Molecular characterization and immune association of Fas-associated death domain protein (FADD) in Tenebrio molitor

Abstract

Fas-associated death domain protein (FADD) is an apoptotic adapter molecule that recruits caspases and facilitates the formation of death-inducing signaling complexes in mammals. In lower animals, FADD forms a complex with immune deficiency and death-related ced-3/Nedd2-like caspase to elicit antimicrobial responses. In this study, we characterized the FADD sequence of Tenebrio molitor (TmFADD) using molecular informatics to understand its role in immune surveillance of the host against microorganisms. The 618 bp open reading frame of TmFADD encodes a polypeptide of 204 amino acids. The protein includes two promiscuous domains, the death-effector domain and the death domain, found at the N- and C-termini, respectively. TmFADD is clustered with the coleopteran FADD sequences in the phylogenetic tree. Homology-based structural analysis showed an α-helical model with a QMEAN score of 0.58. The expression of TmFADD mRNA was conspicuous in the larval and early pupal stages. TmFADD mRNA was distributed in the immune tissues of the larva and was found predominately in the gut of the adult. Infection of the host with Candida albicans, Escherichia coli or Staphylococcus aureus increased the expression of TmFADD mRNA, suggesting that these microorganisms trigger the innate immunity of the host. The silencing of TmFADD transcripts resulted in an increased susceptibility of T. molitor larvae to infection with E. coli, but not to infection with C. albicans or S. aureus. Hence, these results indicate that FADD plays a role in the defense of T. molitor and could open avenues for understanding insect immunity.