Phosphorylation of the multifunctional signal transducer B-cell adaptor protein (BCAP) promotes recruitment of multiple SH2/SH3 proteins including GRB2

Abstract

B-cell adaptor protein (BCAP) is a multimodular, multifunctional signal transducer that regulates signal transduction pathways in leukocytes, including macrophages, B-cells, and T-cells. In particular, BCAP suppresses inflammatory signaling by Toll-like receptors (TLRs). However, how BCAP itself is regulated and what its interaction partners are is unclear. Here, using human immune cell lines, including THP-1 cells, we characterized the complex phosphorylation patterns of BCAP and used a novel protein complex trapping strategy, called virotrap, to identify its interaction partners. This analysis identified known interactions of BCAP with phosphoinositide 3-kinase (PI3K) p85 subunit and NCK adaptor protein (NCK), together with previously unknown interactions of BCAP with Src homology 2 (SH2) and SH3 domain-containing adaptor proteins, notably growth factor receptor-bound protein 2 (GRB2) and CRK-like proto-oncogene, adaptor protein (CRKL). We show that the SH3 domain of GRB2 can bind to BCAP independently of BCAP phosphorylation status, suggesting that the SH2 domains mediate interactions with activated receptor tyrosine kinase complexes including the CD19 subunit of the B-cell receptor. Our results also suggested that the PI3K p85 subunit binds to BCAP via SH3 domains forming an inactive complex that is then activated by sequential binding with the SH2 domains. Taken together, our results indicate that BCAP is a complex hub that processes signals from multiple pathways in diverse cell types of the immune system.

Integrative cBioPortal Analysis Revealed Molecular Mechanisms That Regulate EGFR-PI3K-AKT-mTOR Pathway in Diffuse Gliomas of the Brain

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Phosphoinositide 3-Kinase Signaling in the Tumor Microenvironment: What Do We Need to Consider When Treating Chronic Lymphocytic Leukemia With PI3K Inhibitors?

“…Phosphorylated ITAMs serve as binding sites for Src-homology 2 (SH2) domain-containing proteins such as B-cell PI3K adaptor protein (BCAP) and CD19 ( Figure 2 ). With the help of these proteins, PI3Ks are recruited to the BCR signalosome ( 2 , 3 ). Next, the regulatory subunit interacts with the intracellular section of the activated receptor via its SH2 domain and this event leads to the activation of catalytic p110 isoform which triggers a lipid membrane-associated cascade of phosphorylations (PIP2 to PIP3).…”

Control of T lymphocyte fate decisions by PI3K signaling

“…Coordination of this co-receptor signaling is performed in part by B-cell adapter for PI3K (BCAP), an adaptor protein capable of integrating multiple downstream signaling pathways 12 , 13 . This seems to be mediated by the ability of BCAP to associate with GRB2 and the p85 domain of the inactive form of PI3K (among other proteins) 14 , 15 . Despite its initial discovery in B cells and the absence of its expression in naïve T cells, BCAP is upregulated in T cells following activation, and we will highlight the recent discoveries involving T cells within the scope of this review 16 .…”