B cell antigen receptor (BcR) engagement triggers a cascade of biochemical events that culminate in B cell activation, which entails the proliferation and differentiation of B cells. The BcR utilizes phosphorylation of the Igα and Igβ immunoreceptor tyrosine-based activation motifs (ITAMs) to activate nonreceptor protein tyrosine kinases (PTKs). Sequential activation of three distinct classes of PTKs (the Src-PTKs, Syk, and Btk) is required for regulating downstream signalling pathways. Indeed, deficiencies in any of these three families of PTKs result in defective or aberrant B cell function and development .
The efficient and coordinated generation of a number of second messengers, including IP3 (inositol [1,4,5] trisphosphate) and PI(3,4,5)P3 (phosphatidylinositol [3,4,5] triphosphate), is required for normal B cell function. Phosphoinositide 3-kinase (PI3K) is responsible for the production of PI(3,4,5)P3 and participates in the regulation of cell proliferation, survival, metabolism, cytoskeletal reorganization and membrane trafficking. Heterodimer-type (class Ia) PI3Ks consist of a catalytic subunit (p110) and a regulatory subunit encoded by at least three distinct genes (p85α, p85β and p55γ). p85α is the most abundantly expressed regulatory isoform of P13k and the importance of this subunit for B cell development and proliferation has been recently underscored by gene targeting experiments in mice.
Despite the key role of the p85 subunit in BCR signalling, the mechanism by which the BCR-associated PTKs regulate the PI3K pathway still remains unclear. This mechanism is likely to involve the two SH2 domains of the p85 subunit. Both SH2 domains bind with high affinity to phosphotyrosines in the YxxM motif, and this SH2-dependent binding to docking proteins is thought to be important for recruitment of PI3K to the plasma membrane and for increasing the specific activity of the enzyme. Such docking partners include CD19, a B cell-specific membrane protein containing two YxxM motifs in its cytoplasmic region. Indeed, when stimulated by BCR, CD19 is tyrosine phosphorylated, thereby providing the binding site(s) to the p85 subunit. However, given that B cell developmental and functional defects in p85α -/- mice are more severe than those in CD19-/- mice, this CD19-mediated activation mechanism seems not to fully account for the activation mode of the PI3K.
Based upon the hypothesis that additional adaptor molecules may participate in linking between BCR-associated PTKs and PI3K, Okada et al. have purified a novel protein, termed BCAP, by using the p85 SH2 domain .