THE STRUCTURE AND FUNCTION OF RIBOSOME-INACTIVATING PROTEINS

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