Ricin-like plant toxins are evolutionarily related to single-chain ribosome-inhibiting proteins from Phytolacca

A comparison has been made of the amino-terminal sequences of a number of ribosome-inhibiting proteins (RIPs) and cytotoxins. These include the monomeric enzymes PAP, PAP-S, PAP-II, and dodecandrin and the enzymatic A chains from the heterodimeric toxins ricin and modeccin. We show that these proteins have all evolved from a single ancestor. A statistical analysis is used to show the likely evolutionary relationship among the proteins. A similar analysis was performed on the amino-terminal sequences of ricin, Ricinus agglutinin, and modeccin B chains. These are galactoside-binding proteins associated with the A-chain enzymes. From the two comparisons we propose a scheme for the development of two major classes of proteins. The RIP and sugar-binding genes probably evolved independently. In some plant lines the genes never fused, although the RIP gene replicated and developed into several proteins expressed at various stages of plant maturation. In another line the RIP gene fused with a sugar binding (B-chain) gene to form the class of heterodimeric toxins. In some species this fused gene appears to have multiplied, one or more of the toxin genes mutating to code for a self-dimerizing agglutinin molecule.