Figure 4.

Main types of activity of bacterial protein toxins. First, toxins recognize cell surface receptor(s), which determine their cell specificity (neurotoxins, enterotoxins, leucotoxins, and cytotoxins). Toxins active at the cell membrane use either a transduction, pore formation, or enzymatic hydrolysis mechanism. Intracellularly active toxins exploit short or long endocytic pathways to internalize the catalytic (A) subunit, which interacts with an intracellular target and mediates the cellular effects. Interaction of intracellularly active toxins with cell surface receptors not only drives the cell specificity but also initiates membrane curvature and endocytic vesicle formation. EE, early endosome; LE, late endosome; ER, endoplasmic reticulum.