This is a Toll/interleukin-1 receptor (TIR) domain found in the N-terminal region of B-cell adaptor for phosphoinositide 3-kinase (BCAP). BCAP functions in linking the B-cell receptor (BCR) and the co-receptor CD19 to the activation of PI3K via interaction with the SH2 domains on the regulatory p85 subunit. BCAP TIR associates with the MAL/TIRAP adaptor and the TIR domains of Toll-like receptors (TLRs) [1].

1: Structure of the Toll/Interleukin-1 Receptor (TIR) Domain of the B-cell Adaptor That Links Phosphoinositide Metabolism with the Negative Regulation of the Toll-like Receptor (TLR) Signalosome. Halabi S, Sekine E, Verstak B, Gay NJ, Moncrieffe MC; J Biol Chem. 2017;292:652-660. PMID:27909057