ER. The newly freed A1 fragment is the active enzyme, but it is trapped inside the ER membrane.
The A1 fragment then uses the cell’s own quality control system to escape into the cytoplasm. It is recognized by the ER-associated protein degradation (ERAD) pathway, a system meant to dispose of misfolded proteins. The cell attempts to transport the A1 fragment across the ER membrane into the cytoplasm for destruction, but the toxin refolds into its active structure, evading the destruction machinery.
Once in the cytoplasm, the A1 fragment operates as an N-glycosidase. Its specific target is the ribosome, the cellular factory responsible for building proteins. The A1 fragment attacks the 28S ribosomal RNA (rRNA) molecule, a major component of the large 60S ribosomal subunit. The enzyme precisely removes a single adenine base from a specific position within a conserved region of the 28S rRNA. This removal, known as depurination, permanently inactivates the 60S ribosomal subunit, halting the elongation phase of protein synthesis and leading to programmed cell death.