Upon activation of TLR2/or 4 signaling pathway TIRAP(MAL), a TIR domain–containing adapter protein, undergoes tyrosine phosphorylation (Piao W et al. 2008; Gray P et al. 2006). Bruton's tyrosine kinase (BTK) was shown to mediate the TIRAP phosphorylation (Jefferies CA et al. 2003; Gray P et al. 2006). BTK-specific inhibitor, LFM-A13, blocked the phosphorylation of TIRAP in human monocytic cell line THP-1 stimulated with LPS or macrophage-activating lipopeptide-2 (MALP-2) (Gray P et al. 2006). LFM-A13 also inhibited activation of NFkappaB in LPS-treated THP-1 (Jefferies CA et al. 2003). Besides BTK kinase TIRAP was shown to associate with other kinases such as protein kinase C delta (PKC delta) suggesting their regulatory role in TIRAP activation (Kubo-Murai M et al. 2007).
Tyr-86, Tyr-106 and Tyr-187 were identified as possible phosphorylation sites (Gray P et al. 2006). An additional study has shown that Tyr-86, Tyr-106, and Tyr-159 are important residues, as mutagenesis of these residues impaired TIRAP (MAL) phosphorylation, affected its interaction with BTK and also impaired downstream signaling (Piao W et al. 2008). BTK-mediated phosphorylation of TIRAP leads to recruitment of suppressor of cytokine signaling 1 (SOCS1), which assembles K48-linked polyubiquitin chains resulting in TIRAP's proteosomal degradation, disrupting the TLR complex, and terminating signaling (Mansell A et al. 2006). TIRAP function is also regulated by the cysteine protease caspase-1, which cleaves the protein in a region of the molecule that interacts with MyD88 and TLR4 (Ulrichts P et al. 2010).