Amphioxus tyrosine kinase SYK inhibits NF-κB activation through MyD88-dependent mechanism.

The spleen tyrosine kinase (SYK) plays pivotal roles in the adaptive immune response by recognizing phosphorylated immunoreceptor tyrosine-based activation motif (ITAM) and activating downstream effectors. However, the emergence of SYK predates the advent of adaptive immunity, and its original functions independent of the adaptive immune system remain largely unknown. In this study, we identified and characterized a SYK homolog (bjSYK) in the amphioxus Branchiostoma japonicum, a key model for understanding immune system evolution. Phylogenetic analysis positioned bjSYK basally among deuterostome SYK kinases. Expression analysis revealed its localization in immune-relevant tissues. Contrary to the canonical activating role in vertebrates, functional studies demonstrated that bjSYK acts as a potent suppressor of nuclear factor-kappa B (NF-κB) activation. Specifically, bjSYK directly interacts with bjMyD88 via its SH2 domains binding to a conserved hemi-ITAM motif, suppressing K63-linked polyubiquitination of bjMyD88 and consequently inhibiting bjMyD88-dependent NF-κB signaling. Furthermore, bjSYK also suppresses NF-κB activation through bjTRAF6, a key adaptor downstream of bjMyD88, by inhibiting its ubiquitination. These findings reveal an evolutionarily ancient inhibitory role of bjSYK in NF-κB signaling in basal chordate innate immunity, suggesting that SYK-mediated negative regulation of Toll-like receptor pathways originates in amphioxus and provides a foundation for the context-dependent functions of SYK in vertebrate immunity.