病原菌效应因子调控宿主泛素化修饰的研究进展北大核心

泛素化(ubiquitination)是真核细胞内广泛存在的蛋白质翻译后修饰方式,参与并调控DNA修复、细胞周期、免疫应答、信号通路等真核细胞内几乎所有的生命活动。同时,细胞通过去泛素化酶(deubiquitinases,DUBs)使泛素化修饰成为可逆过程,保证了泛素化系统及其相关生理过程的动态平衡。病原菌感染过程中,宿主细胞可通过泛素化修饰发挥抗细菌感染作用。然而,病原菌可编码并分泌效应因子,靶向宿主泛素(ubiquitin,Ub)系统并调控宿主泛素化修饰过程,干扰宿主细胞的免疫应答,从而促进细菌存活与毒力。本文概述了重要病原菌利用效应因子调控宿主细胞泛素化修饰的研究进展,有助于全面理解病原菌调控宿主泛素化修饰促进感染的机制。     

Intercepting host MAPK signaling cascades by bacterial type III effectors

The evolutionarily conserved MAP kinase (MAPK) cascades play essential roles in plant and animal innate immunity. A recent explosion of research has uncovered a myriad of virulence strategies used by pathogenic bacteria to intercept MAPK signaling through diverse type III effectors injected into host cells. Here, we review the latest literature and discuss the various mechanisms that pathogenic bacteria use to manipulate host MAPK signaling cascades.     

应用竞争力指数分析III型效应子SKWP对青枯菌在寄主植物体内增殖能力的影响

[目的]研究Ⅲ型效应子SKWP对青枯菌OE1-1在寄主植物体内增殖能力的影响。[方法]构建青枯菌RK7197（野生型突变体,带Gm抗性）和SKWP单基因缺失突变体（带PB抗性）,通过竞争力指数分析SKWP各效应子对青枯菌OE1-1在叶片组织内增殖能力的影响。[结果]竞争力指数适合在寄主植物茄子上分析各效应子功能,6个SKWP效应子对OE1-1细菌增殖能力影响不同,SKWP4影响最明显。[结论]竞争力指数可提供一个新视野来分析SKWP各效应子对青枯菌OE1-1在寄主茄子上增殖能力的影响。     

Streptococcus pyogenes and human neutrophils: a paradigm for evasion of innate host defense by bacterial pathogens

Human polymorphonuclear leukocytes (PMNs) are the first line of defense against invading microorganisms. Although most invading bacteria are eliminated by PMNs, some have evolved complex strategies to prevent normal PMN function. This review focuses on the interaction of human PMNs with Streptococcus pyogenes as a paradigm for successful pathogen evasion mechanisms.     

A Salmonella inositol polyphosphatase acts in conjunction with other bacterial effectors to promote host cell actin cytoskeleton rearrangements and bacterial internalization

A central feature of Salmonella pathogenicity is the bacterium's ability to enter into non-phagocytic cells. Bacterial internalization is the consequence of cellular responses characterized by Cdc42- and Rac-dependent actin cytoskeleton rearrangements. These responses are triggered by the co-ordinated function of bacterial proteins delivered into the host cell by a specialized protein secretion system termed type III. We report here that SopB, a Salmonella inositol polyphosphatase delivered to the host cell by this secretion system, mediates actin cytoskeleton rearrangements and bacterial entry in a Cdc42-dependent manner. SopB exhibits overlapping functions with two other effectors of bacterial entry, the Rho family GTPase exchange factors SopE and SopE2. Thus, Salmonella strains deficient in any one of these proteins can enter into cells at high efficiency, whereas a strain lacking all three effectors is completely defective for entry. Consistent with an important role for inositol phosphate metabolism in Salmonella-induced cellular responses, a catalytically defective mutant of SopB failed to stimulate actin cytoskeleton rearrangements and bacterial entry. Furthermore, bacterial infection of intestinal cells resulted in a marked increase in Ins(1,4,5,6)P4, a consumption of InsP5 and the activation of phospholipase C. In agreement with the in vivo findings, purified SopB specifically dephosphorylated InsP5 to Ins(1,4,5,6)P4 in vitro. Surprisingly, the inositol phosphate fluxes induced by Salmonella were not caused exclusively by SopB. We show that the SopB-independent inositol phosphate fluxes are the consequence of the SopE-dependent activation of an endogenous inositol phosphatase. The ability of Salmonella to stimulate Rho GTPases signalling and inositol phosphate metabolism through alternative mechanisms is an example of the remarkable ability of this bacterial pathogen to manipulate host cellular functions.     

