LAUSR

Abstract Interleukin‐1 receptor‐associated kinases (IRAKs) play important roles in MyD88‐dependent TLR signaling, the crucial innate immune pathway in molluscs. In this study, we examined the full‐length IRAK4 genetic sequence in the Pacific oyster (Crassostrea gigas) by molecular cloning. Phylogenetic analysis revealed that CgIRAK4 is most closely related to Mytilus edulis, and forms a clade with other molluscs. CgIRAK4 transcripts are widely expressed in all tissues, with the highest expression observed in the hemocytes and gill. Moreover, CgIRAK4 is significantly upregulated after Oyster herpesvirus‐1 microvariant (OsHV‐1 &mgr;var), Vibrio alginolyticus, and poly I:C challenge. Yeast two‐hybrid and co‐immunoprecipitation assays reveal that the CgIRAK4 death domain is necessary to mediate interaction between CgIRAK4 and two CgMyD88 isoforms. In addition, CgIRAK4 overexpression cannot induce NF‐&kgr;B transcriptional activity, but blocks that induced by CgMyD88 in HEK293T cells. These findings elucidate the mechanisms of MyD88‐dependent TLR signaling in molluscs, and the differences in IRAK‐mediated pathway activation between invertebrates and vertebrates. HighlightsIL‐1 receptor‐associated kinases (IRAKs) mediate TLR/MyD88 signaling in molluscs.CgIRAK4 is broadly expressed in all tissues and induced by pathogenic stimuli.The CgIRAK4 death domain mediates its direct interaction with MyD88.CgIRAK4 blocks NF‐&kgr;B activity induced by exogenous CgMyD88 overexpression.Our data indicate that CgIRAK4 acts as a negative regulator of TLR/MyD88 signaling.