Ricin B chain is a product of gene duplication

The cytotoxin ricin is a heterodimer composed of an A chain which enzymatically inhibits protein synthesis on eukaryotic ribosomes and a lectin B chain which binds to cell surfaces and triggers uptake of the toxin. A low resolution (4 A) x-ray structure revealed that the B chain is a bilobal structure and that each domain binds a galactose sugar. These apparent structural similarities suggested that the protein might have internal symmetry and might have arisen by gene duplication. A subsequent search of the amino acid sequence provided strong evidence for homology between th NH2- and COOH-terminal halves of the B chain and suggested that they may have arisen from a common ancestor. There is no strong relationship between the halves of the A chain and little, if any, significant sequence homology between the A and B chains of ricin.