Iris bulbs express type 1 and type 2 ribosome-inactivating proteins with unusual properties

Two closely related lectins from bulbs of the Dutch iris (Iris hollandica var. Professor Blaauw) have been isolated and cloned. Both lectins, called Iris agglutinin b and Iris agglutinin r, possess N-glycosidase activity and share a high sequence similarity with previously described type 2 ribosome-inactivating proteins (RIP). However, these lectins show only 57% to 59% sequence identity to a previously characterized type 1 RIP from iris, called IRIP. The identification of the iris lectins as type 2 RIP provides unequivocal evidence for the simultaneous occurrence of type 1 and type 2 RIP in iris bulbs and allowed a detailed comparison of type 1 and type 2 RIP from a single plant, which provides further insight into the molecular evolution of RIP. Binding studies and docking experiments revealed that the lectins exhibit binding activity not only toward Gal/N-acetylgalactosamine, but also toward mannose, demonstrating for the first time that RIP-binding sites can accommodate mannose.