Drosophila rely entirely on an innate immune response to combat microbial infection. Diaminopimelic acid-containing peptidoglycan, produced by Gram-negative bacteria, is recognized by two receptors, PGRP-LC and PGRP-LE, and activates a homolog of transcription factor NF-kappaB through the Imd signaling pathway. Here we show that full-length PGRP-LE acted as an intracellular receptor for monomeric peptidoglycan, whereas a version of PGRP-LE containing only the PGRP domain functioned extracellularly, like the mammalian CD14 molecule, to enhance PGRP-LC-mediated peptidoglycan recognition on the cell surface. Interaction with the imd signaling protein was not required for PGRP-LC signaling. Instead, PGRP-LC and PGRP-LE signaled through a receptor-interacting protein homotypic interaction motif-like motif. These data demonstrate that like mammals, drosophila use both extracellular and intracellular receptors, which have conserved signaling mechanisms, for innate immune recognition.

All metazoan guts are in permanent contact with the microbial realm. However, understanding of the exact mechanisms by which the strength of gut immune responses is regulated to achieve gut-microbe mutualism is far from complete. Here we identify a signaling network composed of complex positive and negative mechanisms that controlled the expression and activity of dual oxidase (DUOX), which 'fine tuned' the production of microbicidal reactive oxygen species depending on whether the gut encountered infectious or commensal microbes. Genetic analyses demonstrated that negative and positive regulation of DUOX was required for normal host survival in response to colonization with commensal and infectious microbes, respectively. Thus, the coordinated regulation of DUOX enables the host to achieve gut-microbe homeostasis by efficiently combating infection while tolerating commensal microbes.

Condensins are key mediators of chromosome condensation across organisms. Like other condensins, the bacterial MukBEF condensin complex consists of an SMC family protein dimer containing two ATPase head domains, MukB, and two interacting subunits, MukE and MukF. We report complete structural views of the intersubunit interactions of this condensin along with ensuing studies that reveal a role for the ATPase activity of MukB. MukE and MukF together form an elongated dimeric frame, and MukF's C-terminal winged-helix domains (C-WHDs) bind MukB heads to constitute closed ring-like structures. Surprisingly, one of the two bound C-WHDs is forced to detach upon ATP-mediated engagement of MukB heads. This detachment reaction depends on the linker segment preceding the C-WHD, and mutations on the linker restrict cell growth. Thus ATP-dependent transient disruption of the MukB-MukF interaction, which creates openings in condensin ring structures, is likely to be a critical feature of the functional mechanism of condensins.

Drosophila peptidoglycan recognition protein (PGRP)-LCx and -LCa are receptors that preferentially recognize meso-diaminopimelic acid (DAP)-type peptidoglycan (PGN) present in Gram-negative bacteria over lysine-type PGN of Gram-positive bacteria and initiate the IMD signaling pathway, whereas PGRP-LE plays a synergistic role in this process of innate immune defense. How these receptors can distinguish the two types of PGN remains unclear. Here the structure of the PGRP domain of Drosophila PGRP-LE in complex with tracheal cytotoxin (TCT), the monomeric DAP-type PGN, reveals a buried ionic interaction between the unique carboxyl group of DAP and a previously unrecognized arginine residue. This arginine is conserved in the known DAP-type PGN-interacting PGRPs and contributes significantly to the affinity of the protein for the ligand. Unexpectedly, TCT induces infinite head-to-tail dimerization of PGRP-LE, in which the disaccharide moiety, but not the peptide stem, of TCT is positioned at the dimer interface. A sequence comparison suggests that TCT induces heterodimerization of the ectodomains of PGRP-LCx and -LCa in a closely analogous manner to prime the IMD signaling pathway, except that the heterodimer formation is nonperpetuating.Innate immune defenses against pathogens are initiated by pattern recognition receptors that bind conserved stereotypical, rather than particular, molecular structures present in a wide spectrum of microorganisms but absent in the host (1). A representative example of such structures is peptidoglycan (PGN), 3 the major constituent of the cell wall of both Gram-positive and -negative bacteria. The peptidoglycan recognition protein (PGRP) family is a class of pattern recognition receptors that bind, and sometimes cleave, PGN. A total of 13 and four PGRP family members have been identified in Drosophila and humans, respectively (2-5). PGRPs are often characterized based on their polypeptide length. Short form PGRPs, such as PGRP-SA and -SD, contain a single PGRP domain (ϳ180 amino acids) and, in most cases, a signal sequence, leading to the secretion of the proteins. Long form PGRPs, such as PGRP-LC and -LE, contain other domain(s) in addition to the PGRP domain, often including a transmembrane region (6). The PGRP domain is similar in structure to N-acetylmuramoyl-L-alanine amidases, such as T7 lysozyme, and some PGRPs are similarly catalytic, whereas others lack a critical cysteine residue in the catalytic triad (7). Thus, these PGRPs lack catalytic activity but function instead as pattern recognition receptors and/or as antimicrobials. For example, murine PGRP-S is directly antimicrobial and contributes to the neutrophil-mediated killing of bacteria (8, 9), whereas PGRP-SA, -SD, -LC, and -LE are key pattern recognition receptors involved in activation of the Drosophila immune response through the Toll or IMD (immune deficiency) signaling pathway (10 -16).In particular, genetic studies revealed that circulating PGRP-SA and PGRP-SD detect Gram-positive bacteria and activate the Toll pa...

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