Molecular characterization and expression analysis of target of rapamycin (TmTOR) in coleopteran insect Tenebrio molitor

Target of rapamycin (TOR) is an evolutionarily conserved serine-threonine kinase that affects various cellular functions including growth, development, ageing, immunity and autophagy. Inhibition in TOR activity stimulates autophagy, a self-eating process that ensures survival by maintaining metabolic homeostasis and energy retention at both cellular and organismal levels. To understand the interplay between TOR and autophagy during development and immunity, we screened and identified a TOR homologue from the coleopteran insect, Tenebrio molitor (TmTOR). The full-length cDNA of TmTOR contained an open reading frame (ORF) of 7,197 bp encoding a protein of 2,398 amino acids, and a 5- and 3- untranslated region (UTR) of 156 and 457 bp, respectively. Deduced amino acid sequence of TmTOR shows characteristic Huntington, EF3A, ATM, TOR (HEAT) repeat, focal adhesion kinase targeting (FAT), rapamycin-binding, phosphatidylinositol 3-/4-kinase and FRAP, ATM and TRRAP C-terminal (FATC) domains known to be conserved among TOR orthologs. TmTOR showed high sequence identity (92%) with Tribolium castaneumTOR (TcTOR) and the two were placed in the same cluster of the phylogenetic tree. TmTOR shows high mRNA expression level in the fat body and integument of T.molitor larvae. Significantly, a reduced expression of TmTOR mRNA during pupation correlated to an increase in the area of autolysosomes. Intriguingly, T.molitor larvae showed an increase in TmTOR transcripts upon a challenge with various bacterial pathogens. This suggests a negative role of TmTOR in the regulation of autophagy during development.