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J Biol Chem, Vol. 273, Issue 48, 31795-31800, November 27, 1998

Binding of Anti-CD23 Monoclonal Antibody to the Leucine Zipper Motif of FcRII/CD23 on B Cell Membrane Promotes Its Proteolytic Cleavage
EVIDENCE FOR AN EFFECT ON THE OLIGOMER/MONOMER EQUILIBRIUM

Olivier Munoz, Chrystelle Brignone, Nicole Grenier-Brossette, Jean-Yves Bonnefoy^§, and Jean-Louis Cousin

From INSERM U343, Hopital de l'Archet, B.P. 79, F-06202 Nice cedex 03, France,  INSERM U364, Faculté de Médecine (Pasteur) F-06107 Nice cedex 02, France, and the ^§ Glaxo Institute for Molecular Biology, chemin des Aulx, 1228 Plan-les Ouates, Geneva, Switzerland

In the present study we have compared the binding of two monoclonal antibodies to CD23, EBVCS1 and mAb25, which recognize the stalk and the lectin domain, respectively, on the CD23 molecule. At 4 °C, EBVCS1 binds to about 10% of the receptors recognized by mAb25 on the B cell surface. At 37 °C, whereas mAb25 reaches its maximal binding within a few seconds, EBVCS1 requires 60 min to bind to the same extent. Stabilization of the oligomeric structure of CD23 with IgE strongly affects in a dose-dependent fashion the number of binding sites seen by EBVCS1 but not the t_1/2 to reach them, suggesting that EBVCS1 binds to the coiled coil region through an allosteric mechanism. EBVCS1 rapidly down-modulates the membrane CD23 expression with a coincident increase of CD23-soluble fragments in the culture medium, an effect that is inhibited by IgE. In contrast, mAb25, as well as IgE, protects CD23 from proteolytic cleavage and stimulates its endocytosis. These results suggest that EBVCS1 unravels the coiled coil structure of CD23, rendering it more susceptible to proteolytic attack. This supports the oligomeric model proposed previously (Gould, H., Sutton, B., Edmeades, R., and Beavil, A. (1991) Monogr. Allergy 29, 28-49). The biological significance of these observations is discussed.

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