Crystal structure of mistletoe lectin I from Viscum album

The crystal structure of the ribosome-inactivating protein (RIP) mistletoe lectin I (ML-I) from Viscum album has been solved by molecular replacement techniques. The structure has been refined to a crystallographic R-factor o f 24.5% using X-ray diffraction data to 2.8 Angstrom resolution. The hetero dimeric 63-kDa protein consists of a toxic A subunit which exhibits RNA-gly cosidase activity and a galactose-specific lectin B subunit. The overall pr otein fold is similar to that of ricin hom Ricinus communis; however, unlik e ricin, ML-I is already medically applied as a component of a commercially available misteltoe extract with immunostimulating potency and for the tre atment of human cancer. The three-dimensional structure reported here revea led structural details of this pharmaceutically important protein. The comp arison to the structure of ricin gives more insights into the functional me chanism of this protein, provides structural details for further protein en gineering studies, and may lead to the development of more effective therap eutic RIPs. (C) 1999 Academic Press.