Structural basis for preferential recognition of diaminopimelic acid-type peptidoglycan by a subset of peptidoglycan...

Area of Science:

• Molecular Biology
• Immunology
• Structural Biology

Background:

• Drosophila peptidoglycan recognition proteins (PGRPs) are crucial for innate immunity.
• PGRP-LCx and PGRP-LCa recognize Gram-negative bacterial peptidoglycan (PGN) to activate the IMD pathway.
• PGRP-LE acts synergistically in this immune response, but the mechanism of PGN discrimination is unclear.

Purpose of the Study:

• To elucidate the structural basis by which Drosophila PGRP-LE distinguishes between different types of peptidoglycan (PGN).
• To understand the interaction between PGRP-LE and tracheal cytotoxin (TCT), a monomeric DAP-type PGN.

Main Methods:

• X-ray crystallography was used to determine the structure of the PGRP domain of Drosophila PGRP-LE in complex with TCT.
• Sequence comparison analysis was performed to identify conserved residues and infer dimerization mechanisms.

Main Results:

• A unique buried ionic interaction between the carboxyl group of DAP and a conserved arginine residue in PGRP-LE was identified, crucial for ligand binding affinity.
• Tracheal cytotoxin (TCT) induces non-perpetuating head-to-tail dimerization of PGRP-LE, with the disaccharide moiety at the dimer interface.
• Sequence comparison suggests a similar, non-perpetuating heterodimerization mechanism for PGRP-LCx and PGRP-LCa.

Conclusions:

• The study reveals a novel binding mechanism for DAP-type PGN by PGRP-LE involving a conserved arginine residue.
• TCT-induced dimerization of PGRP-LE and analogous heterodimerization of PGRP-LCx/LCa are key events in priming the Drosophila IMD signaling pathway.
• These findings provide structural insights into the molecular recognition of bacterial PGN by insect immune receptors.