A ubiquitin-proteasome pathway represses the Drosophila immune deficiency signaling cascade.
BACKGROUND
The inducible production of antimicrobial peptides is a major immune response in Drosophila. The genes encoding these peptides are activated by NF-kappaB transcription factors that are controlled by two independent signaling cascades: the Toll pathway that regulates the NF-kappaB homologs, Dorsal and DIF; and the IMD pathway that regulates the compound NF-kappaB-like protein, Relish. Although numerous components of each pathway that are required to induce antimicrobial gene expression have been identified, less is known about the mechanisms that either repress antimicrobial genes in the absence of infection or that downregulate these genes after infection.


RESULTS
In a screen for factors that negatively regulate the IMD pathway, we isolated two partial loss-of-function mutations in the SkpA gene that constitutively induce the antibacterial peptide gene, Diptericin, a target of the IMD pathway. These mutations do not affect the systemic expression of the antifungal peptide gene, Drosomycin, a target of the Toll pathway. SkpA encodes a homolog of the yeast and human Skp1 proteins. Skp1 proteins function as subunits of SCF-E3 ubiquitin ligases that target substrates to the 26S proteasome, and mutations affecting either the Drosophila SCF components, Slimb and dCullin1, or the proteasome also induce Diptericin expression. In cultured cells, inhibition of SkpA and Slimb via RNAi increases levels of both the full-length Relish protein and the processed Rel-homology domain.


CONCLUSIONS
In contrast to other NF-kappaB activation pathways, the Drosophila IMD pathway is repressed by the ubiquitin-proteasome system. A possible target of this proteolytic activity is the Relish transcription factor, suggesting a mechanism for NF-kappaB downregulation in Drosophila.
Khush
Ranjiv S
RS
Centre de Génétique Moléculaire, Centre National de la Recherche Scientifique, 91198 Gif-sur-Yvette, France. khush@stanford.edu
Cornwell
William D
WD
Uram
Jennifer N
JN
Lemaitre
Bruno
B
D000595:Amino Acid Sequence; D000818:Animals; D000890:Anti-Infective Agents / Q000737:chemistry / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000235:genetics; D003433:Crosses, Genetic; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D029721:Drosophila Proteins / Q000235:genetics; D004331:Drosophila melanogaster / Q000235:genetics / Q000276:immunology*; D005020:Ethyl Methanesulfonate; D005260:Female; D006801:Humans; D007153:Immunologic Deficiency Syndromes / Q000235:genetics; D019476:Insect Proteins / Q000737:chemistry / Q000235:genetics*; D008297:Male; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000378:metabolism*; D016296:Mutagenesis; D009153:Mutagens; D046988:Proteasome Endopeptidase Complex; D011994:Recombinant Proteins / Q000096:biosynthesis; D044786:S-Phase Kinase-Associated Proteins; D044843:SKP Cullin F-Box Protein Ligases; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D025801:Ubiquitin / Q000378:metabolism*
D000890:Anti-Infective Agents; D018797:Cell Cycle Proteins; C054756:DptA protein, Drosophila; D029721:Drosophila Proteins; D019476:Insect Proteins; D009097:Multienzyme Complexes; D009153:Mutagens; D011994:Recombinant Proteins; D044786:S-Phase Kinase-Associated Proteins; C434749:SkpA protein, Drosophila; D025801:Ubiquitin; D005020:Ethyl Methanesulfonate; D044843:SKP Cullin F-Box Protein Ligases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex