The B Lymphocyte Adaptor Molecule of 32 Kilodaltons (Bam32) Regulates B Cell Antigen Receptor Internalization

The B lymphocyte adaptor molecule of 32 kDa (Bam32) is an adaptor that plays an indispensable role in BCR signaling. In this study, we found that upon BCR ligation, Bam32 is recruited to the plasma membrane where it associates with BCR complexes and redistributes and internalizes with BCRs. BCR ligation induced colocalization of Bam32 with lipid rafts, clathrin, and actin filaments. An inhibitor of Src family protein tyrosine kinases (PTKs) blocked both BCR-induced tyrosine phosphorylation of Bam32 and BCR internalization. Moreover, BCR internalization is impaired in Bam32-/- and Lyn-/- cells, and expression of Bam32 with a mutation of its tyrosine phosphorylation site (Y139F) inhibited BCR internalization. These data suggest that Bam32 functions downstream of Src family PTKs to regulate BCR internalization. Bam32 deficiency does not affect tyrosine phosphorylation of clathrin or the association of clathrin with lipid rafts upon BCR cross-linking. However, BCR-induced actin polymerization is impaired in Bam32-/- cells. Collectively, these findings indicate a novel role of Bam32 in connecting Src family PTKs to BCR internalization by an actin-dependent mechanism.