Plant and microbial toxic proteins as hemilectins: emphasis on canatoxin

Ribosome-inactivating plant toxic proteins and ADP-ribosylating microbial toxins share a common structural organization. These proteins present domains displaying different biological properties: a target cell membrane-binding component (B-subunit or haptomer) and an enzymatically active component (A-subunit or effectomer). Interactions of these toxins with the target cells are mediated by the hemilectin-like haptomer, which recognizes and specifically binds to a given glycoderivative present at the cell surface. After binding the holoprotein is internalized via endocytosis. Inside the endocytic compartment the toxin is processed to release its effectomer moiety which catalytically modifies a cytoplasmic component, and this step accounts for its toxic effect. The structural relationships between toxic hemilectins and plant lectins are discussed, with emphasis on the example of canatoxin and concanavalin A, both present in the seeds of the jack bean Canavalia ensiformis. Contrary to other plant toxic proteins, which inhibit protein synthesis, canatoxin-induced toxicity includes central nervous system-mediated effects. In vivo as well as in vitro canatoxin acts as lipoxygenase-mediated secretagogue in several types of cells: blood platelets, mast cells, pancreatic islets and synaptosomes. Elucidation of structure vs biological activity relationships of canatoxin and other toxic proteins may provide data for their utilization as pharmacological tools and as therapeutic agents.