Clustering of peptidoglycan recognition protein-SA is required for sensing lysine-type peptidoglycan in insects

doi:10.1073/pnas.0610924104

Clustering of peptidoglycan recognition protein-SA is required for sensing lysine-type peptidoglycan in insects

*National Research Laboratory of Defense Proteins, College of Pharmacy, Pusan National University, Busan 609-735, Korea;
^†Division of Molecular Life Science, Ewha Woman's University, Seoul 120-750, Korea; and
^‡Division of Molecular and Life Sciences, Department of Life Sciences, Center for Biomolecular Recognition, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Korea

Edited by Russell F. Doolittle, University of California at San Diego, La Jolla, CA, and approved March 2, 2007 (received for review December 13, 2006)

Abstract

Recognition of lysine-type peptidoglycan by peptidoglycan recognition protein (PGRP)-SA provokes the activation of the Toll and prophenoloxidase pathways. Here we reveal that a soluble fragment of lysine-type peptidoglycan, a long glycan chain with short stem peptides, is a potent activator of the Drosophila Toll pathway and the prophenoloxidase activation cascade in the beetle Tenebrio molitor. Using this peptidoglycan fragment, we present biochemical evidence that clustering of PGRP-SA molecules on the peptidoglycan is required for the activation of the prophenoloxidase cascade. We subsequently highlight that the lysozyme-mediated partial digestion of highly cross-linked lysine-type peptidoglycan dramatically increases the binding of PGRP-SA, presumably by inducing clustering of PGRP-SA, which then recruits the Gram-negative bacteria-binding protein 1 homologue and a modular serine protease containing low-density lipoprotein and complement control protein domains. The crucial role of lysozyme in the prophenoloxidase activation cascade is further confirmed in vivo by using a lysozyme inhibitor. Taken together, we propose a model whereby lysozyme presents a processed form of lysine-type peptidoglycan for clustering of PGRP-SA that recruits Gram-negative bacteria-binding protein 1 and the modular serine protease, which leads to the activation of both the Toll and prophenoloxidase pathways.

Footnotes

^§To whom correspondence should be addressed. E-mail: brlee{at}pusan.ac.kr
Author contributions: J.-W.P., W.-J.L., and B.-L.L. designed research; J.-W.P., C.-H.K., J.-H.K., B.-R.J., K.-B.R., S.-J.K., H.-H.L., J.-H.R., and J.-H.L. performed research; B.-H.O. contributed new reagents/analytic tools; J.-W.P., W.-J.L., N.-C.H., and B.-L.L. analyzed data; and N.-C.H. and B.-L.L. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0610924104/DC1.
Abbreviations:

PG,

peptidoglycan;

PGRP,

PG recognition protein;

GNBP1,

Gram-negative bacteria-binding protein 1.