2009年8月3日，北京生命科學(xué)研究所邵峰博士實(shí)驗(yàn)室在Proc Natl Acad Sci USA雜志在線發(fā)表題為題為“A Legionella type IV effector activates the NF-κB pathway by phosphorylating the IκB family of inhibitors”的文章,。該文章首次報(bào)道了肺炎軍團(tuán)菌通過其四型分泌系統(tǒng)向宿主細(xì)胞注入一個(gè)新的能激活宿主中具有抗凋亡作用的NF-κB免疫炎癥調(diào)節(jié)信號(hào)通路及其具體分子作用機(jī)制,。
肺炎軍團(tuán)菌（Legionella pneumophila）感染肺泡巨噬細(xì)胞，并在宿主細(xì)胞內(nèi)大量繁殖和裂解細(xì)胞,，隨后擴(kuò)散侵襲其它的巨噬細(xì)胞產(chǎn)生新一輪的感染最終引發(fā)嚴(yán)重的肺炎,。肺炎軍團(tuán)菌通過其四型分泌系統(tǒng)分泌毒力效應(yīng)蛋白分子進(jìn)入宿主細(xì)胞內(nèi)，進(jìn)而阻斷或調(diào)節(jié)宿主免疫防御相關(guān)的信號(hào)通路,。目前關(guān)于肺炎軍團(tuán)菌四型分泌系統(tǒng)效應(yīng)蛋白直接調(diào)節(jié)宿主免疫信號(hào)通路的研究還鮮有報(bào)道,。
邵峰小組在這篇文章中篩選了100多個(gè)可能的軍團(tuán)菌四型分泌系統(tǒng)效應(yīng)蛋白，發(fā)現(xiàn)其中只有一個(gè)叫做LegK1的蛋白,，在導(dǎo)入真核細(xì)胞后顯示出非常強(qiáng)的激活NF-κB信號(hào)通路的活性,。同時(shí)，LegK1對(duì)其它包括MAPK激酶和干擾素(IFNb)在內(nèi)的相關(guān)免疫信號(hào)通路沒有激活作用,。LegK1編碼了一個(gè)類真核的絲氨酸/蘇氨酸蛋白激酶,。作者也證明了LegK1的激酶活性對(duì)激活NF-κB信號(hào)通路是必不可少的，并且LegK1確實(shí)能通過肺炎軍團(tuán)菌四型分泌系統(tǒng)被注入到宿主巨噬細(xì)胞內(nèi),。IKK激酶復(fù)合物介導(dǎo)的IκB家族蛋白的磷酸化是NF-κB信號(hào)通路激活的關(guān)鍵步驟,。邵峰小組通過RNA干擾的方法發(fā)現(xiàn)knockdown 諸多已知的IKK上游的信號(hào)分子對(duì)LegK1激活NF-κB信號(hào)通路沒有影響。作者進(jìn)一步發(fā)現(xiàn)LegK1在IKK遺傳缺失的細(xì)胞中仍然能夠激活NF-κB信號(hào)通路,。這些結(jié)果在后續(xù)的基于細(xì)胞提取物的無細(xì)胞體系重組實(shí)驗(yàn)中也得到了驗(yàn)證,。深入的生物化學(xué)研究表明LegK1能夠和宿主中的IKK激酶一樣直接磷酸化IκBa蛋白的32位和36位的絲氨酸，從而導(dǎo)致IκBa的泛素化和降解,，進(jìn)而釋放NF-κB進(jìn)入細(xì)胞核內(nèi)并激活轉(zhuǎn)錄,。有趣并值得注意的是，NFκB2（也叫p100）蛋白在生化上也屬于IκB家族,，同樣受IKK磷酸化調(diào)節(jié),，但NFκB2介導(dǎo)的是與天然免疫不相關(guān)的非經(jīng)典NF-κB信號(hào)通路。作者發(fā)現(xiàn)LegK1也能磷酸化NFκB2并直接誘導(dǎo)其成熟為活性形式的p52蛋白,。根據(jù)上述實(shí)驗(yàn)結(jié)果,，作者認(rèn)為肺炎軍團(tuán)菌四型分泌系統(tǒng)效應(yīng)蛋白LegK1是模擬了宿主的IKK激酶來激活宿主的NF-κB信號(hào)通路，并進(jìn)一步推測這種激活作用可能起到了抗凋亡作用使得宿主細(xì)胞在感染后不會(huì)立刻啟動(dòng)具有保護(hù)性的凋亡程序,。
這篇文章不僅揭示了肺炎軍團(tuán)菌如何通過其四型分泌系統(tǒng)來調(diào)節(jié)宿主NF-κB免疫信號(hào)通路的具體分子作用機(jī)制,，同時(shí)LegK１也是目前為止第一個(gè)報(bào)道的能直接激活宿主NF-κB信號(hào)通路的病原菌效應(yīng)蛋白分子。（生物谷Bioon.com）
生物谷推薦原始出處：
PNAS August 3, 2009, doi: 10.1073/pnas.0907200106
A Legionella type IV effector activates the NF-κB pathway by phosphorylating the IκB family of inhibitors
Jianning Gea,b,1, Hao Xua,b,1, Ting Lib, Yan Zhoua,b, Zhibin Zhanga,b, Shan Lib, Liping Liub and Feng Shaob,2
aCollege of Life Sciences, Beijing Normal University, Beijing 100875, China; and
bNational Institute of Biological Sciences, Beijing 102206, China
NF-κB is critical in innate immune defense responses against invading microbial pathogens. Legionella pneumophila infection of lung macrophages causes Legionnaire's disease with pneumonia symptoms. A set of NF-κB-controlled genes involved in inflammation and anti-apoptosis are up-regulated in macrophages upon L. pneumophila infection in a Legionella Dot/Icm type IV secretion system-dependent manner. Among ≈100 Dot/Icm substrates screened, we identified LegK1 as the sole Legionella protein that harbors a highly potent NF-κB-stimulating activity. LegK1 does not affect MAPK and IFN pathways. Activation of the NF-κB pathway by LegK1 requires its eukaryotic-like Ser/Thr kinase activity and is independent of upstream components in the NF-κB pathway, including TRAFs, NIK, MEKK3, and TAK1. Cell-free reconstitution revealed that LegK1 stimulated NF-κB activation in the absence of IKKα and IKKβ, and LegK1 efficiently phosphorylated IκBα on Ser-32 and Ser-36 both in vitro and in cells. LegK1 seems to mimic the host IKK as LegK1 also directly phosphorylated other IκB family of inhibitors including p100 in the noncanonical NF-κB pathway. Phosphorylation of p100 by LegK1 led to its maturation into p52. Thus, LegK1 is a bacterial effector that directly activates the host NF-κB signaling and likely plays important roles in modulating macrophage defense or inflammatory responses during L. pneumophila infection.
