Fig. 4

Immune deficiency (IMD) pathway members in malacostracans. a PGRPs recognises Gram-negative bacteria and activate the IMD pathway through the RHIM motifs. Although the IMD pathway is typically activated by PGRPs in Drosophila melanogaster, PGRPs are not necessary for IMD signalling and it was posited that an unknown protein is present upstream of the IMD signalling cascade. Like DIF from the Toll pathway, in the IMD pathway, differential activation of another NF-κB transcription factor, Relish, occurs. Relish is phosphorylated through the activation of IκB kinase (IKK) complexes and transforming growth factor-β-activated kinase 1 (TAK1). The caspase-8 homolog death-related ced-3/Nedd2-like protein (DREDD) and FAS-associated death domain (FADD) proteins are required for IKK and TAK1 activation and Relish is cleaved through DREDD. Caspar, a homologue of mammalian Fas-associating factor 1 that is essential for antifungal immunity, negatively regulates the IMD-mediated immune response by preventing nuclear translocation of Relish. Caspar also suppresses the IMD pathway through targeting Dredd-dependent cleavage of Relish. Phylogenetic trees of b IMD, c Caspar, d Relish and e DREDD are constructed using the maximum-likelihood method from an amino acid multiple sequence alignment. Taxa labels are depicted as their respective colour codes. Bootstrap support values (n = 1000) for all trees can be found in Additional file 26: Figure S14. Scale bar represents substitution per site