The Journal of Immunology, Vol 155, Issue 8 3769-3779, Copyright © 1995 by American Association of Immunologists

ARTICLES

The ligand-induced membrane IgM association with the cytoskeletal matrix of B cells is not mediated through the Ig alpha beta heterodimer

JH Hartwig, LS Jugloff, NJ De Groot, SA Grupp and J Jongstra-Bilen
Department of Immunology, University of Toronto, Ontario, Canada.

The B cell Ag receptor complex consists of membrane-associated Ig (mIg), Ig alpha, and Ig beta, associated molecules that have been implicated in transducing the activation signal that occurs upon receptor cross-linking. The role of the Ig alpha beta heterodimer in mediating binding to the cytoskeleton is unknown. We studied the ligand- induced association of mIgM with the cytoskeleton following receptor cross-linking in mIgM-expressing B lymphoma lines by biochemical assays, FACS analysis, and electron microscopy. Cytoskeletal association is not detected in unstimulated cells, but occurs rapidly upon anti-IgM-mediated cross-linking. Ig alpha is absent from the cytoskeleton-mIgM complex. To further analyze the possible role of Ig alpha beta in cytoskeletal binding, a surface Ig alpha beta-negative plasmacytoma line was transfected with a mutant form of mIgM (IgM- MutA). IgM-MutA is expressed on the surface despite the lack of Ig alpha beta, and the cytoskeletal binding occurred to a similar extent as in Ig-alpha-positive cell lines. In another transfectant expressing a mutated form of human mIgM (YS:VV), which does not have the capacity to bind to Ig alpha beta, the association of the receptor with the cytoskeleton appeared to be more extensive (100%) and faster than with mouse mIgM. These data indicate that Ig-associated Ig alpha beta proteins are not required for mIgM association with the cytoskeleton.

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