Khush Ranjiv S, Cornwell William D, Uram Jennifer N, Lemaitre Bruno
Centre de Génétique Moléculaire, Centre National de la Recherche Scientifique, 91198 Gif-sur-Yvette, France.
Curr Biol. 2002 Oct 15;12(20):1728-37. doi: 10.1016/s0960-9822(02)01214-9.
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.
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.
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.
抗菌肽的诱导产生是果蝇的一种主要免疫反应。编码这些肽的基因由NF-κB转录因子激活，而NF-κB转录因子受两个独立的信号级联控制：调节NF-κB同源物Dorsal和DIF的Toll途径；以及调节复合NF-κB样蛋白Relish的IMD途径。尽管已经确定了诱导抗菌基因表达所需的每条途径的众多成分，但对于在未感染时抑制抗菌基因或在感染后下调这些基因的机制了解较少。
在筛选负向调节IMD途径的因子时，我们在SkpA基因中分离出两个功能部分丧失的突变，这些突变组成型地诱导抗菌肽基因双翅菌素（Diptericin）的表达，双翅菌素是IMD途径的一个靶标。这些突变不影响抗真菌肽基因果蝇霉素（Drosomycin）的全身表达，果蝇霉素是Toll途径的一个靶标。SkpA编码酵母和人类Skp1蛋白的同源物。Skp1蛋白作为SCF-E3泛素连接酶的亚基，将底物靶向26S蛋白酶体，影响果蝇SCF成分Slimb和dCullin1或蛋白酶体的突变也会诱导双翅菌素的表达。在培养细胞中，通过RNA干扰抑制SkpA和Slimb会增加全长Relish蛋白和加工后的Rel同源结构域的水平。
与其他NF-κB激活途径相反，果蝇IMD途径受到泛素-蛋白酶体系统的抑制。这种蛋白水解活性的一个可能靶标是Relish转录因子，这提示了果蝇中NF-κB下调的一种机制。
