Identification and characterization of novel short-type BmPGRP-S4 from the silkworm, Bombyx mori, involved in innate immunity

Peptidoglycan recognition proteins (PGRPs) are pattern recognition receptors that can recognize bacterial peptidoglycans and trigger the innate immune response of insects. Here, we identified and characterized a novel short-type Bombyx mori peptidoglycan recognition proteins short-4 (BmPGRP-S4) in a lepidopteran insect, Bombyx mori. BmPGRP-S4 exhibited a cDNA sequence length of 600 bp, encoding 199 aa with a protein molecular weight of 22 kDa. Multiple sequence alignment revealed that BmPGRP-S4 contains a conserved PGRP domain. Quantitative real-time polymerase chain reaction analysis showed that BmPGRP-S4 is highly expressed in the early developmental stages of silkworm larvae and presents tissue-specific expression in hemocytes. Interestingly, BmPGRP-S4 expression is significantly induced by bacterial infection in the midgut, fat body, and hemocytes. Furthermore, a dual luciferase reporter gene assay revealed that BmPGRP-S4 can activate the expression of the antimicrobial peptide genes lebocin, moricin, cecropin D, cecropin B, and attacin. Taken together, these results suggest that BmPGRPS4 plays an important role in the innate immune response of silkworms.