Structural and Functional Analysis of PGRP-LC Indicates Exclusive Dap-Type PGN Binding in Bumblebees

Area of Science:

• Immunology
• Entomology
• Biochemistry

Background:

• Peptidoglycan recognition proteins (PGRPs) are vital for insect immunity, recognizing bacterial peptidoglycan (PGN).
• While fruit flies have multiple PGRP-LC isoforms, bumblebees possess a single PGRP-LC, suggesting a simplified yet effective immune system.

Purpose of the Study:

• To investigate the function and binding characteristics of bumblebee PGRP-LC (Bl-PGRP-LC) in immune response.
• To compare the PGN-binding affinity of Bl-PGRP-LC with that of honeybee Am-PGRP-LC.

Main Methods:

• Bacterial infection assays with Escherichia coli.
• Direct binding assays for Bl-PGRP-LC and Dap-type PGNs.
• Gene expression analysis (qRT-PCR) for immune-related genes.
• Site-directed mutagenesis of Bl-PGRP-LC.

Main Results:

• Bumblebee PGRP-LC directly binds Dap-type PGNs and is upregulated upon E. coli infection, alongside abaecin and defensin.
• Bl-PGRP-LC shows significantly higher affinity for Dap-type PGN compared to honeybee Am-PGRP-LC.
• Mutagenesis identified His390 as the key residue for Dap-type PGN binding in Bl-PGRP-LC, forming a hydrogen bond with MurNAc.

Conclusions:

• Bumblebee PGRP-LC plays a direct role in recognizing Gram-negative bacterial PGN, contributing to innate immunity.
• The unique binding mechanism and high affinity of Bl-PGRP-LC highlight distinct evolutionary adaptations in insect immune systems.