Fly Fights with Both Hands
DOI: 10.1371/journal.pbio.0020313


Published September 7, 2004

Copyright: © 2004 Public Library of Science. This is an open-access article
distributed under the terms of the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.

Defending against attack is one of the most important challenges facing any
organism. But while sticks and stones may break the bones of a lion,
microscopic threats such as bacteria require different weapons. And it's not
just we humans who have this problem-insects are prey to bacterial
infections too. Their immune systems, however, rely on a far simpler set of
defenses than those found in mammals. Exactly how one insect immune system
recognizes bacteria, and how it fights off the invader, is the subject of a
new study in this issue by Johann Deisenhofer and colleagues.

The fruitfly, Drosophila, has long been known to use a set of molecular
sentries called "peptidoglycan recognition proteins," or PGRPs, that
circulate in the fly's bloodstream. When a PGRP recognizes a bacterial
invader, it triggers a cascade of events whose ultimate product is a group
of antimicrobial compounds that attack and kill the bacteria.

While the family of PGRPs has been extensively studied, exactly how they
recognize their target bacteria has been less clear. At the cellular level,
recognition requires contact, and the part of the bacterium the PGRP
recognizes is, as its name implies, the peptidoglycan. A peptidoglycan is a
special sort of molecular polymer found primarily on bacterial cell walls.
Peptidoglycan forms when chains of sugar molecules (the glycans) are
cross-linked by amino acids (the peptides) to form a meshwork that helps
keep the bacterium from bursting under the osmotic strain of its contents.

There are several types of peptidoglycans that differ in their precise sugar
and amino acid constituents and in their ability to trigger the Drosophila
defensive reaction. Deisenhofer and colleagues set out to determine whether
this difference in triggering ability of particular peptidoglycans was
linked to differences in the PGRPs that recognize them. To do this, they
determined the three-dimensional structure of one PGRP, called PGRP-SA. They
worked out not only the overall shape of PGRP-SA, but also which amino acids
sat where on the convoluted surface of the protein.

Full Text at PLoS Biology
http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371/journal.pbio.0020313

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