The structure and function of ribosome-inactivating proteins

Many plants produce ribosome-inactivating proteins (RIPs) - enzymes that act on ribosomes in a highly specific way, thereby inhibiting protein synthesis. Some RIPs can bind to and enter cells, making them among the most toxic substances known. More commonly, however, RIPs are unable to enter healthy cells, and are therefore poorly cytotoxic, Their role in plants is currently a matter of debate, although it has been suggested that they may have defensive functions, or apr involvement in programmed senescence, in certain organs, The RIPs are frequently produced in large amounts - up to 10% of total protein - that are far in excess of the amounts required to inhibit protein synthesis. In most RIP-producing plants, the ribosomes are sensitive to the enzyme, and the two are physically separated.