Zhu Jinsong, Miura Ken, Chen Li, Raikhel Alexander S
Department of Entomology, University of California, Riverside, CA 92521, USA.
Proc Natl Acad Sci U S A. 2003 Jan 21;100(2):544-9. doi: 10.1073/pnas.0235695100. Epub 2003 Jan 9.
In anautogenous mosquitoes, egg maturation requires a blood meal. As a consequence, mosquitoes are vectors of numerous devastating human diseases. Blood feeding triggers a 20-hydroxyecdysone (20E) hormonal cascade, which activates yolk protein precursor (YPP) genes in the female fat body, an insect metabolic tissue. An important adaptation for anautogeny is the previtellogenic arrest preventing activation of YPP genes. Equally essential is termination of their expression, so that another arrest is achieved after a batch of eggs is laid. Here, we report that mosquito Seven-up (AaSvp), a chicken ovalbumin upstream promoter-transcription factor homologue, is involved in regulating the cyclicity of vitellogenic ecdysteroid-mediated signaling through heterodimerization with a retinoid X receptor homologue Ultraspiracle (USP), the obligatory functional ecdysteroid receptor (EcR) partner. AaSvp inhibits 20E-dependent activation of the vitellogenin (Vg) gene in transfection assays. Two-hybrid and GST pull-down analyses demonstrate that in vitro AaSvp interacts with both AaUSP and AaEcR. However, the coimmunoprecipitation using fat body nuclear extracts reveals that at 33-36 h postblood meal, when the 20E titer sharply declines and YPP gene expression ceases, AaSvp replaces AaEcR in USP heterodimers. The chromatin immunoprecipitation assay indicates that protein-protein interaction rather than binding competition for the Vg ecdysteroid response element accounts for the inhibition of Vg expression by AaSvp.
在自育型蚊子中，卵子成熟需要吸食血液。因此，蚊子是众多毁灭性人类疾病的传播媒介。吸食血液会触发20-羟基蜕皮激素（20E）激素级联反应，从而激活雌性脂肪体（一种昆虫代谢组织）中的卵黄蛋白前体（YPP）基因。自育的一个重要适应性特征是卵黄生成前停滞，可防止YPP基因的激活。同样重要的是其表达的终止，以便在产下一批卵后实现另一次停滞。在此，我们报告称，蚊子的七上蛋白（AaSvp）是一种鸡卵清蛋白上游启动子转录因子同源物，它通过与类视黄醇X受体同源物超气门蛋白（USP）（功能性蜕皮激素受体（EcR）的必需伴侣）异源二聚化，参与调节卵黄生成蜕皮类固醇介导信号的周期性。在转染实验中，AaSvp抑制卵黄蛋白原（Vg）基因的20E依赖性激活。双杂交和谷胱甘肽S-转移酶下拉分析表明，在体外AaSvp与AaUSP和AaEcR均相互作用。然而，使用脂肪体核提取物进行的免疫共沉淀显示，在吸食血液后33-36小时，当20E滴度急剧下降且YPP基因表达停止时，AaSvp在USP异源二聚体中取代了AaEcR。染色质免疫沉淀分析表明，AaSvp对Vg表达的抑制是由于蛋白质-蛋白质相互作用，而非对Vg蜕皮类固醇反应元件的结合竞争。
