Mistletoe lectin I (ML-I) from Viscum album belongs to the ribosomeinactivating proteins type II. The bifunctional protein exhibits RNA N-glycosidase activity with ...
Crystal structure of Mistletoe lectin I in complex with lactose and galactose R. Krauspenhaar1, M. Perbandt1, V. Kornilov2, N. Konareva3, I. Mikailova4, S. Stoeva5, R. Wacker5, M. Franz5, T. P. Singh6, A. Mikhailov3, W. Voelter5 and C. Betzel1 1
Institute of med. Biochem. a. Molecularbiol., University Hospital, c/o DESY, Building 22a, Notkestr. 85, 22603 Hamburg, Germany 2
3
Institute of Cell Biophysics, Russian Academy of Sciences, Pushino 142292, Russian Federation
Institute of Crystallography of Russian Academy of Sciences, Leninsky Prospect 59, Moscow 117333, Russian Federation 4
5
Tambov State University, International Street 33, Tambov 392622, Russian Federation
Department of Physiological Biochemistry, Institute of Physiological Chemistry, University of Tübingen, Hoppe-Seyler-Str. 4, 72076 Tübingen, Germany 6
Department of Biophysics, All India Institute od Medical Sciences, 110029 New Dehli, India
Mistletoe lectin I (ML-I) from Viscum album belongs to the ribosomeinactivating proteins type II. The bifunctional protein exhibits RNA N-glycosidase activity with its A subunit and has lectin activity in the B subunit. The A chain has toxic potential by inhibiting protein biosynthesis in eucaryotic cells when translocated through target cell membranes via binding to galactose terminated glycolipids and glycoproteins on the cell surface. ML-I is the main component of commercially available mistletoe extracts applied for the treatment of human cancer. Crystals of Mistletoe lectin I in complex with lactose and galactose diffracted to about 2.3 Å and data sets of both sugar complexes were collected using cryo conditions. Crystals of ML-I in complex with lactose belong to the space group P6522 with the unit cell parameters a=b= 107.01, c= 312.11, α=β= 90°, γ= 120°. Crystals of ML-I in complex with galactose belong also to space group P6522 having the unit cell parameters a=b= 106.83, c= 310.95, α=β= 90°, γ= 120°. Saccharide binding is performed by two galactose-binding sites in domain α1 and 2γ of ML-I Bchain (figure 1), seperated 62 Å from each other. Galactose recognition is based on mainly two key features [1, 2]. Epimeric specifity is mediated by hydrogen bonds to the sugar functional groups 3’OH and 4’OH [3]. Additionally aromatic stacking between an aromatic ring amino acid site chain (Trp 38 and Tyr 248) towards the apolar site of the galactose moiety is prominent for gal-specific lectins. ML-I belongs to the group II carbohydrate-binding proteins which bind their ligands in shallow preformed pockets on the protein surface [4]. No hydrogen bonds to the glucose moiety of lactose and the protein can be detected so interactions between the lectin and both saccharide moieties are the same. An additional structural feature found in the both sugar complexed as well as in native ML-I is that glycerol is prone to bind to two galactose-binding site in the native structure and to the potential third sugar-binding site predicted for ricin, a well investigated RIP protein type II, underlines that glycerol mimics the sugar moiety and it is more favoured than water molecules as reported for rat liver mannose-binding protein [5].
With regard to drug design for more efficient binding introducing additional galactose-binding site in the B-chain of ML-I the potential third site in subunit 1β is highly preferable and prone for galactose-binding. The structures of ML-I in complex with lactose and galactose provide an insight in the sugar recognition process of a galactose specific lectin and give deeper information for target specific drug design as vehicle through cell membranes.
Figure 1: Cα-trace of Mistletoe lection I in comlex with lactose. Lactose bound to galactose-binding site I and II, sugar residues in the glycosylation sites and active site residues are shown as ball-and-stick. The potential third galactose-binding site is shown with the bound glycerol moiety as ball-and-stick.
References [1] Elgavish, S. and Shaanan, B. (1998) J. Mol. Biol. 277, 917-932. [2] Weiss, W. I. and Drickamer, K. (1996) Annu. Rev. Biochem. 65, 441-73. [3] Drickamer, K. (1997) Structure 5, 465-468. [4] Rini, J. M. (1995). Annu. Rev. Biophys. Biomol. Struct. 24, 551-577. [5] Ng Kenneth K.-S., Drickamer, K. and Weis, W.I. (1996) J. Biol. Chem. 271, 663-674.