Enhanced bacterial virulence through exploitation of host glycosaminoglycans

Present in the extracellular matrix and membranes of virtually all animal cells, proteoglycans (PGs) are among the first host macromolecules encountered by infectious agents. Because of their wide distribution and direct accessibility, it is not surprising that pathogenic bacteria have evolved mechanisms to exploit PGs for their own purposes, including mediating attachment to target cells. This is achieved through the expression of adhesins that recognize glycosaminoglycans (GAGs) linked to the core protein of PGs. Some pathogens, such as Bordetella pertussis and Chlamydia trachomatis, may express more than one GAG-binding adhesin. Bacterial interactions with PGs may also facilitate cell invasion or systemic dissemination, as observed for Neisseria gonorrhoeae and Mycobacterium tuberculosis respectively. More-over, pathogenic bacteria can use PGs to enhance their virulence via a shedding of PGs that leads to there lease of effectors that weaken the host defences.The exploitation of PGs by pathogenic bacteria is thus a multifaceted mechanistic process directly related to the potential virulence of a number of microorganisms.     

Salmonella type III effectors PipB and PipB2 are targeted to detergent-resistant microdomains on internal host cell membranes

The intracellular pathogen, Salmonella enterica, translocates type III effectors across its vacuolar membrane into host cells. Herein we describe a new Salmonella effector, PipB2, which has sequence similarity to another type III effector, PipB. In phagocytic cells, PipB2 localizes to the Salmonella-containing vacuole (SCV) and tubular extensions from the SCV, Salmonella-induced filaments (Sifs). We used the specific targeting of PipB2 in macrophages to characterize Sifs in phagocytic cells for the first time. In epithelial cells, PipB2 has a unique localization pattern, localizing to SCVs and Sifs and additionally to vesicles at the periphery of infected cells. We further show that the N-terminal 225-amino-acid residues of PipB2 are sufficient for type III translocation and association with SCVs and Sifs, but not peripheral vesicles. Subcellular fractionation demonstrated that both PipB and PipB2 associate with host cell membranes and resist extraction by high salt, high pH and to a significant extent, non-ionic detergent. Furthermore, PipB and PipB2 are enriched in detergent-resistant microdomains (DRMs), also known as lipid rafts, present on membranes of SCVs and Sifs. The enrichment of Salmonella effectors in DRMs on these intracellular membranes probably permits specific interactions with host cell molecules that are concentrated in these signalling platforms.     

Multiple factors contribute to inefficient prenylation of Rab27a in Rab prenylation diseases

Post-translational geranylgeranylation of Rab GT-Pases is essential for their membrane association and function as regulators of intracellular vesicular transport. The reaction is catalyzed by Rab geranylgeranyltransferase (RGGT) and is assisted by the Rab escort proteins (REP), which form stable complexes with newly synthesized GDP-bound Rabs. Two genetic diseases involve the Rab geranylgeranylation machinery: choroideremia, an X-linked retinal degeneration resulting from loss-of-function mutations in REP1, and gunmetal, a mouse model of Hermansky-Pudlak syndrome resulting from mutations in the alpha-subunit of RGGT. A small subset of Rab proteins is selectively under-prenylated in both diseases, most notably Rab27a. Here we analyze why Rab27a is selectively affected in diseases of Rab geranylgeranylation. Semi-quantitative immunoblotting suggests that mass action, i.e. the amount of Rab27a relative to other Rabs, is unlikely to be a factor as the expression level of Rab27a is similar to other Rabs not affected in these diseases. In vitro binding assays and fluorescence resonance energy transfer detected by fluorescence lifetime imaging microscopy in intact cells demonstrate that Rab27a binds equally well to both REP1 and REP2, suggesting differential affinity of Rab27a for REP isoforms is not an important factor. However, steady-state kinetic analysis of the geranylgeranylation reaction indicates that REP2-Rab27a has lower affinity for RGGT compared with REP1-Rab27a. Furthermore, we show that Rab27a has relatively low GTPase activity, presumably decreasing the affinity of the REP interaction in vivo. We suggest that the restricted phenotypes observed in these diseases result from multiple contributing factors.     

Specific threonine phosphorylation of a host target by two unrelated type III effectors activates a host innate immune receptor in plants

The Arabidopsis NB-LRR immune receptor RPM1 recognizes the Pseudomonas syringae type III effectors AvrB or AvrRpm1 to mount an immune response. Although neither effector is itself a kinase, AvrRpm1 and AvrB are known to target Arabidopsis RIN4, a negative regulator of basal plant defense, for phosphorylation. We show that RIN4 phosphorylation activates RPM1. RIN4(142-176) is necessary and, with appropriate localization sequences, sufficient to support effector-triggered RPM1 activation, with the threonine residue at position 166 being critical. Phosphomimic substitutions at T166 cause effector-independent RPM1 activation. RIN4 T166 is phosphorylated in vivo in the presence of AvrB or AvrRpm1. RIN4 mutants that lose interaction with AvrB cannot be coimmunoprecipitated with RPM1. This defines a common interaction platform required for RPM1 activation by phosphorylated RIN4 in response to pathogenic effectors. Conservation of an analogous threonine across all RIN4-like proteins suggests a key function for this residue beyond the regulation of RPM1.