Infection-induced molecular pattern recognition proteins in larvae of Protaetia brevitarsis seulensis (Coleoptera: Cetoniidae)

Abstract

We cloned and sequenced full-length peptidoglycan recognition protein (PGRP)-like cDNAs, named PS PGRP-SA(a)like, PS PGRP-SA(b)-like, PS PGRP-SB1-like and PS PGRP-SC-like, from Protaetia brevitarsis seulensis. The amino acid sequences of PS PGRPs share 32.03−47.93% homology with those of PGRP family members in insects and mammals, including humans. We identified a conserved consensus sequence for amidase activity (His; H-Tyr; Y-His; H-Thr; T-Cys; C) and residues for binding peptidoglycan (PGN), one of the major bacterial cell wall components, including Asp (D) and Phe (F) for Lys-type PGN; and Gly(G), Trp (W) and Arg (R) for DAP-type PGN. The topological structures of PS PGRP-SA(a)-like, PS PGRP-SA(b)-like and PS PGRP-SC-like proteins are structurally similar to those of Drosophila melanogaster PGRP-SA, which has three α-helices and six β-strands. The β-strands are located in a central region and helix α1 on the back and peripheral α2 and α3 helices are on the front. The three α-helices and six β-strands are also present in PS PGRP-SB1-like, but the topological structure differs from that of typical PGRP. Significantly increased levels of PS PGRP-SA (a)-like and PS PGRP-SA (b)-like mRNA were recorded when Gram-positive bacteria or yeast cells were injected into larvae. PS PGRP-SB1-like mRNA levels were up-regulated by infection by all three pathogens; however, expression of PS PGRP-SC-like mRNA was increased 20or 30-fold only shortly after injection with Gram-negative bacteria. * Corresponding author; e-mail: saeyoullcho@kangwon.ac.kr InTroDUCTIon Micro-associated molecular patterns (MAMPs) and pathogen-associated molecular patterns (PAMPs), such as peptidoglycan (PGN), lipopolysaccharide (LPS), β-glucans, lipoproteins, CpG dinucleotides and flagellin, are molecular markers recognized by the insect innate immune system. There are various cellular immune responses of insect blood cells (haemocytes) and humoral immune responses mediated by various effector molecules, including antimicrobial peptides (AMPs) and the phenol oxidase (PO) cascade is part of the insect immune system (Janeway et al., 2002; Hoffmann, 2003; Cho & Cho, 2019). Humoral immune responses involving Toll and immune deficiency (IMD) pathways are mainly activated by insect pattern recognition receptors (PRRs) (Wang et al., 2019). Therefore, humoral immune responses are activated when MAMPs are recognized by insect PRRs. Peptidoglycan recognition proteins (PGRPs), C-type lectin receptors (CLRs), fibrinogen-related proteins (FREPs), thioester-containing proEur. J. Entomol. 119: 77–84, 2022 doi: 10.14411/eje.2022.008