Abstract:
Toll-like receptors (TLRs) are a large family of pattern recognition receptors, which are involved in triggering host
immune responses against various pathogens by detecting their evolutionarily conserved pathogen associated molecular patterns (PAMPs). TLR21 is a non-mammalian type TLR, which recognizes unmethylated CpG DNA, and is
considered as a functional homolog of mammalian TLR9. In this study, we attempted to identify and characterize
a novel TLR21 counterpart from rock bream (Oplegnathus fasciatus) designated as RbTLR21, at molecular level. The
complete coding sequence of RbTLR21 was 2919 bp in length, which encodes a polypeptide of 973 amino acids
with a predicted molecular mass of 112 kDa and a theoretical isoelectric point of 8.6. The structure of the deduced
RbTLR21 protein is similar to that of the members of typical TLR family, and includes the ectodomain, which consists
of 16 leucine rich repeats (LRRs), a transmembrane domain, and a cytoplasmic Toll/interleukin-1 receptor (TIR) domain. According to the pairwise sequence analysis data, RbTLR21 was homologous to that of the orange-spotted
grouper (Epinephelus coioides) with 76.9% amino acid identity. Furthermore, our phylogenetic analysis revealed
that RbTLR21 is closely related to E. coioides TLR21. The RbTLR21 was ubiquitously expressed in all the tissues tested,
but the highest expression was found in spleen. Additionally, upon stimulation with Streptococcus iniae, rock bream
iridovirus (RBIV), and Edwardsiella tarda, RbTLR21 mRNA was significantly up-regulated in spleen tissues.
Collectively, our findings suggest that RbTLR21 is indeed an ortholog of the TLR21 family and may be important
in mounting host immune responses against pathogenic infections.
