A number of bacterial and plant toxins act by inhibiting protein synthesis in
eukaryotic cells. The toxins of the Shiga and ricin family inactivate 60S
ribosomal subunits by an N-glycosidic cleavage which releases a specific
adenine base from the sugar-phosphate backbone of 28S rRNA [1,2,3]. The toxins
which are known to function in this manner are:
Shiga toxin from Shigella dysenteriae . This toxin is composed of one
copy of an enzymatically active A subunit and five copies of a B subunit
responsible for binding the toxin complex to specific receptors on the
target cell surface.
Shiga-like toxins (SLT) are a group of Escherichia coli toxins very
similar in their structure and properties to Shiga toxin. The sequence of
two types of these toxins, SLT-1  and SLT-2 , is known.
Ricin, a potent toxin from castor bean seeds. Ricin consists of two
glycosylated chains linked by a disulfide bond. The A chain is
enzymatically active. The B chain is a lectin with a binding preference for
galactosides. Both chains are encoded by a single polypeptidic precursor.
Ricin is classified as a type-II ribosome-inactivating protein (RIP); other
members of this family are agglutinin, also from castor bean, and abrin
from the seeds of the bean Abrus precatorius .
Single chain ribosome-inactivating proteins (type-I RIP) from plants.
Examples of such proteins are: barley protein synthesis inhibitors I and
II, mongolian snake-gourd trichosanthin, sponge gourd luffin-A and -B,
garden four-o'clock MAP, common pokeberry PAP-S and soapwort saporin-6 .
All these toxins are structurally related. A conserved glutamic residue has
been implicated  in the catalytic mechanism; it is located near a conserved
arginine which also plays a role in catalysis . The signature we developed
for these proteins includes these catalytic residues.
PROSITE is copyrighted by the SIB Swiss Institute of Bioinformatics and
distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives
(CC BY-NC-ND 4.0) License, see prosite_license.html.