Ricin inhibition of in vitro protein synthesis by plant ribosomes

Abstract

In vitro translation systems were prepared with supernatant factors from wheat germ and 80S ribosomes from wheat germ, barley embryos, watermelon cotyledons, pea cotyledons, and castor bean endosperm. Ricin A-chain, which strongly inhibits protein synthesis by mammalian ribosomes, inhibited all ofthe plant ribosomal systems by 50% when present at 25-45 ,ug/ ml--23,000 times the concentration needed to inhibit mammalian systems. Ricinus communi agglutinin A-chain, a protein similar to ricin A-chain, inhibited translation by the plant systems 50% at concentrations 5-10 times those of the ricin A-chain. Ribosomes from castor bean endosperm, the source of ricin and the agglutinin, were just as susceptible to the inhibitors as were ribosomes from the other four plants. Compartmentation of the inhibitors within vacuoles derived from protein bodies of the endosperm appears to be responsible for protecting cytoplasmic protein synthesis during germination of castor beans.Ricin and Ricinus communis agglutinin (RCA) are two glycoproteins present in the protein bodies of the endosperm tissue of seeds of castor bean (Ricinus communis) (1). Ricin, a potent toxin for animal cells, is composed oftwo subunits. The B-chain interacts with cell surfaces, allowing the toxin to enter cells, whereas the A-chain acts on ribosomes, thus inhibiting protein synthesis (2). In cell-free translation systems, the A-chain alone has a greater effect on translation than does the complete ricin molecule (2). RCA is structurally similar to ricin, as determined by immunologic relatedness (3) and amino-terminal sequences (4). However, RCA is composed of two A-chains and two Bchains and is much less toxic than ricin (5). The RCA A-chain can inhibit cell-free protein synthesis (4, 5), but Cawley et aL (4) have noted that ricin A-chain is 2 to 5 times more effective than RCA A-chain.Although there is general agreement in the literature on the ability of ricin A-chain to inhibit in vitro synthesis by mammalian 80S ribosomes, the information available for plant systems is confined to conflicting results on wheat germ. Olsnes et aL (2) cited unpublished work that demonstrated that a cellfree system from wheat germ "was strongly inhibited." Later, Cawley et aL (6) found no effect when ricin A-chain was added at 50 kg/ml to a poly(U)-directed wheat germ system, whereas ricin A-chain at 10 ng/ml inhibited a similarly prepared rat liver system by 50%. Recently, Lugnier and Rether (7) have demonstrated that ricin at 24 ug/ml inhibited a poly(U)-directed wheat germ system by 65%.The purpose of the present investigation was to study the effects ofricin, RCA, and their A-chains on cell-free translation systems composed of soluble factors from wheat germ and 80S ribosomes from wheat germ, barley embryos, watermelon cotyledons, pea cotyledons, and castor bean endosperm-the source ofricin and RCA. The results suggest some general conclusions about the response of plant translation systems to the two lectins.MATERIALS AND METHODS Plant Materials. Castor bean...

How Ricin and Shiga Toxin Reach the Cytosol of Target Cells: Retrotranslocation from the Endoplasmic Reticulum

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A maize ribosome-inactivating protein is controlled by the transcriptional activator Opaque-2.

“…In general, nonplant ribosomes seem to be susceptible to RIP inactivation. However, susceptibility of plant ribosomes depends on the source of both the ribosome and the RIP, with homologous ribosomes having at least some resistance to inactivation (Owens et al, 1973;Harley and Beevers, 1982;Reisbig and Bruland, 1983;Battelli et al, 1984). We tested the capacity of purified b-32 to inactivate both maize and wheat germ cell-free translation systems.…”

Structural Insights into the Neutralization Mechanism of Monoclonal Antibody 6C2 against Ricin

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