Plant Physiol, February 2001, Vol. 125, pp. 866-876

Iris Bulbs Express Type 1 and Type 2 Ribosome-Inactivating Proteins with Unusual Properties¹

Laboratory for Phytopathology and Plant Protection, Katholieke Universiteit Leuven, Willem de Croylaan 42, 3001 Leuven, Belgium (Q.H., E.J.M.V.D., Y.C., W.P.); Institute of Plant Biochemistry, P.O. Box 110432, D-06018 Halle, Germany (B.H.); and Institut de Pharmacologie et Biologie Structurale, Unité Propre de Recherche Centre National de la Recherche Scientifique 9062, 205 Route de Narbonne, 31077 Toulouse cedex, France (A.B., P.R.)

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.