2.8-Å crystal structure of a nontoxic type-II ribosome-inactivating protein, ebulin 1

Ebulin 1 is a type-II ribosome-inactivating protein (RIP) isolated from the leaves of Sambucus ebulus L, As with other type-II RIP, ebulin is a disulfide-linked heterodimer composed of a toxic A chain and a galactoside-specific lectin B chain, A normal level of ribosome-inactivating N-glycosidase activity, characteristic of the A chain of type-II RIP, has been demonstrated for ebulin 1, However, ebulin is considered a nontoxic type-II RIP due to a reduced cytotoxicity on whole cells and animals as compared with other toxic type-II RIP like ricin, The molecular cloning, amino acid sequence, and the crystal structure of ebulin 1 are presented and compared with ricin, Ebulin 1 is shown to bind an A-chain substrate analogue, pteroic acid, in the same manner as ricin, The galactoside-binding ability of ebulin 1 is demonstrated crystallographically with a complex of the B chain with galactose and with lactose, The negligible cytotoxicity of ebulin 1 is apparently due to a reduced affinity for galactosides. An altered mode of galactoside binding in the 2 gamma subdomain of the lectin B chain primarily causes the reduced affinity. (C) 2001 Wiley-Liss, Inc.