Monomeric and oligomeric complexes of the B cell antigen receptor

The current structural model of the B cell antigen receptor (BCR) describes it as a symmetric protein complex in which one membrane-bound immunoglobul in molecule (mlg) is noncovalently bound on each side by an Ig-alpha/Ig-bet a heterodimer. Using peptide-tagged Ig-alpha proteins, blue native polyacry lamide gel electrophoresis (BN-PAGE), and biosynthetical labeling of B cell s, we find that the mIg:Ig-alpha/Ig-beta complex has a stoichiometry of 1:1 and not 1:2. An anti-flag stimulation of B cells coexpressing Flag-tagged and wild-type Ig-alpha proteins results in the phosphorylation of both Ig-a lpha proteins, suggesting that on the surface of living B cells, several BC R monomers are in contact with each other. A BN-PAGE analysis after limited detergent lysis provides further evidence for an oligomeric BCR structure.