T. Michel et al., Drosophila Toll is activated by Gram-positive bacteria through a circulating peptidoglycan recognition protein, NATURE, 414(6865), 2001, pp. 756-759
Microbial infection activates two distinct intracellular signalling cascade
s in the immune-responsive fat body of Drosophila(1,2). Gram-positive bacte
ria and fungi predominantly induce the Toll signalling pathway, whereas Gra
m-negative bacteria activate the Imd pathway(3,4). Loss-of-function mutants
in either pathway reduce the resistance to corresponding infections(4,5).
Genetic screens have identified a range of genes involved in these intracel
lular signalling cascades(6-12), but how they are activated by microbial in
fection is largely unknown. Activation of the transmembrane receptor Toll r
equires a proteolytically cleaved form of an extracellular cytokine-like po
lypeptide, Spatzle(13), suggesting that Toll does not itself function as a
bona fide recognition receptor of microbial patterns. This is in apparent c
ontrast with the mammalian Toll-like receptors(14) and raises the question
of which host molecules actually recognize microbial patterns to activate T
oll through Spatzle. Here we present a mutation that blocks Toll activation
by Gram-positive bacteria and significantly decreases resistance to this t
ype of infection. The mutation semmelweis (seml) inactivates the gene encod
ing a peptidoglycan recognition protein (PGRP-SA). Interestingly, seml does
not affect Toll activation by fungal infection, indicating the existence o
f a distinct recognition system for fungi to activate the Toll pathway.