A unique echinoderm NLR triggers Vibrio phagocytosis by promoting microtubule severing to facilitate microfilament polymerization

<h2>Abstract</h2> Mammal nucleotide-binding leucine-rich repeat (NLR) immune receptors detect intracellular pathogen effectors and activate immunity. The origin of mammalian NLRs is thought to be closely related to NLR proteins in metazoans and bacteria, where some novel NLRs encoding NACHT domains combined with other domains have been discovered. However, their functions remain largely unknown. Here we report the first AjNLR-related protein (AjNLR-new) in <i>Apostichopus japonicus</i> with an extracellular NACHT domain, which functions as a membrane receptor of <i>Vibrio splendidus</i> (AJ01) to regulate phagocytosis. Mechanistically, the regulation of AJ01 phagocytosis by AjNLR-new relies on its intracellular Ig domain recruiting the microtubule-severing protein AjFidgetin, thereby inducing microtubule severing. This leads to microtubule depolymerize around AJ01, forming vacuoles. Surprisingly, at the site of the microtubule vacuole, F-actin polymerize extensively to envelop AJ01, which depends on the direct interaction between the Ig domain of AjNLR-new and actin. Intriguingly, F-actin polymerization during this process is significantly inhibited by interference with AjNLR-new and AjFidgetin or treatment with the microtubule stabiliser (paclitaxel), whereas the microfilament aggregation inhibitor CK666 does not affect microtubule depolymerization. Ultimately, the phagocytosed AJ01 was cleared via the lysosomal pathway. We have not only revealed a novel mechanism by which AjNLR-new induces microtubule disassembly through the recruitment of AjFidgetin, thereby promoting F-actin polymerisation and facilitating phagocytosis of AJ01, but have also further further enriched and advanced the structural and functional diversity of proteins encoding the NACHT domain.