Modulation of innate immune signaling by a Coxiella burnetii eukaryotic-like effector protein

Abstract

<jats:p>The Q fever agent<jats:italic>Coxiella burnetii</jats:italic>uses a defect in organelle trafficking/intracellular multiplication (Dot/Icm) type 4b secretion system (T4SS) to silence the host innate immune response during infection. By investigating<jats:italic>C. burnetii</jats:italic>effector proteins containing eukaryotic-like domains, here we identify NopA (nucleolar protein A), which displays four regulator of chromosome condensation (RCC) repeats, homologous to those found in the eukaryotic Ras-related nuclear protein (Ran) guanine nucleotide exchange factor (GEF) RCC1. Accordingly, NopA is found associated with the chromatin nuclear fraction of cells and uses the RCC-like domain to interact with Ran. Interestingly, NopA triggers an accumulation of Ran-GTP, which accumulates at nucleoli of transfected or infected cells, thus perturbing the nuclear import of transcription factors of the innate immune signaling pathway. Accordingly, qRT-PCR analysis on a panel of cytokines shows that cells exposed to the<jats:italic>C. burnetii nopA</jats:italic>::Tn or a Dot/Icm-defective<jats:italic>dotA</jats:italic>::Tn mutant strain present a functional innate immune response, as opposed to cells exposed to wild-type<jats:italic>C. burnetii</jats:italic>or the corresponding<jats:italic>nopA</jats:italic>complemented strain. Thus, NopA is an important regulator of the innate immune response allowing<jats:italic>Coxiella</jats:italic>to behave as a stealth pathogen.</jats:p>

Nucleocytoplasmic Trafficking Perturbation Induced by Picornaviruses

“…Interestingly, non-virus pathogens were also shown to target the nucleocytoplasmic traffic machinery. Bacteria such as Salmonella , Coxiella, and Orientia counteract innate immune defenses and notably NFκB activation by targeting exportins, importins, or RAN [ 79 , 80 , 81 ]. Thus, pathogens as different as picornaviruses and bacteria evolved diverse manners to target the NPC and to perturb the nucleocytoplasmic traffic, probably in part with the common goal to escape innate immunity.…”

Undercover Agents of Infection: The Stealth Strategies of T4SS-Equipped Bacterial Pathogens

“…Similar to Orientia , C. burnetii also perturbs nucleocytoplasmic traffic to dampen the NF-κB-dependent transcriptional response to infection. The effector protein NopA (for nucleolar protein A) localises at the nucleoli of infected cells, interacts with the eukaryotic small GTPase Ran, and sequesters it within the nucleus [ 54 ] ( Figure 2 ). As nucleocytoplasmic transport depends on a protein concentration gradient of Ran between the cytoplasm and the nucleus [ 55 ], NopA activity effectively perturbs the nuclear import of proteins, including the transcription factors p65 and interferon regulatory transcription factor 3 (IRF3) ( Figure 2 ).…”

“…As nucleocytoplasmic transport depends on a protein concentration gradient of Ran between the cytoplasm and the nucleus [ 55 ], NopA activity effectively perturbs the nuclear import of proteins, including the transcription factors p65 and interferon regulatory transcription factor 3 (IRF3) ( Figure 2 ). Consequently, cytokines expression is strongly attenuated in cells infected by wild-type Coxiella , whereas this effect is completely lost in cells infected either with nopA or T4SS-defective dotA mutants [ 54 ]. Notably, despite their subcellular localisation, neither Ank1, Ank6, nor NopA encode canonical nuclear localisation signals (NLS), suggesting that bacterial effector proteins have evolved alternative strategies to target subcellular compartments.…”

Evolution and function of bacterialRCC1repeat effectors

“…Among the few characterised C. burnetii effectors one is called NopA (Nucleolar protein A) (Burette et al, 2020). NopA is an Icm/Dot‐translocated effector, which contains 4 RCC1 repeats in the C‐terminal part.…”

“…The nucleolar accumulation of Ran(GTP) perturbs nucleocytoplasmic transport and the import of the transcription factor NF‐κB. As a result, the production of cytokines and innate immune signalling is impaired by the C. burnetii effector NopA (Burette et al, 2020).…”