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Glycobiology Advance Access published online on July 21, 2005
Glycobiology, doi:10.1093/glycob/cwj020
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© The Author 2005. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oupjournals.org
Received June 14, 2005
Revised July 15, 2005
Accepted July 18, 2005

Article

Monomer/dimer equilibrium of the AB-type lectin from mistletoe enables combination of toxin/agglutinin activities in one protein: Analysis of native and citraconylated proteins by ultracentrifugation/gel filtration and cell biological consequences of dimer destabilization

Marta Jiménez 1, José L. Sáiz 1, Sabine André 2, Hans-J. Gabius 2, and Dolores Solís 1*

1 Instituto de Química Física Rocasolano, CSIC, 28006 Madrid, Spain
2 Institut für Physiologische Chemie, Tierärztliche Fakultät, Ludwig-Maximilians-Universität, 80539 München, Germany

* To whom correspondence should be addressed.
Dolores Solís, E-mail: d.solis{at}iqfr.csic.es


  Abstract

The biological activity of a lectin is influenced by its quaternary structure. Viscumin is special among the family members of toxic AB-type plant lectins, because it triggers mitogenicity, toxicity and agglutination. Its activity profile is dependent on the concentration, motivating a thorough inspection of the status of quaternary structure. Over a broad range of protein concentrations (0.01-25 mg/mL) viscumin occurs as a dimer. At high concentrations the solutions exhibited non-ideality, selfassociation and polydispersity in sedimentation equilibrium and velocity experiments accounted for by irreversible aggregation. Calculation of viscumin’s overall shape based on sedimentation velocity data resulted in an elongated dimer form resembling that of crystallized agglutinin. Appearance of monomers was restricted to concentrations in the submicrogram/mL level, as demonstrated by FPLC gel filtration analysis. In order to shift the equilibrium to the monomer for comparative cell biological assays we performed chemical modification under conditions protecting the lectin activity. Citraconylation was effective to destabilize the dimer. Binding studies by FACScan analysis revealed a reduction in cell association upon modification and a tendency for increased sensitivity toward haptenic inhibitors at µg/mL concentrations. Nonetheless, growth inhibition continued to be potent for the ricin-like monomer despite reduced extent of binding. Occurrence of a concentration-dependent monomer/dimer equilibrium appears to achieve the same objectives as the development of two separate protein entities in Ricinus communis, an alternative strategy to emergence of a monomeric toxin and cell-crosslinking dimeric agglutinin.

Keywords: agglutinin/cytotoxicity/lectin/mistletoe/ribosome-inactivating protein.
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M. Jimenez, S. Andre, H.-C. Siebert, H.-J. Gabius, and D. Solis
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
Glycobiology, October 1, 2006; 16(10): 926 - 937.
[Abstract] [Full Text] [PDF]


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