Gene structure, cDNA characterization and RNAi-based functional analysis of a myeloid differentiation factor 88 homolog in Tenebrio molitor larvae exposed to Staphylococcus aureus infection.

Myeloid differentiation factor 88 (MyD88), an intracellular adaptor protein involved in Toll/Toll-like receptor (TLR) signal processing, triggers activation of nuclear factor-kappaB (NF-κB) transcription factors. In the present study, we analyzed the gene structure and biological function of MyD88 in a coleopteran insect, Tenebrio molitor (TmMyD88). The TmMyD88 gene was 1380 bp in length and consisted of five exons and four introns. The 5'-flanking sequence revealed several putative transcription factor binding sites, such as STAT-4, AP-1, cJun, cfos, NF-1 and many heat shock factor binding elements. The cDNA contained a typical death domain, a conservative Toll-like interleukin-1 receptor (TIR) domain, and a C-terminal extension (CTE). The TmMyD88 TIR domain showed three significantly conserved motifs for interacting with the TIR domain of TLRs. TmMyD88 was grouped within the invertebrate cluster of the phylogenetic tree and shared 75% sequence identity with the TIR domain of Tribolium castaneum MyD88. Homology modeling of the TmMyD88 TIR domain revealed five parallel β-strands surrounded by five α-helices that adopted loop conformations to function as an adaptor. TmMyD88 expression was upregulated 7.3- and 4.79-fold after 12 and 6h, respectively, of challenge with Staphylococcus aureus and fungal β-1,3 glucan. Silencing of the TmMyD88 transcript by RNA interference led to reduced resistance of the host to infection by S. aureus. These results indicate that TmMyD88 is required for survival against Staphylococcus infection.