CRYSTALLIZATION OF THE RIBOSOME-INACTIVATING PROTEIN-ML1 FROM VISCUM-ALBUM (MISTLETOE) COMPLEXED WITH BETA-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 P6122 or P6522, 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.