Glycobiology Advance Access published online on June 14, 2006
Glycobiology, doi:10.1093/glycob/cwl017
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1 Instituto de Química Física Rocasolano, CSIC, Serrano 119, E-28006 Madrid, Spain
* To whom correspondence should be addressed. Viscumin of mistletoe (Viscum album L.) has a concentration-dependent activity profile unique for plant AB-toxins. It starts with lectin-dependent mitogenicity and then covers toxicity and cell agglutination, associated with shifts in the monomer/dimer equilibrium. Each lectin subunit harbours two sections for ligand contact. In the dimer, the B-chains’ sites in subdomain 2
Received February 8, 2006
Revised June 5, 2006
Accepted June 12, 2006
Article
AB-type lectin (toxin/agglutinin) from mistletoe: differences in affinity of the two galactoside-binding Trp/Tyr-sites and regulation of their functionality by monomer/dimer equilibrium
Marta Jiménez 1,
Sabine André 2,
Hans-C. Siebert 2,
Hans-J. Gabius 2,
and
Dolores Solís 1 *
2 Institut für Physiologische Chemie, Tierärztliche Fakultät, Ludwig-Maximilians-Universität, Veterinärstrasse 13, D-80539 München, Germany
Dolores Solís, E-mail: d.solis{at}iqfr.csic.es
Abstract 
(designated as the Tyr-sites) appear fully accessible, whereas Trp-sites in subdomain 1
are close to the dimer interface. It is unclear whether both types of site operate similarly in binding glycoligands in solution. By systematically covering a broad range of lactose/lectin ratio in isothermal titration calorimetry, evidence for two sites showing dissimilar binding affinity was obtained. Intriguingly, the site with higher affinity was only partially occupied. To assign the observed properties to the Trp/Tyr-sites, we next performed chemically induced dynamic nuclear polarization measurements of Trp and Tyr accessibility. A Tyr signal but no distinct Trp peaks were recorded when testing the dimer. Lactose-quenchable Trp peaks became visible upon destabilization of the dimer by citraconylation, intimating Trp involvement in ligand contact in the monomer. Fittingly, Tyr acetylation but not mild Trp oxidation reduced the dimer’s haemagglutination activity and extent of binding to asialofetuin-Sepharose 4B. Altogether, the results attribute lectin activity in the dimer primarily to Tyr-sites. Full access to Trp-sites is gained upon dimer dissociation. Thus, the monomer/dimer equilibrium of viscumin regulates the operativity of these sites. Their structural divergence affords the possibility for differences in ligand selection when comparing monomer (Tyr- and Trp-sites) vs dimer (primarily Tyr-sites).
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