Aj-rel and Aj-p105, two evolutionary conserved NF-κB homologues in sea cucumber (Apostichopus japonicus) and their involvement in LPS induced immunity.

The nuclear factor κB (NF-κB) has been evolutionary conserved from insects to mammals and plays a major regulatory role in the initiation of physiological responses. In this study, we identified and characterized a primitive and functional NF-κB pathway active in the immune defence of the sea cucumber (Apostichopus japonicus). The ancient NF-κB homologues, Aj-rel and Aj-p105, share numerous signature motifs with their vertebrate orthologues, notably the Rel Homology Domain, Rel Protein Signature DNA Binding Motif, Nuclear Localization Signal and the Ankyrin Repeats for Aj-p105. Phylogenetic analyses indicate that these homologues belong to class I and II of NF-κB respectively. We examined the dimerization of Aj-rel and Aj-p105 and our results demonstrated that Aj-rel forms heterdimers with Aj-p105 and the degradation product of Aj-p105, namely Aj-p50. We further observed that LPS stimulation led to the degradation of Aj-p105 and the nuclear translocation of Aj-rel and Aj-p50. Taken together, our data indicate that the NF-κB signaling cascade is active in sea cucumber and plays a crucial role in regulating their immune defence. Our results increase the available information on sea cucumber immunity and provide new information for use in the study of the comparative and evolutionary aspects of immunity.