Signaling adapter that contributes to B-cell development by linking B-cell receptor (BCR) signaling to the phosphoinositide 3-kinase (PI3K)-Akt signaling pathway. Has a complementary role to the BCR coreceptor CD19, coupling BCR and PI3K activation by providing a docking site for the PI3K subunit PIK3R1. Alternatively, links Toll-like receptor (TLR) signaling to PI3K activation, a process preventing excessive inflammatory cytokine production. Also involved in the activation of PI3K in natural killer cells. May be involved in the survival of mature B-cells via activation of REL. Homooligomer (By similarity). Interacts (phosphorylated on tyrosine residues within YXXM motifs) with PIK3R1 (via SH2 domain); required for BCR- and TLR-mediated activation of phosphoinositide 3-kinase. Interacts (via polyproline C-terminal region) with ABI1 (via SH3 domain); the interaction promotes phosphorylation of PIK3AP1 by ABL1. May interact with MYD88 and TIRAP (By similarity). Expressed in natural killer (NK) cells. The DBB domain is required for dimerization. Constitutively phosphorylated. Phosphorylated on tyrosine residues in C-terminal region by ABL1. Phosphorylated on tyrosine residues within the YXXM motifs by BTK and SYK (By similarity). Isoform 1 and isoform 2 are phosphorylated on tyrosine residues, most likely within the YXXM motifs, via CD19 activation (By similarity). Toll-like receptor activation induces appearance of a phosphorylated form associated with membranes (By similarity).