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. 2012 Oct 23:3:237.
doi: 10.3389/fpls.2012.00237. eCollection 2012.

How complex are intracellular immune receptor signaling complexes?

Vera Bonardi  1 , Jeffery L Dangl
Affiliations

How complex are intracellular immune receptor signaling complexes?

Vera Bonardi et al. Front Plant Sci. .

Abstract

Nucleotide binding leucine-rich repeat proteins (NLRs) are the major class of intracellular immune receptors in plants. NLRs typically function to specifically recognize pathogen effectors and to initiate and control defense responses that severely limit pathogen growth in plants (termed effector-triggered immunity, or ETI). Despite numerous reports supporting a central role in innate immunity, the molecular mechanisms driving NLR activation and downstream signaling remain largely elusive. Recent reports shed light on the pre- and post-activation dynamics of a few NLR-containing protein complexes. Recent technological advances in the use of proteomics may enable high-resolution definition of immune protein complexes and possible activation-relevant post-translational modifications of the components in these complexes. In this review, we focus on research aimed at characterizing pre- and post-activation NLR protein complexes and the molecular events that follow activation. We discuss the use of new or improved technologies as tools to unveil the molecular mechanisms that define NLR-mediated pathogen recognition.

Keywords: NLR; disease resistance; effector; immune system; plant; protein complex.

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Figures

FIGURE 1
FIGURE 1
Schematic representation of intramolecular interactions of plant NLRs. (A) Domain modularity of plant NLRs. (B) Intramolecular interactions maintain the NLR in an “off” state through the inhibitory function of the LRR domain (top). Effector recognition results in a conformational change that allows nucleotide cycling and NLR activation (middle). Catalytic activity of the NB domain triggers a second conformational change that exposes the N-terminal domain (bottom).
FIGURE 2
FIGURE 2
Pre- and post-activation status of NLR immune complexes in plants. The order of the NLRs described reflects the presentation in the text. N exists as monomers prior to activation. p50 sequesters the chloroplastic protein NRIP1 and allows association of NRIP1 to the TIR domain of N, and dimerization of N. RPS5 dimerizes in its resting state and is associated with PBS1 through the RPS5 CC domain. AvrPphB targets and cleaves PBS1, activating RPS5. MLA10 exists in inactive homodimers and recognition of the specific pathogen effector triggers nucleotide-binding/hydrolysis/exchange-dependent conformational changes that allow the recruitment of WRKY transcription factors. L6 is in an inactive monomeric state and upon AvrL567 recognition through the LRR domain, L6 self-associates into dimers through TIR–TIR domain interactions. RPS2 associates with RIN4 prior to activation; no evidence for RPS2 homodimerization exists. AvrRpt2 targets and cleaves and this relieves RIN4-dependent suppression of RPS2 activity. Resting state RPM1 is in a heteromeric protein complex that comprises the guardee RIN4. Moreover RIN4 also associates with RIPK, but whether resting state RIN4, RIPK, and RPM1 are part of the same protein complex, or not, remains unknown. AvrB or AvrRpm1 enhance RIPK-mediated phosphorylation of RIN4, and this drives nucleotide-binding/hydrolysis/exchange-dependent activation of RPM1. Prf forms homodimers that bridge Pto to Fen, or possibly another Pto-family kinase. AvrPto targets Pto and recognition results in a conformational change that activates Prf signaling. AvrPtoB is an E3 ubiquitin ligase that initiates the degradation of Fen, moreover AvrPtoB recognition by Pto results in the phosphorylation of the E3 ligase domain of AvrPtoB by Pto, thus Pto is resistant to AvrPtoB-mediated degradation. No evidence for RPS4 self-association exists, thus the RPS4 inactive state is thought to contain monomeric RPS4, EDS1, and SRFR1. Cleavage of AvrRps4 releases the C-terminus AvrRps4C that interacts with EDS1, thus altering the endomembrane-associated receptor complex. Post-delivery effector processing is a common event, however it is not detailed in this review. Release of the EDS1-containing RPS4 complexes to the cytoplasm and to the nucleus is thought to activate two different defense branches: cell death, and bacterial growth-restriction respectively, and these may occur in different cellular compartments.
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