Crystallization of the Ribosome Inactivating Protein ML1 from Viscum Album (Mistletoe) Complexed with β- d-Galactose

A ribosome inactivating protein (ML1) from the mistletoe plant ( Viscum album) has been crystallized. The crystals, grown in the presence of β- d-galactose, are hexagonal, space group P6 122 or P6 522, a = b = 111·0 Å, c = 309·3 Å with 24 molecules per unit cell (assuming 33% solvent by weight). The protein of molecular mass 63 kDa is a heterodimer consisting of two chains, A and B, joined by a disulfide bond. The A-chain, 29 kDa, inhibits protein synthesis by depurinating an adenine residue (A4324) in a highly conserved RNA loop of the 28 S ribosomal subunit. The toxicity of the protein is mediated by the B-chain, 34 kDa, which has lectin activity, interacting with sugar residues of glycoproteins and glycolipids on the surface of target cells.