PTRAP adaptor protein scaffolds Syk for TLR4 signalling

Abstract

Spleen tyrosine kinase (Syk) is a non-receptor protein tyrosine kinase, which is known to relay adaptive and innate immune signalling. The recruitment of Syk to cell surface receptors has been well characterised in ITAM-containing immunoreceptors like BCR and TCR. In these cases, Syk is activated by binding of its two SH2 domains to appropriately spaced phosphorylated tyrosine motifs. However, no conserved ITAM motif is identified in TLR4 itself. The underlying mechanism for Syk activation and recruitment to TLR4 is still unclear. One possibility is that the Syk-TLR association is indirect, relying on a scaffolding protein. In our study, we identified the transmembrane adaptor SCIMP, a member of the pTRAP family, as a scaffold for Syk binding and activation via pull-down and co-immunoprecipitation methods. And by using site-directed mutagenesis and domain truncation, we further pinpointed two specific phosphorylation sites of the adaptor protein that are responsible for interaction with Syk-SH2 domains. Use of pharmacological inhibitors also showed evidence for a functional role for Syk in TLR phosphorylation, signalling and cytokine outputs in macrophages. Therefore, we identified SCIMP as a novel Syk scaffold, which can also contribute to inflammation in autoimmune and chronic diseases.