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J. Biol. Chem., Vol. 283, Issue 27, 18545-18552, July 4, 2008

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CD20 Homo-oligomers Physically Associate with the B Cell Antigen Receptor

DISSOCIATION UPON RECEPTOR ENGAGEMENT AND RECRUITMENT OF PHOSPHOPROTEINS AND CALMODULIN-BINDING PROTEINS^*

From the Department of Biochemistry and Molecular Biology, Faculty of Medicine, University of Calgary, Calgary, Alberta T2N 4N1, Canada

B cell antigen receptor (BCR) signaling initiates sustained cellular calcium influx necessary for the development, differentiation, and activation of B lymphocytes. CD20 is a B cell-restricted tetraspanning protein organized in the plasma membrane as multimeric molecular complexes involved in BCR-activated calcium entry. Using coprecipitation of native CD20 with tagged or truncated forms of the molecule, we provide here direct evidence of CD20 homo-oligomerization into tetramers. Additionally, the function of CD20 was explored by examining its association with surface-labeled and intracellular proteins before and after BCR signaling. Two major surface-labeled proteins that coprecipitated with CD20 were identified as the heavy and light chains of cell surface IgM, the antigen-binding components of the BCR. After activation, BCR-CD20 complexes dissociated, and phosphoproteins and calmodulin-binding proteins were transiently recruited to CD20. These data provide new evidence of the involvement of CD20 in signaling downstream of the BCR and, together with the previously described involvement of CD20 in calcium influx, the first evidence of physical coupling of the BCR to a calcium entry pathway.

Received for publication, January 30, 2008 , and in revised form, April 17, 2008.

^* This work was supported by an operating grant from the Canadian Institutes of Health Research. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Present address: Centre de recherche-Centre Hospitalier de I'Universitéde Montréal, Hopital Saint-Luc, Montreal, Quebec H2X 1P1, Canada.

² Present address: Dept. of Biochemistry, Tianjin Medical University, Tianjin 300070, China.

³ To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Biology, University of Calgary, Health Sciences Center, 3330 Hospital Dr. N.W., Calgary, Alberta T2N 4N1, Canada. Fax: 403-283-1267; E-mail: jdeans{at}ucalgary.ca.

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