THE LIGAND-INDUCED MEMBRANE IGM ASSOCIATION WITH THE CYTOSKELETAL MATRIX OF B-CELLS IS NOT MEDIATED THROUGH THE IG ALPHA-BETA HETERODIMER

The B cell Ag receptor complex consists of membrane-associated Ig (mIg ), Ig alpha, and Ig beta, associated molecules that have been implicat ed in transducing the activation signal that occurs upon receptor cros s-linking. The role of the Ig alpha beta heterodimer in mediating bind ing to the cytoskeleton is unknown, We studied the ligand-induced asso ciation of mIgM with the cytoskeleton following receptor cross-linking in mIgM-expressing B lymphoma lines by biochemical assays, FACS analy sis, and electron microscopy. Cytoskeletal association is not detected in unstimulated cells, but occurs rapidly upon anti-IgM-mediated cros s-linking. Ig alpha is absent from the cytoskeleton-mIgM complex. To f urther analyze the possible role of Ig alpha beta in cytoskeletal bind ing. a surface Ig alpha beta-negative plasmacytoma line was transfecte d 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 bindin g 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 associa tion of the receptor with the cytoskeleton appeared to be more extensi ve (100%) and faster than with mouse mIgM. These data indicate that Ig -associated Ig alpha beta proteins are not required for mIgM associati on with the cytoskeleton.