A novel B lymphocyte-associated adaptor protein, Bam32, regulates antigen receptor signaling downstream of phosphatidylinositol 3-kinase

We have identified and characterized a novel src homology 2 (SH2) and pleck strin homology (PH) domain-containing adaptor protein, designated Bam32 (fo r B cell adaptor molecule of 32 kD). cDNAs encoding the human and mouse Bam 32 coding sequences were isolated and the human bam32 gene was mapped to ch romosome 4q25-q27. Bam32 is expressed by B lymphocytes, but not T lymphocyt es or nonhematopoietic cells. Human germinal center B cells show increased Bam32 expression, and resting B cells rapidly upregulate expression of Bam3 2 after ligation of CD40, but not immunoglobulin M. Bam32 is tyrosine-phosp horylated upon B cell antigen receptor (BCR) ligation or pervanadate stimul ation and associates with phospholipase C gamma 2. After BCR ligation, Bam3 2 is recruited to the plasma membrane through its PH domain. Membrane recru itment requires phosphatidylinositol 3-kinase (PI3K) activity and an intact PI(3,4,5)P-3-binding motif, suggesting that membrane association occurs th rough binding to 3-phosphoinositides. Expression of Bam32 in B cells leads to a dose-dependent inhibition of BCR-induced activation of nuclear factor of activated T cells (NF-AT), which-is blocked by deletion of the PH domain or mutation of the PI(3,4,5)P-3-binding motif. Thus, Bam32 represents a no vel B cell-associated adaptor that regulates BCR signaling downstream of PI 3K.