Immunotoxins containing single chain ribosome-inactivating proteins

Abstract
Proteins1 that catalytically inactivate eukaryotic ribosomes have been identified in the extracts of a wide variety of plants [1–3]. Such ribosome-inactivating proteins can be divided into two classes. One class comprises proteins that consist of two nonidentical subunits, each of Mr about 30,000 (A and B chains), that are joined by a disulfide bond [1, 2, 4]. Ricin, which has been known for many years, is the best studied example of this class, which also includes abrin, modeccin, volkensin, and viscumin [3, 4]. The B chains of these proteins have the property of binding to cell-surface carbohydrates and promote the uptake of their A chain into cells [1–4]. Entry of the A chain of these toxins into the cytoplasm of a cell then results in the death of the cell by inactivation of its ribosomes. Members of this class of ribosome-inactivating proteins are extremely cytotoxic for cultured cell lines and are very poisonous for animals.

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