The Uptake and Effects of Ricin in Humans
Introduction
Cells have semi-permeable membranes which are highly selective when sequestering materials,
but occasionally virulent cells can emulate beneficial materials and fool the safety mechanisms
of the body. Consequently, this material finds its way into healthy functioning cells, potentially
causing apoptosis, or massive cell death. In order to deceive cell machinery, these cells employ
a variety of tactics; one of which involves utilizing multiple protein chains, each providing a
different function. Through protein folding, these constituent chains combine to form a complex
quaternary structure, or a three dimensional structure made up of two or more individual
proteins. A subset of these multi chain proteins are known as the A-B family of toxins,
consisting of two proteins, an A-moiety with enzymatic function, and a B-moiety with receptor
binding function.
Among the A-B family of toxins, ricin belongs to a type of
Credit: S. Olsnes, J.V. Kozlov/ Toxicon 39
(2001) 1723-1728
protein family known as lectins, which bind to carbohydrates
and provide a variety of functions, due to its’ A-chain. Ricin
also acts as a ribosome inactivating protein (RIP), or a protein
synthesis inhibitor, due to its’ B-chain. However, without both protein
chains, ricin cannot function properly and becomes completely
harmless in the body. As a result, ricin undergoes very specific
processing during cell uptake. This technical document will describe
how both protein subunits function and the mechanism by which ricin
enters and effects cells.
A-Chain
The A chain works as an enzyme which binds to rRNA and elicits a
conformational change, modifying the output of protein synthesis. As
shown in figure 1, ricin attacks the 28 S portion of the rRNA, removing
an adenine and thus debilitating protein functioning. To be specific,
the A-Chain does not explicitly break the RNA chain, but however acts
as a catalyst, necessitating depurination, or removal of the adenine
Figure 1: The 28 S ribosomal
residue. Thus, ricin fosters a chemical reaction which removes the
RNA is shown. As indicated
ricin(R) removes an adenine
adenine residue, and leaves rRNA susceptible to hydrolysis, which
residue.
renders the protein unable to perform synthesis. Due to implicitly
effecting rRNA, a single molecule of ricin can influence a few thousand
ribosomes per minute, making small doses very toxic with a high efficacy.
B-Chain
Whereas the A-chain acts as an RIP, the B-chain contains the leptin component of the poison,
allowing ricin to bind to receptors on the cell, transporting the protein into the cell. Specifically,
the B-chain contains three binding peptide chains on the outside. These binding peptides are
shown in white on the periphery of the B-chain displayed in figure 2, with the small white
peptide chain between the A-B chains linking the two subunits. Only two of the three binding
peptides are capable of binding to galactose, which subsequently binds to receptor ligands, or
signaling receptors which trigger cellular functioning. Though only two of these chains have
this functioning, an average cell contains millions of binding sites, and so, ricin has copious
amounts of entry points into an average cell.
Uptake, Routing and Inhibition
Credit: Dr. Cyr, Penn State Biology
Figure 2: Ricin A-B chains; white network structures are
peptide chain binding catalyst.
After reaching the ER, ricin needs to
undergo some changes before it becomes
activated, and so, the toxic A-chain and the
lectin B-chain are cleaved in order to
prepare the A-chain for transportation into
the cytosol, where it will be primed for
action. While in the ER, the newly
independent A-chain unfolds and embeds
itself into the ER, which normally signals to
a cell that it requires ubiquination, a cellular
process which often yields the degradation of
proteins or molecules no longer functioning
properly. Identified as just another misfolded
protein, the cell targets the A-chain of ricin,
and transports it to the cytosol using protein
complexes used for ubiquination.
Once the binding of the B-chain to an outer
membrane receptor on a cell, via a ligand, cell
signaling will trigger endocytosis, bringing a
properly functioning ricin protein into the cell.
Upon entering a cell, ricin aims to make it to the
endoplasmic reticulum (ER), where much of
protein synthesis occurs, but it cannot travel
straight to its final destination. Enveloped in an
endosome, or membranous enclosure, from
endocytosis, ricin travels to the trans-Golgi
network, which acts as a hub for accepting and
sending material throughout the cell. Binding to
a carrier protein, ricin then makes its’ way from
the Golgi apparatus to the ER. This process can
be shown in Figure 3, which displays both
normal cell processing, anterograde transport,
and the retrograde transport ricin follows.
Credit: Dr. Cyr, Penn State Biology
Figure 3: Red arrows depict anterograde transport; Blue arrows
depict retrograde transport.
However, once in the cytosol, the A-chain does not go through the normal degradation process,
due to ricin possessing a unique structure lacking binding sites necessary for the breakdown
process to occur. Instead, the A-chain undergoes a sorting process, which results in a newly
folded state which has the catalytic conformation needed for protein inactivation. The properly
folded ricin A-chain then depurinates adenine residues within rRNA rapidly, rendering
ribosomes non-functioning and resulting in toxicity. Finally in its’ active state, a single
molecule of ricin will inhibit around 1500 ribosomes per minute, resulting in apoptosis for
virtually all cells effected.