2009年8月3日,，北京生命科學研究所邵峰博士實驗室在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。該文章首次報道了肺炎軍團菌通過其四型分泌系統(tǒng)向宿主細胞注入一個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)信號通路及其具體分子作用機制,。
肺炎軍團菌（Legionella pneumophila）感染肺泡巨噬細胞,，并在宿主細胞內(nèi)大量繁殖和裂解細胞，隨后擴散侵襲其它的巨噬細胞產(chǎn)生新一輪的感染最終引發(fā)嚴重的肺炎,。肺炎軍團菌通過其四型分泌系統(tǒng)分泌毒力效應蛋白分子進入宿主細胞內(nèi),，進而阻斷或調(diào)節(jié)宿主免疫防御相關的信號通路。目前關于肺炎軍團菌四型分泌系統(tǒng)效應蛋白直接調(diào)節(jié)宿主免疫信號通路的研究還鮮有報道,。
邵峰小組在這篇文章中篩選了100多個可能的軍團菌四型分泌系統(tǒng)效應蛋白,，發(fā)現(xiàn)其中只有一個叫做LegK1的蛋白，在導入真核細胞后顯示出非常強的激活NF-κB信號通路的活性,。同時,，LegK1對其它包括MAPK激酶和干擾素(IFNb)在內(nèi)的相關免疫信號通路沒有激活作用。LegK1編碼了一個類真核的絲氨酸/蘇氨酸蛋白激酶,。作者也證明了LegK1的激酶活性對激活NF-κB信號通路是必不可少的,，并且LegK1確實能通過肺炎軍團菌四型分泌系統(tǒng)被注入到宿主巨噬細胞內(nèi)。IKK激酶復合物介導的IκB家族蛋白的磷酸化是NF-κB信號通路激活的關鍵步驟,。邵峰小組通過RNA干擾的方法發(fā)現(xiàn)knockdown 諸多已知的IKK上游的信號分子對LegK1激活NF-κB信號通路沒有影響,。作者進一步發(fā)現(xiàn)LegK1在IKK遺傳缺失的細胞中仍然能夠激活NF-κB信號通路。這些結果在后續(xù)的基于細胞提取物的無細胞體系重組實驗中也得到了驗證,。深入的生物化學研究表明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,，從而導致IκBa的泛素化和降解，進而釋放NF-κB進入細胞核內(nèi)并激活轉錄,。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介導的是與天然免疫不相關的非經(jīng)典NF-κB信號通路。作者發(fā)現(xiàn)LegK1也能磷酸化NFκB2并直接誘導其成熟為活性形式的p52蛋白,。根據(jù)上述實驗結果,，作者認為肺炎軍團菌四型分泌系統(tǒng)效應蛋白LegK1是模擬了宿主的IKK激酶來激活宿主的NF-κB信號通路，并進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胞在感染后不會立刻啟動具有保護性的凋亡程序,。
這篇文章不僅揭示了肺炎軍團菌如何通過其四型分泌系統(tǒng)來調(diào)節(jié)宿主NF-κB免疫信號通路的具體分子作用機制,，同時LegK１也是目前為止第一個報道的能直接激活宿主NF-κB信號通路的病原菌效應蛋白分子。（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.
